Methods for Biological Material Labeling and Medical Imaging CROSS-REFERENCES TO RELATED APPLICATIONS [0001] This application is based on, claims benefit of, and claims priority to U.S. Application No.63/230,318 filed on August 6, 2021, which is hereby incorporated by reference herein in its entirety for all purposes. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH [0002] Not Applicable. BACKGROUND OF THE INVENTION 1. Field of the Invention [0003] This invention relates to methods of labeling of a biological material for medical imaging, methods for preparing a labeling agent, and methods for medical imaging of a subject using a biological material labeled with a labeling agent. 2. Description of the Related Art [0004] In cell therapy, cells are injected into the body as a therapy against diseases such as some cancers, or to regenerate damaged tissues. This type of treatment benefits from the ability to image cells as they migrate though the body in order to evaluate therapy progress. A number of radioisotopic cell labeling methods have traditionally been used for single-photon emission computerized tomography (SPECT) and positron emission tomography (PET) imaging-based cell tracking (see Nguyen et al., [ Ref.81]) (The full citation for all numbered references (i.e., ending with Ref. XX) mentioned throughout the specification can be found in the “References” section of the specification (i.e., ¶ [00149]). As described below, the citation of any document or reference is not to be construed as an admission that it is prior art with respect to the present invention.) However, a PET-based approach offers superior quantification and imaging sensitivity characteristics over a SPECT- based approach, which are critical for tracking of small numbers of administered cells (see Nguyen et al., [Ref.81]). In this regard, ⁸⁹ Zr has emerged as an attractive PET radionuclide for cell labeling applications due to its high spatial resolution and 78.4- hour half-life that may allow monitoring of administered cells up to a 2-3 week period. U.S. Patent Application Publication No.2018/0043041 to Aditya Bansal et al. discloses methods of ex vivo labeling of a biological material for in vivo imaging, methods of labeling a biological material in vivo, methods for preparing a labeling agent, and methods for in vivo imaging of a subject using a biological material labeled with a labeling agent. With respect to labeling agents, U.S.2018/0043041 discloses a method of synthesizing ⁸⁹ Zr-labeled p-isothiocyanato-benzyl- desferrioxamine ( ⁸⁹ Zr-DBN), a labeling agent that uses ⁸⁹ Zr as a long-half-life radionuclide (3.3 days), enabling PET imaging of labeled injected biological material for several weeks. The ⁸⁹ Zr-DBN can act as a general Zr-89 labeling synthon and can be produced at a centralized facility and shipped to various sites and labs that need to perform labeling of biological materials such as, without limitation, cells, liposomes, DNA aptamers, RNA aptamers, viruses, nanoparticles, extracellular vesicles, microorganisms, antibodies, proteins, peptides, scaffolds, polymers, and nucleic acids. [0005] Even with the technical advances provided by the methods and labeling agents of U.S.2018/0043041, the labeling of biological materials (such as some viruses) at very low concentrations can be difficult and/or unsuccessful. Therefore, there is a need for even more improved labeling agents which make possible the successful labeling of biological materials at very low concentrations. SUMMARY OF THE INVENTION [0006] The present disclosure provides a new way of synthesizing a labeling agent, such as ⁸⁹ Zr-labeled p-isothiocyanato-benzyl-desferrioxamine ( ⁸⁹ Zr-DBN), using a purification step that results in 2-3-fold increased molar activity and labeling efficiency, which makes possible the successful labeling of biological materials (such as some viruses or extracellular vesicles) at very low concentrations where an unpurified labeling agent would be difficult to radiolabel and can thus result in poor labeling. [0007] In one aspect, the present disclosure provides a method for preparing a labeling agent. The method comprises: (a) providing a compound including a chelating moiety and a conjugation moiety; (b) contacting the compound with a radionuclide to create a radiolabeled preparation having a first molar activity measured at an end of step (b); and (c) purifying the radiolabeled preparation to prepare a labeling agent having a second molar activity measured at an end of step (c), wherein the second molar activity is greater than the first molar activity. The second molar activity can be at least two times greater than the first molar activity. [0008] In some aspects, a second molar activity can be at least two times greater than a first molar activity. [0009] In some aspects, a first molar activity can be in a range of 1 to 50 GBq/µmol. [0010] In some aspects, a second molar activity can be in a range of 100 to 500 GBq/µmol. [0011] In some aspects, a radionuclide can be selected from the group consisting of ¹¹ C, ¹³ N, ¹⁵ O, ¹⁸ F, ^34m Cl, ³⁸ K, ⁴⁵ Ti, ⁵¹ Mn, ⁵² Mn, ^52m Mn, ⁵² Fe, ⁵⁵ Co, ⁶⁰ Cu, ⁶¹ Cu, ⁶² Cu, ⁶⁴ Cu, ⁶⁶ Ga, ⁶⁸ Ga, ⁷¹ As, ⁷² As, ⁷⁴ As, ⁷⁵ Br, ⁷⁶ Br, ⁸² Rb, ⁸⁶ Y, ⁸⁹ Zr, ⁹⁰ Nb, ^94m Tc, ^99m Tc, ^110m In, ¹¹¹ In, ¹¹⁸ Sb, ¹²⁰ I, ²⁰³ Pb ^{, 121} I, ¹²² I, ¹²³ I, and ¹²⁴ I. [0012] In some aspects, step (b) can include contacting a compound with a solution of a halide including a radionuclide cation. [0013] In some aspects, a radionuclide cation can be ⁸⁹ Zr ⁺⁴ . [0014] In some aspects, a halide can be chloride (Cl-). [0015] In some aspects, step (b) can include contacting a compound with ⁸⁹ Zr- chloride in a hydrochloride solution. [0016] In some aspects, step (b) can include contacting a compound with ⁸⁹ Zr- chloride in a hydrochloride solution at a pH in a range of 7 to 9. [0017] In some aspects, step (c) can include purifying a radiolabeled preparation using reverse phase chromatography. [0018] In some aspects, step (c) can include purifying a radiolabeled preparation using gradient elution. [0019] In some aspects, gradient elution can use at least two different solvents. [0020] In some aspects, one of the solvents can include water and trifluoroacetic acid, and another of the solvents can include acetonitrile and trifluoroacetic acid. [0021] In some aspects, a chelating moiety can be a hydroxamic acid group. [0022] In some aspects, a hydroxamic acid group can be a desferrioxamine group. [0023] In some aspects, a conjugation moiety can include an isothiocyanate group. [0024] In some aspects, a conjugation moiety can include a benzyl group. [0025] In some aspects, a labeling agent can be a ⁸⁹ Zr-isothiocyanato-benzyl- desferrioxamine. [0026] In some aspects, a labeling agent can have a radiochemical stability greater than 60% measured at 72 hours after step (c). [0027] In some aspects, a method can include (d) adding a stabilizer to a labeling agent. [0028] In some aspects, a stabilizer can be ascorbic acid. [0029] In some aspects, a labeling agent can have a radiochemical stability greater than 80% measured at 72 hours after step (d). [0030] In some aspects, step (b) can include creating a radiolabeled preparation at a radiochemical yield of at least 95%. [0031] In another aspect, the present disclosure provides a method of labeling of a biological material for imaging. The method comprises contacting a biological material with the labeling agent prepared by the method of this disclosure such that the biological material becomes labeled for imaging, wherein the biological material is selected from cells, liposomes, DNA aptamers, RNA aptamers, viruses, nanoparticles, extracellular vesicles, microorganisms, antibodies, proteins, peptides, scaffolds, polymers, and nucleic acids. [0032] In some aspects, a radiolabeling yield when contacting a biological material with a labeling agent can be at least 5% when the biological material is contacted with the labeling agent at a concentration of the biological material of 0.1 mg/mL. [0033] In some aspects, a radiolabeling yield when contacting a biological material with a labeling agent can be at least 15% when the biological material is contacted with the labeling agent at a concentration of the biological material of 0.5 mg/mL. [0034] In some aspects, a radiolabeling yield when contacting a biological material with a labeling agent can be at least 30% when the biological material is contacted with the labeling agent at a concentration of the biological material of 1.0 mg/mL. [0035] In some aspects, a first radiolabeling yield when contacting a first amount of a biological material with a first quantity of a labeling agent can be greater than a second radiolabeling yield when contacting a second amount of the biological material with a second quantity of a radiolabeled preparation created in step (b). The first amount and the second amount can be the same. The first quantity and the second quantity can be the same. [0036] In some aspects, a first radiolabeling yield when contacting a first amount of a biological material with a first quantity of a labeling agent can be at least two times greater than a second radiolabeling yield when contacting a second amount of the biological material with a second quantity of a radiolabeled preparation created in step (b). The first amount and the second amount can be the same. The first quantity and the second quantity can be the same. [0037] In some aspects, a biological material can be selected from antibodies. The biological material can include an antibody. [0038] In some aspects, a biological material can be selected from proteins. The biological material can include a protein. [0039] In some aspects, a biological material can be selected from cells. The biological material can include a cell. [0040] In some aspects, a biological material can be selected from viruses. The biological material can include a virus. [0041] In some aspects, a biological material can be selected from stem cells. The biological material can include a stem cell. [0042] In some aspects, a biological material can be selected from white blood cells. The biological material can include a white blood cell. [0043] In another aspect, the present disclosure provides a method for in vivo imaging of a subject. The method can include (a) administering to the subject a biological material labeled with a labeling agent prepared by a method of this disclosure, (b) waiting a time sufficient to allow the biological material to accumulate at a tissue site to be imaged, and (c) imaging the tissues with a non-invasive imaging technique. The biological material can be selected from cells, liposomes, DNA aptamers, RNA aptamers, viruses, nanoparticles, microorganisms, antibodies, proteins, peptides, scaffolds, polymers, and nucleic acids. [0044] In some aspects, a non-invasive imaging technique can be selected from positron emission tomography imaging, positron emission tomography with computed tomography imaging, or positron emission tomography with magnetic resonance imaging. [0045] In another aspect, the present disclosure provides a method of imaging a subject by emission tomography. The method can include (a) administering to the subject a biological material labeled with a labeling agent prepared by a method of the present disclosure, (b) using a plurality of detectors to detect gamma rays emitted from the subject and to communicate signals corresponding to the detected gamma rays, and (c) reconstructing from the signals a series of medical images of a region of interest of the subject. The biological material can be selected from cells, liposomes, DNA aptamers, RNA aptamers, viruses, nanoparticles, microorganisms, antibodies, proteins, peptides, scaffolds, polymers, and nucleic acids. [0046] In another aspect, the preset disclosure provides an imaging method. The imaging method can include acquiring an image of a subject to whom a detectable amount of a biological material labeled with a labeling agent prepared by a method of the presented disclosure has been administered. The biological material can be selected from cells, liposomes, DNA aptamers, RNA aptamers, viruses, nanoparticles, microorganisms, antibodies, proteins, peptides, scaffolds, polymers, and nucleic acids. [0047] In some aspects, a method can include acquiring an image using positron emission tomography imaging, positron emission tomography with computed tomography imaging, or positron emission tomography with magnetic resonance imaging. [0048] In some aspects, a biological material can be selected from antibodies. [0049] In some aspects, a biological material can be selected from proteins. [0050] In some aspects, a biological material can be selected from cells. [0051] In some aspects, a biological material can be selected from viruses. [0052] In some aspects, a biological material can be selected from stem cells. [0053] In some aspects, a biological material can be selected from white blood cells. [0054] In yet another aspect, the present disclosure provides a method for determining radiolabeling efficiency when a biological material is contacted with a labeling agent including a radionuclide to produce a radiolabeled biological material. The method comprises separating the radiolabeled biological material produced when the biological material is contacted with the labeling agent from free radionuclide and unconjugated labeling agent using instant thin layer chromatography. [0055] In some aspects, a labeling agent can be the labeling agent prepared by a method of this disclosure. [0056] In some aspects, a biological material can be selected from cells, liposomes, DNA aptamers, RNA aptamers, viruses, nanoparticles, microorganisms, antibodies, proteins, peptides, scaffolds, polymers, and nucleic acids. [0057] In some aspects, a biological material can be selected from antibodies. [0058] In some aspects, instant thin layer chromatography can use an acid- alcohol mixture as a mobile phase and a gel as a solid phase. [0059] In some aspects, an acid-alcohol mixture can include citric acid and methanol. [0060] In some aspects, a gel can include silica. [0061] These and other features, aspects, and advantages of the present invention will become better understood upon consideration of the following detailed description, drawings and appended claims. BRIEF DESCRIPTION OF THE DRAWINGS [0062] FIG.1 is a schematic of a positron emission tomography (PET) system. [0063] FIG.2 shows a comparison of HPLC traces of unpurified and purified [ ⁸⁹ Zr]Zr-DBN. [0064] FIG.3 shows a comparison of radiolabeling yield of antibody (IgG) with purified and unpurified [ ⁸⁹ Zr]Zr-DBN as a function of protein (IgG) concentration. [0065] FIG.4 shows a comparison of HPLC trace and peak analysis of purified [ ⁸⁹ Zr]Zr-DBN with and without addition of ascorbic acid. [0066] FIG.5 shows a comparison of HPLC trace and peak analysis of purified [ ⁸⁹ Zr]Zr-DBN with and without addition of ascorbic acid at 72 hours post HPLC purification. [0067] FIG.6 shows an rTLC analysis of [ ⁸⁹ Zr]Zr-DBN in 100mM DTPA (pH 7) solution. [0068] FIG.7 shows a concentration calibration curve for DFO-NCS. [0069] FIG.8 shows an rTLC analysis of [ ⁸⁹ Zr]Zr-chloride in 20mM citric acid (pH 4.9–5.1) 1:1 methanol (v:v) solution. [0070] FIG.9 shows an rTLC analysis of [ ⁸⁹ Zr]Zr-DBN in 20mM citric acid (pH 4.9–5.1) 1:1 methanol (v:v) solution [0071] FIG.10 shows an rTLC analysis of [ ⁸⁹ Zr]Zr-IgG and [ ⁸⁹ Zr]Zr-DBN in 20mM citric acid (pH 4.9–5.1) 1:1 methanol (v:v) solution. [0072] FIG.11 shows in panels A-C: HPGe spectrums of ⁸⁹ Zr produced at timepoint ⁸⁸ Zr and ⁸⁸ Y. Panel A: HPGe spectrum of ⁸⁹ Zr. Panel B: HPGe spectrum of ⁸⁹ Zr 60 days after the EOB. Panel C: HPGe spectrum of ⁸⁹ Zr 96 days after the EOB (produced at 15.2 MeV beam energy). [0073] FIG.12 shows an HPLC trace of [ ⁸⁹ Zr]Zr-chloride. [0074] FIG.13 shows chemical structures of [ ⁸⁹ Zr]Zr-DFO-Bn-NCS. [0075] FIG.14 shows percentage of ⁸⁹ Zr complexation with DFO-Bn-NCS, at different reaction times. [0076] FIG.15 shows radiolabeling of white blood cells (WBCs) with [ ⁸⁹ Zr]Zr-DFO- Bn-NCS. [0077] FIG.16 shows radiolabeling of stem cells with [ ⁸⁹ Zr]Zr-DFO-Bn-NCS. [0078] FIG.17 shows representative PET images showing distribution of WBCs labeled with [ ⁸⁹ Zr]Zr-DFO-NCS in athymic nude mice at different time points post- injection. [0079] FIG.18 shows standardized uptake value (SUV) and biodistribution of WBCs labeled with [ ⁸⁹ Zr]Zr-DFO-NCS in major organs of athymic nude mice at day 7 post-injection.. [0080] FIG.19 shows standardized uptake value (SUV) and biodistribution of WBCs labeled with [ ⁸⁹ Zr]Zr-DFO-NCS in athymic nude mice at day 7 post-injection. [0081] FIG.20 shows representative PET images showing distribution of stem cells labeled with [ ⁸⁹ Zr]Zr-DFO-NCS in athymic nude mice at different time points post-injection. [0082] FIG.21 shows standardized uptake value (SUV) and biodistribution of stem cells labeled with [ ⁸⁹ Zr]Zr-DFO-NCS in lung, liver and spleen of athymic nude mice at day 7 post-injection. [0083] FIG.22 shows standardized uptake value (SUV) and biodistribution of stem cells labeled with [ ⁸⁹ Zr]Zr-DFO-NCS, in lung, liver and spleen of athymic nude mice at day 7 post-injection. [0084] FIG.23 shows representative PET images showing distribution of [ ⁸⁹ Zr]ZrCl ₄ in athymic nude mice at different time points post-injection. [0085] FIG.24 shows standardized uptake value (SUV) and distribution of [ ⁸⁹ Zr]ZrCl ₄ in athymic nude mice at day 7 post-injection. DETAILED DESCRIPTION OF THE INVENTION [0086] The present disclosure provides a new way of synthesizing a labeling agent, such as ⁸⁹ Zr-DBN, using a purification step that results in 2-3 fold increased molar activity and labeling efficiency, which makes possible the successful labeling of biological materials (such as some viruses) at very low concentrations where an unpurified labeling agent would be difficult to radiolabel and can thus result in poor labeling. [0087] Referring now to FIG.1, a PET system 100 that can be used with the labeled biological material of the present invention comprises an imaging hardware system 110 that includes a detector ring assembly 112 about a central axis or bore 114. An operator workstation 116 including a commercially available processor running a commercially available operating system communicates through a communications link 118 with a gantry controller 120 to control operation of the imaging hardware system 110. [0088] The detector ring assembly 112 is formed of a multitude of radiation detector units 122 that produce a signal responsive to detection of a photon on communications line 124 when an event occurs. A set of acquisition circuits 126 receive the signals and produce signals indicating the event coordinates (x, y) and the total energy associated with the photons that caused the event. These signals are sent through a cable 128 to an event locator circuit 130. Each acquisition circuit 126 also produces an event detection pulse that indicates the exact moment the interaction took place. Other systems utilize sophisticated digital electronics that can also obtain this information regarding the precise instant in which the event occurred from the same signals used to obtain energy and event coordinates. [0089] The event locator circuits 130 in some implementations, form part of a data acquisition processing system 132 that periodically samples the signals produced by the acquisition circuits 126. The data acquisition processing system 132 includes a general controller 134 that controls communications on a backplane bus 136 and on the general communications network 118. The event locator circuits 130 assemble the information regarding each valid event into a set of numbers that indicate precisely when the event took place and the position in which the event was detected. This event data packet is conveyed to a coincidence detector 138 that is also part of the data acquisition processing system 132. [0090] The coincidence detector 138 accepts the event data packets from the event locator circuit 130 and determines if any two of them are in coincidence. Coincidence is determined by a number of factors. First, the time markers in each event data packet must be within a predetermined time window, for example, 0.5 nanoseconds or even down to picoseconds. Second, the locations indicated by the two event data packets must lie on a straight line that passes through the field of view in the scanner bore 114. Events that cannot be paired are discarded from consideration by the coincidence detector 138, but coincident event pairs are located and recorded as a coincidence data packet. These coincidence data packets are provided to a sorter 140. The function of the sorter in many traditional PET imaging systems is to receive the coincidence data packets and generate memory addresses from the coincidence data packets for the efficient storage of the coincidence data. In that context, the set of all projection rays that point in the same direction (θ) and pass through the scanner's field of view (FOV) is a complete projection, or "view". The distance (R) between a particular projection ray and the center of the FOV locates that projection ray within the FOV. The sorter 140 counts all of the events that occur on a given projection ray (R, θ) during the scan by sorting out the coincidence data packets that indicate an event at the two detectors lying on this projection ray. The coincidence counts are organized, for example, as a set of two- dimensional arrays, one for each axial image plane, and each having as one of its dimensions the projection angle θ and the other dimension the distance R. This θ by R map of the measured events is call a histogram or, more commonly, a sinogram array. It is these sinograms that are processed to reconstruct images that indicate the number of events that took place at each image pixel location during the scan. The sorter 140 counts all events occurring along each projection ray (R, θ) and organizes them into an image data array. [0091] The sorter 140 provides image datasets to an image processing / reconstruction system 142, for example, by way of a communications link 144 to be stored in an image array 146. The image arrays 146 hold the respective datasets for access by an image processor 148 that reconstructs images. The image processing/reconstruction system 142 may communicate with and/or be integrated with the work station 116 or other remote work stations. [0092] The PET system 100 provides an example emission tomography system for acquiring a series of medical images of a subject during an imaging process after administering a pharmaceutically acceptable composition including labeled biological materials as described herein. The system includes a plurality of detectors configured to be arranged about the subject to acquire gamma rays emitted from the subject over a time period relative to an administration of the composition to the subject and communicate signals corresponding to acquired gamma rays. The system also includes a reconstruction system configured to receive the signals and reconstruct therefrom a series of medical images of the subject. In one version of the system, a second series of medical images is concurrently acquired using an x-ray computed tomography imaging device. In one version of the system, a second series of medical images is concurrently acquired using a magnetic resonance imaging device. [0093] Administration to the subject of a pharmaceutical composition including radiolabeled biological materials for in vivo detection of the accumulated biological materials in a target region of the subject can be accomplished intravenously, intraarterially, intrathecally, intramuscularly, intradermally, subcutaneously, or intracavitary. A "subject" is a mammal, preferably a human. In the method of the invention, sufficient time is allowed after administration of a detectable amount of the radiolabeled biological materials such that the radiolabeled biological materials can accumulate in a target region of the subject. A "detectable amount" means that the amount of the detectable radiolabeled biological materials that is administered is sufficient to enable detection of accumulation of the radiolabeled biological materials in a subject by a medical imaging technique. [0094] One non-limiting example method of imaging according to the invention involves the use of an intravenous injectable composition including radiolabeled biological materials. A positron emitting atom of the radiolabeled biological materials gives off a positron, which subsequently annihilates and gives off coincident gamma radiation. This high energy gamma radiation is detectable outside the body using positron emission tomography imaging, or positron emission tomography concurrent with computed tomography imaging (PET/CT). With PET/CT, the location of the injected and subsequently accumulated labeled biological materials within the body can be identified. [0095] In one embodiment, the present invention provides a method for preparing a labeling agent. The method comprises: (a) providing a compound including a chelating moiety and a conjugation moiety; (b) contacting the compound with a radionuclide to create a radiolabeled preparation having a first molar activity measured at an end of step (b); and (c) purifying the radiolabeled preparation to prepare a labeling agent having a second molar activity measured at an end of step (c), wherein the second molar activity is greater than the first molar activity. The second molar activity can be at least two times greater than the first molar activity. The second molar activity can be at least two times greater than the first molar activity. The first molar activity can be in a range of 1 to 50 GBq/µmol. The second molar activity can be in a range of 100 to 500 GBq/µmol. [0096] In the method, the chelating moiety can be a hydroxamic acid group. The hydroxamic acid group can be a desferrioxamine group. The conjugation moiety can include an isothiocyanate group. The conjugation moiety can include a benzyl group. The labeling agent can be ⁸⁹ Zr-isothiocyanato-benzyl-desferrioxamine. [0097] The radionuclide can selected from the group consisting of ¹¹ C, ¹³ N, ¹⁵ O, ¹⁸ F, ^34m Cl, ³⁸ K, ⁴⁵ Ti, ⁵¹ Mn, ⁵² Mn, ^52m Mn, ⁵² Fe, ⁵⁵ Co, ⁶⁰ Cu, ⁶¹ Cu, ⁶² Cu, ⁶⁴ Cu, ⁶⁶ Ga, ⁶⁸ Ga, ⁷¹ As, ⁷² As, ⁷⁴ As, ⁷⁵ Br, ⁷⁶ Br, ⁸² Rb, ⁸⁶ Y, ⁸⁹ Zr, ⁹⁰ Nb, ^94m Tc, ^99m Tc, ^110m In, ¹¹¹ In, ¹¹⁸ Sb, ¹²⁰ I, ¹²¹ I, ¹²² I, ¹²³ I, and ¹²⁴ I. Step (b) of the method can comprise contacting the compound with a solution of a halide including a radionuclide cation. The radionuclide cation can be ⁸⁹ Zr ⁺⁴ . The halide can be chloride (Cl-). Step (b) of the method can comprise contacting the compound with ⁸⁹ Zr-chloride in a hydrochloride solution. Step (b) of the method can comprise contacting the compound with ⁸⁹ Zr-chloride in a hydrochloride solution at a pH in a range of 7 to 9. [0098] Step (c) of the method can comprise purifying the radiolabeled preparation using reverse phase chromatography. Step (c) of the method can comprise purifying the radiolabeled preparation using gradient elution. The gradient elution may use at least two different solvents. One of the solvents can comprise water and trifluoroacetic acid, and another of the solvents can comprise acetonitrile and trifluoroacetic acid. [0099] The labeling agent produced by the method can have a radiochemical stability greater than 60% measured at 72 hours after step (c). In step (d), the method may further comprise adding a stabilizer to the labeling agent. The stabilizer can be ascorbic acid. The labeling agent produced by the method can have a radiochemical stability greater than 80% measured at 72 hours after step (d). In the method, step (b) can comprise creating the radiolabeled preparation at a radiochemical yield of at least 95%, or least 97%. [00100] In another embodiment, the present invention provides a method of labeling of a biological material for imaging. The method comprises contacting a biological material with the labeling agent prepared by the method of the invention such that the biological material becomes labeled for imaging, wherein the biological material is selected from cells, liposomes, DNA aptamers, RNA aptamers, viruses, nanoparticles, microorganisms, antibodies, proteins, peptides, scaffolds, polymers, and nucleic acids. A radiolabeling yield when contacting the biological material with the labeling agent can be at least 5% when the biological material is contacted with the labeling agent at a concentration of the biological material of 0.1 mg/mL. A radiolabeling yield when contacting the biological material with the labeling agent can be at least 15% when the biological material is contacted with the labeling agent at a concentration of the biological material of 0.5 mg/mL. A radiolabeling yield when contacting the biological material with the labeling agent can be at least 30% when the biological material is contacted with the labeling agent at a concentration of the biological material of 1.0 mg/mL. [00101] A first radiolabeling yield when contacting a first amount of the biological material with a first quantity of the labeling agent can be greater than a second radiolabeling yield when contacting a second amount of the biological material with a second quantity of the radiolabeled preparation created in step (b), wherein the first amount and the second amount are the same, and wherein the first quantity and the second quantity are the same. A first radiolabeling yield when contacting a first amount of the biological material with a first quantity of the labeling agent can be at least two times greater than a second radiolabeling yield when contacting a second amount of the biological material with a second quantity of the radiolabeled preparation created in step (b), wherein the first amount and the second amount are the same, and wherein the first quantity and the second quantity are the same. [00102] In yet another embodiment, the present disclosure provides a method for determining radiolabeling efficiency when a biological material is contacted with a labeling agent including a radionuclide to produce a radiolabeled biological material. The method comprises separating the radiolabeled biological material produced when the biological material is contacted with the labeling agent from free radionuclide and unconjugated labeling agent using instant thin layer chromatography. The labeling agent can be the labeling agent prepared by the method of the invention. The biological material can be selected from cells, liposomes, DNA aptamers, RNA aptamers, viruses, nanoparticles, microorganisms, antibodies, proteins, peptides, scaffolds, polymers, and nucleic acids. The instant thin layer chromatography may use an acid-alcohol mixture as a mobile phase and a gel as a solid phase. The acid-alcohol mixture can comprise citric acid and methanol. The gel can comprise silica. In some cases, while the solid phase can be a gel, in other configurations, the solid phase can be a solid (e.g., alumina coated substrate), a liquid supported on a solid, etc. EXAMPLES [00103] The following Examples are provided in order to demonstrate and further illustrate certain embodiments and aspects of the present invention and are not to be construed as limiting the scope of the invention. Example 1 Overview of Example 1 [00104] Due to the advent of various biologics like antibodies, proteins, cells, viruses, and extracellular vesicles as biomarkers for disease diagnosis, progression, and as therapeutics, there exists a need to have a simple and ready to use radiolabeling synthon to enable noninvasive imaging trafficking studies. Previously, we reported [ ⁸⁹ Zr]zirconium-p-isothiocyanatobenzyl-desferrioxamine ([ ⁸⁹ Zr]Zr-DBN) as a synthon for the radiolabeling of biologics to allow PET imaging of cell trafficking. In Example 1, we focused on improving the molar activity (A _m ) of [ ⁸⁹ Zr]Zr-DBN, by developing a new reverse phase HPLC method to purify [ ⁸⁹ Zr]Zr-DBN. First, 4.78±0.33 GBq (129.3±8.9 mCi) of ⁸⁹ Zr was produced at 40 µA for 180 minutes (3 hours) proton irradiation decay corrected to the end of bombardment with a saturation yield of 4.56±0.31 MBq/µA. Second, after reverse phase HPLC purification, the molar activity of [ ⁸⁹ Zr]Zr-DBN was found to be in 165-316 GBq/µmol range. The high molar activity of [ ⁸⁹ Zr]Zr-DBN also allowed radiolabeling of low concentration of proteins in relatively higher yield. The stability of [ ⁸⁹ Zr]Zr-DBN was measured over time with and without the presence of ascorbic acid. The HPLC method of [ ⁸⁹ Zr]Zr-DBN purification can be adopted in the routine production of [ ⁸⁹ Zr]Zr-DBN as a ready to use labeling synthon. Introduction to Example 1 [00105] ⁸⁹ Zr has emerged as a preferred positron emission tomography (PET) isotope for the radiolabeling of various antibodies, viruses, cells, exosomes, extracellular vesicles (EVs) and nanoparticles (NPs) due to its long half-life (78.4 hours) and suitable PET imaging characteristics (β ⁺ _max -0.9 MeV, 22.7%) [Ref.1-4]. To provide a stable labeling strategy, the macromolecules are normally covalently conjugated with a suitable chelator by using primary amines, hydroxyls or carboxylic groups present on the molecules [Ref.5]. During the conjugation reaction, appropriately functionalized (activated esters, isothiocyanates, reactive ketones and other easily reactive functional groups) chelators are used in excess (typically 3-6 fold) as compared to the biologics/macromolecules to ensure adequate availability of the chelators on the biologics/macromolecules post-conjugation to enable efficient radiolabeling with ⁸⁹ Zr [Ref.6-13]. The molar activity (A _m ) of the final radiolabeled biologic/macromolecule depends on the initial starting radioactivity, molar activity of the ⁸⁹ Zr, radiolabeling yield and amount (mmol or mg) of the biologics/macromolecules present in the final formulation. To achieve high molar activity (A _m ), a high concentration of ⁸⁹ Zr radioactivity (MBq/μL) is needed, so that only a small mass (μg or ng range) of biologics/macromolecules can be used for an efficient radiolabeling, otherwise more mass of the biologics/macromolecules will be needed to compensate for the dilution caused by the higher volume of radioactivity. The production of ⁸⁹ Zr from the cyclotron has been well documented using both liquid targets and solid targets [Ref.8-10, 13]. Among the various reported methods of ⁸⁹ Zr production from solid targets, yttrium foil, yttrium-sputtering, yttrium-pellet and yttrium deposition are commonly known [Ref.8, 11-12]. In Example 1, we aimed to improve the radiolabeling yield of ⁸⁹ Zr labeled biologics/macromolecules by developing a new method of purification of [ ⁸⁹ Zr]zirconium-p-isothiocyanatobenzyl-desferrioxamine ([ ⁸⁹ Zr]Zr-DBN [Ref.2-3]) as a ready to use radiolabeling synthon for the direct radiolabeling of the biologics/macromolecules with a high molar activity [ ⁸⁹ Zr]Zr-DBN. 2. Materials and Methods 2.1 Targetry Details [00106] A PETtrace cyclotron (GE HealthCare, Waukesha, WI, USA) was used in this study. To perform Zr-89 production, an Advanced Cyclotron Systems Inc. (ACSI) target holder was used and placed after the switching magnet at a 30 degree angle with respect to beamline on a PETtrace cyclotron; the proton beam energy was degraded using 0.1, 0.2, and 0.3 mm aluminum foils to 15.2, 13.9, and 12.3 MeV, respectively, as estimated from TRIM program. Variable thickness (0.1 mm and 0.127 mm) of yttrium foils were purchased from the Alfa-Aesar (50x50 mm, 99.9%) Haverhill, MA, USA. 2.2 Chemicals [00107] The trace metal grade concentrated nitric acid (67-70 % as HNO ₃ ) and hydrochloric acid (34-37% as HCl) were purchased from the Fisher Chemicals part of the Thermo Fisher Scientific (Waltham MA, USA). Sodium bicarbonate, acetonitrile (HPLC grade) and trifluoroacetic acid (TFA, 99%) were purchased from Sigma- Aldrich (St. Louis, MO, USA). The hydroxamate resin was synthesized as previously described by the Pandey et al. [Ref.10]. The i-TLC paper was purchased from Agilent Technologies (Palo Alto, CA, USA). The labeling precursor p-SCN-Bn- Deferoxamine (B-705, ≥94%) was purchased from Macrocyclics, Plano, TX, USA. Deionized water was obtained from Barnstead Nanopure water purification system from Thermo Fisher Scientific, Waltham, MA, USA. 2.3 Instrumentation [00108] The radioactive samples were counted using a Wizard 2480 gamma counter (Perkin Elmer, Waltham, MA, USA). Radionuclidic purity was evaluated using a high-purity germanium gamma spectrometer (Canberra, Meriden, CT, USA) running Genie 2000 software. The radioactivity readings were recorded using a CRC dose calibrator (489 setting for ⁸⁹ Zr and 465 for ⁸⁸ Y, CRC-55tPET, Capintec, Ramsey, NJ, USA). 2.4 Purification of Cyclotron Produced ⁸⁹ Zr and Radiosynthesis of [ ⁸⁹ Zr]Zr-DBN [00109] The cyclotron-produced ⁸⁹ Zr was purified following previously reported methods [Ref.8-10]. The purified [ ⁸⁹ Zr]Zr-oxalate was converted to [ ⁸⁹ Zr]Zr-chloride using anion exchange column Chromafix 30-PS-HCO ₃ SPE 45 mg cartridge (Macherey-Nagel, Düren, Germany) following the method of Larenkov et al. [Ref.14]. The cartridge was activated with a 6.0 mL acetonitrile followed by 10 mL saline and 10 mL deionized water wash with air drying steps in between each solvent. The ⁸⁹ Zr was trapped on an activated Chromafix 30-PS-HCO ₃ SPE (45 mg) cartridge and oxalate was removed with 50 mL deionized water. Finally, ⁸⁹ Zr was eluted as [ ⁸⁹ Zr]Zr-chloride with 0.5 mL 1N HCl in a 97.4±1.1 (n=10) elution efficiency. The eluted [ ⁸⁹ Zr]Zr-chloride was then dried using a Savant™ SpeedVac™ High Capacity Concentrator (Thermo Fisher Scientific Inc., Logan, UT, USA) at 0.42 torr and 65°C. The dried [ ⁸⁹ Zr]Zr-chloride was resuspended in ~180 µL of 0.1N HCl and then neutralized to pH ~8.0 with ~18 µL 1M Na ₂ CO ₃ . To this, 2.5 mM DFO-Bz-NCS in DMSO (Macrocyclics, Plano, TX, USA) was added to give a final concentration of ~54 µM DFO-Bz-NCS, and chelation of ⁸⁹ Zr proceeded at 37°C for ~30 minutes in a thermomixer at 550 rpm. The chelation efficiency was determined by silica gel iTLC (Agilent Technologies Inc., Santa Clara, CA, USA) with 100 mM DTPA pH 7 as the mobile phase. [ ⁸⁹ Zr]Zr-DBN showed an R _f =0, whereas [ ⁸⁹ Zr]Zr-chloride had an R _f ^=^0.9 (see Figure 6). 2.5 HPLC Purification of [ ⁸⁹ Zr]Zr-DBN [00110] The [ ⁸⁹ Zr]Zr-DBN reaction mixture was diluted to ~1.0 mL with deionized water (neutralized with sodium carbonate to final pH 7.0) before HPLC purification. The purification was performed at room temperature using a reverse phase Agilent Zorbax 300-SB-C-18 (5 µm; 9.4 X 250mm) column (Agilent Technologies Inc., Santa Clara, CA, USA) and 1.0 mL injection loop size. The gradient elution was performed with solvent A (deionized H ₂ O + 0.1% TFA) and solvent B (Acetonitrile + 0.1% TFA). The flow rate was set at 1.8 mL/min and absorbance was set at 220 nm. The purification was performed using 0-5 min (static 5% solvent B), 5-10 min (gradient, 5-34% solvent B), 10-95 min (gradient, 34-41.5% solvent B), 95-100 min (gradient, 41.5-85 % solvent B), 100-110 min (gradient, 85- 5% solvent B) and 110-115 min (static, 5% solvent B) gradient program. The total separation time was ~35 minutes. Blank runs were performed in between the sample injections. Concentration of nonradioactive (UV) Zr-DBN was estimated using a calibration curve (see Figure 7). 2.6 Radiolabeling of Antibody with Purified [ ⁸⁹ Zr]Zr-DBN [00111] The purified [ ⁸⁹ Zr]Zr-DBN (~7.2 mL) was collected at the appropriate retention time in a glass test tube and dried using the concentrator at 0.42 torr and at room temperature. For radiolabeling of an example antibody, different concentrations (0.1, 0.5, and 1.0 mg/mL) of human IgG were prepared in phosphate buffered saline (PBS) from a stock solution (~10 mg/mL) of ChromPure Human IgG, whole molecule (Jackson Immuno Research Inc., West Grove, PA, USA). The pH of the different human IgG solutions was adjusted to pH 9.0 using appropriate volumes of 0.5 M Na ₂ CO ₃ . Immediately after adjusting the pH, 200 µL of pH adjusted human IgG solution was added to ~ 3.7MBq of [ ⁸⁹ Zr]Zr-DBN. The final pH was adjusted with additional volumes of 0.5 M Na ₂ CO ₃ to achieve a pH of 9.0. The IgG radiolabeling reaction was performed at 37°C for ~30 minutes in a thermomixer at 550 rpm. After 30 minutes of reaction, the extent of radiolabeling was quantified using silica gel iTLC (Agilent Technologies Inc., Santa Clara, CA, USA) with 20 mM citric acid (pH 4.9– 5.1) : methanol (1:1, V:V) as the mobile phase. On iTLC, the [ ⁸⁹ Zr]Zr-DBN-IgG showed an R _f ^=^0.0, whereas [ ⁸⁹ Zr]Zr-chloride and [ ⁸⁹ Zr]Zr-DBN had an R _f ^=^0.99 (see Figures 8-10). 2.7 Stability of Purified [ ⁸⁹ Zr]Zr-DBN [00112] The purified and concentrated [ ⁸⁹ Zr]Zr-DBN was stored at -20°C and stability was tested at 0 hours, 24 hours, and 72 hours post HPLC purification and concentration in comparison with unpurified [ ⁸⁹ Zr]Zr-DBN which was also stored at -20°C. To test the stability, the frozen [ ⁸⁹ Zr]Zr-DBN was allowed to come to room temperature and reconstituted with ~100 µL DMSO. The reconstituted [ ⁸⁹ Zr]Zr-DBN (~ 3.7 MBq) was diluted with ~900 µL neutralized deionized water (pH 7.0) and pH was further adjusted to pH 7.0 using 1M Na ₂ CO ₃ . The stability of reconstituted and neutralized [ ⁸⁹ Zr]Zr-DBN was tested using the same HPLC method that was used for [ ⁸⁹ Zr]Zr-DBN purification. 2.8 Effect of Ascorbic Acid on Stability of Purified [ ⁸⁹ Zr]Zr-DBN [00113] The HPLC purified [ ⁸⁹ Zr]Zr-DBN (~7.2 mL) was divided into two equal parts of ~3.6 mL each. In one-part, 25 µL of 200 mg/mL ascorbic acid (Sigma Aldrich, St. Louis, MO, USA) was added. Both ascorbic acid treated, and untreated fractions were concentrated at 0.42 torr and room temperature. The concentrated solutions were stored at -20°C and stability was tested at 0 hours, 24 hours and 48 hours using the same HPLC method used for [ ⁸⁹ Zr]Zr-DBN purification. 2.9. Measurement of Trace Metal Impurity and Radionuclidic Impurities [00114] The presence of trace metals (Y, Fe, Al) in the purified samples were analyzed using inductively coupled Plasma spectrometer using PerkinElmer Elan or PerkinElmer NexION 350D ICP-MS spectrometers (Waltham MA, USA) equipped with Elemental Scientific Inc. (ESI) SC2-DX autosamplers (Omaha NE, USA). The mass spectrometers were equipped with ESI microflow PFA-ST nebulizers and quartz cyclonic spray chambers including a baffle. Radionuclidic purity was measured via high purity germanium gamma spectrometer (HPGe). Purified [ ⁸⁹ Zr]Zr- oxalate samples were counted in a dose calibrator (at Zr-89 setting, 489) and analyzed by HPGe, at different time points over the period of 3-4 months from the day of purification to estimate the presence of ⁸⁸ Zr (T1/283.4 days) due to possible ⁸⁹ Y(p, 2n) ⁸⁸ Zr reaction and its daughter radionuclide ⁸⁸ Y (T1/2106.6 days) in the final formulation (Figure 11, panels A-C). Using the manufacturer’s ionization response versus gamma energy of the dose calibrator (Capintec/Mirion, Florham Park, NJ, USA), the radioactivities of ⁸⁸ Zr and ⁸⁸ Y were estimated at time of measurement, and decay corrected to estimate ⁸⁸ Zr produced at EOB. 2.10. Statistical Analysis [00115] All values are given as mean ± standard deviation (SD). Statistical significance of differences was determined by two-tailed student’s T-test. P values < 0.05 were considered statistically significant. Results and Discussion 3.1. Production of ⁸⁹ Zr from Cyclotron using Solid Target [00116] To enhance our cyclotron production capability of ⁸⁹ Zr, we switched our production method from liquid target to solid target [Ref.9-10]. For the simplicity of ⁸⁹ Zr production, we chose to use a yttrium foil as a target material. To optimize ⁸⁹ Zr production yield, we tested various thicknesses of yttrium foil (0.1-0.25 mm), different beam current (25-40 µA) and different irradiation durations (120-180 minutes). The production yields are listed in Table 1. We noticed an expected 2.33-fold improvement in saturation yield from 2.07±0.60 MBq/µA to 4.84±0.88 MBq/µA on changing yttrium foil thickness from 0.1 mm to 0.25 mm along with beam current from 25-30 µA to 40 µA for a 120-minute proton irradiation. To reduce overall yttrium content in our final solution, the ⁸⁹ Zr production yield was optimized with 0.2 mm thick yttrium foil (two foils of 0.1 mm thickness) at 40 µA beam current for 180 minutes of irradiation. We found a saturation yield of 4.56±0.31 MBq/µA with radioactivity of 4.78±0.33 GBq (129.3±8.9 mCi) of ⁸⁹ Zr, decay corrected to end of the beam. Following previously developed methods, irradiated yttrium foil was dissolved slowly in 2 mL of 6N-HNO ₃ at room temperature. After complete dissolution, the solution was diluted with 7 mL of deionized water and loaded slowly onto the hydroxamate resin (100 mg) column. After loading, the hydroxamate resin was washed with 20 mL of 2N-HCl to remove trace quantities of yttrium salt followed by10 mL of deionized water to remove any leftover acid before eluting ⁸⁹ Zr with 3.0 mL of 1M oxalic acid. Table 1: Cyclotron production of ⁸⁹ Zr *EOB: End of bombardment [00117] The purified ⁸⁹ Zr solution was analyzed for the presence of trace quantities of Y, Zr, Fe and Al using MP-AES and results showed no unexpected levels of any of tested metal ions. In addition to metal ion impurity, we also tested the radionuclidic purity using HPGe for the presence of ⁸⁸ Zr or its daughter nuclei ⁸⁸ Y. Based on our analysis, we found 0.014 ± 0.006% (n=5) of total non ⁸⁹ Zr related radioactivity as a radionuclidic impurity at end of purification, which includes both ⁸⁸ Zr due to potential (p,2n) reaction and ⁸⁸ Y (T _1/2 106.6 days) the daughter nuclei of ⁸⁸ Zr (T1/283.4 days). 3.2 Radiosynthesis and HPLC Purification of [ ⁸⁹ Zr]Zr-DBN [00118] The [ ⁸⁹ Zr]Zr-DBN was synthesized as reported previously by reacting [ ⁸⁹ Zr]Zr-chloride (neutralized to pH ~8.0) and 10.7 moles of DFO-Bz-NCS in DMSO at 37°C for ~30 minutes. The percentage of radiolabeling was determined by silica gel iTLC using 100 mM DTPA (pH 7) as the mobile phase. Based on iTLC, radiolabeling yield was found to be >95%. In previous studies, we used unpurified [ ⁸⁹ Zr]Zr-DBN for labeling of various biologics (proteins, cells, viruses, EVs. etc.) but noticed some biologics (having low protein concentration) were more sensitive and gave poor radiolabeling yield with unpurified [ ⁸⁹ Zr]Zr-DBN. Therefore, in Example 1, we attempted to purify [ ⁸⁹ Zr]Zr-DBN to remove unlabeled p-SCN-Bn-desferoxamine (DBN) and also to increase molar activity (A _m ) of [ ⁸⁹ Zr]Zr-DBN to enhance radiolabeling yield of biologics having low protein concentration with the purified [ ⁸⁹ Zr]Zr-DBN. To purify [ ⁸⁹ Zr]Zr-DBN, initially, we attempted various solid phase cartridges to separate [ ⁸⁹ Zr]Zr-DBN with DBN but in vain. We evaluated reverse phase HPLC (Zorbax 300-SB-C-18.5 µm; 9.4 X 250 mm) to separate [ ⁸⁹ Zr]Zr-DBN with DBN, and after trying various solvents and their combinations, we finally settled on a gradient elution method, which was comprised of solvent A (deionized water with + 0.1% TFA) and solvent B (Acetonitrile + 0.1% TFA) with a flow rate of 1.8 mL/min as described in methods section above. Using this newly developed HPLC method, we successfully separated [ ⁸⁹ Zr]Zr-DBN with DBN (see Figure 2). The unlabeled DBN (DFO-NCS) eluted at the retention time of 33.5 minutes, whereas labeled [ ⁸⁹ Zr]Zr-DBN eluted at 27.1 minutes, showing good separation. The [ ⁸⁹ Zr]Zr- DBN was collected and concentrated (SpeedVac) to remove acetonitrile and trifluoroacetic acid before using it for the radiolabeling of the antibody/protein (IgG). The molar activities of purified and unpurified [ ⁸⁹ Zr]Zr-DBN were measured and presented in Table 2. It is important to mention here that the same concentration of (mg/mL) of labeling protein (IgG), starting radioactivity (mCi or MBq) of [ ⁸⁹ Zr]Zr-DBN and same labeling conditions (pH, temperature and reaction time) were used to radiolabel protein (IgG) with both purified and unpurified [ ⁸⁹ Zr]Zr-DBN. To avoid any potential confound from the starting ⁸⁹ Zr, we always used same batch of cyclotron produced and purified ⁸⁹ Zr for the synthesis of [ ⁸⁹ Zr]Zr-DBN and antibody labeling experiments with or without the [ ⁸⁹ Zr]Zr-DBN HPLC purification step. 3.3 Radiolabeling of Antibody (IgG) with HPLC Purified [ ⁸⁹ Zr]Zr-DBN [00119] The radiolabeling of IgG was performed at various concentrations of antibody (0.1-1.0 mg/mL) to study radiolabeling efficiency as a function of conjugatable protein concentration. To measure the radiolabeling efficiency, we developed a new iTLC system in which both free ⁸⁹ Zr and unconjugated [ ⁸⁹ Zr]Zr-DBN were separated from the radiolabeled protein [ ⁸⁹ Zr]Zr-DBN-IgG. The system employs 20 mM citric acid (pH 4.9–5.1), methanol (1:1, v/v) as a mobile phase and silica gel iTLC as the solid phase. In this system, we independently confirmed the R _f ’s of [ ⁸⁹ Zr]Zr-chloride and [ ⁸⁹ Zr]Zr-DBN to be 0.99 (solvent front) and ^0.0 (origin) for radiolabeled IgG protein ([ ⁸⁹ Zr]Zr-DBN-IgG, see Figures 8-10). The purified [ ⁸⁹ Zr]Zr- DBN gave 2.5 fold higher radiolabeling yield than unpurified [ ⁸⁹ Zr]Zr-DBN at 0.1 mg/mL concentration of protein (IgG), and a similar trend of 2.4 fold higher radiolabeling yield was noted for 0.5 mg/mL concentration of IgG protein (IgG) (Table 2, Figure 3). However, we noticed a lower impact of HPLC purification of [ ⁸⁹ Zr]Zr- DBN on radiolabeling yield for 1.0 mg/mL concentration of IgG protein (1.4 fold higher yield with purified [ ⁸⁹ Zr]Zr-DBN) (Table 2). These data suggest that the beneficial effect of purification of [ ⁸⁹ Zr]Zr-DBN on labeling yield is dependent on protein concentration with more benefit for labeling of biologics having low protein concentrations (see Figure 3). Table 2: Comparison of Radiolabeling Yield of Protein (IgG) with Molar Activity (A _m ) of Purified and Unpurified [ ⁸⁹ Zr]Zr-DBN *P value <0.05 with respect to unpurified [ ⁸⁹ Zr]Zr-DBN 3.4 Stability of Purified [ ⁸⁹ Zr]Zr-DBN Overtime and Effect of Ascorbic Acid [00120] Encouraged by high protein (IgG) radiolabeling yield with the purified [ ⁸⁹ Zr]Zr-DBN, especially at lower protein concentrations, we thought to evaluate the radiochemical stability of [ ⁸⁹ Zr]Zr-DBN over time following HPLC purification and concentration. We noticed appearance of additional small radioactivity peaks at 18.8 minutes, 21.2 minutes and 24.5 minutes retention time other than the expected 27.1 minutes peak for the [ ⁸⁹ Zr]Zr-DBN on HPLC analysis immediately after concentrating the collected HPLC fractions for the [ ⁸⁹ Zr]Zr-DBN (Figure 4, panel a). Based on the retention time of the additional peaks, it was evident that none of the additional peaks resulted from demetallation of ⁸⁹ Zr since free ⁸⁹ Zr appears at ~6 minutes retention time on the HPLC analysis (see Figure 12). Therefore, based on our experience, we suspected, it could be due to the radiation induced decomposition of [ ⁸⁹ Zr]Zr-DBN [Ref.15]. Therefore, without further characterizing newly appeared peaks on HPLC, we treated the collected HPLC fraction with 25 µL of 200 mg/mL ascorbic acid before the concentration step [Ref.15]. As expected, the addition of this small quantity of ascorbic acid did help and reduced the total areas under the curve for the additional peaks present at 18.8 minutes, 21.2 minutes and 24.5 minutes retention times while enhancing the 27.1 minutes peak for [ ⁸⁹ Zr]Zr-DBN by ~12% (Figure 4, panel b). Addition of ascorbic acid improved stability of [ ⁸⁹ Zr]Zr-DBN at 72 hours from 66% to >82% (Figure 5, panels a & b). We noted that the relative percentage of radioactivity peaks in some of the fractions were not changed even after the concentration step while others showed less protection with ascorbic acid. Conclusions [00121] In Example 1, we produced high quantities of ⁸⁹ Zr (4783±330 MBq, 129.3±8.9 mCi) with a saturation yield of 4.56±0.31 MBq/µA using yttrium foil via proton irradiation, and then successfully developed a reverse phase HPLC method for the purification of [ ⁸⁹ Zr]Zr-DBN and a new iTLC system for instant monitoring of radiolabeling yield of antibodies with [ ⁸⁹ Zr]Zr-DBN. The successful production of high molar activity of [ ⁸⁹ Zr]Zr-DBN has allowed 1.4-2.7 fold higher radiolabeling yield at various concentrations of antibody protein (0.1-1.0 mg/mL) in comparison to unpurified (low molar activity) [ ⁸⁹ Zr]Zr-DBN. The long-term (up to 72 hours) stability of purified [ ⁸⁹ Zr]Zr-DBN was also studied with and without addition of ascorbic acid. Thus, present Example 1 enables the translation of [ ⁸⁹ Zr]Zr-DBN as a ready to use synthon for on-demand and on site radiolabeling of various biologics. Example 2 Overview of Example 2 [00122] In Example 2, we investigated the direct cell labeling and tracking of white blood cells and stem cells in healthy athymic mice by evaluating different ⁸⁹ Zr- labeled synthons. Cell based therapies are evolving as an effective new approach to treat various diseases. To understand the safety, efficacy and mechanism of action of cell-based therapies, it is imperative to follow their biodistribution noninvasively. The positron-emission-tomography (PET) based non-invasive imaging of cell trafficking offers such a potential. In Example 2, we evaluated a ready to use direct cell radiolabeling synthon - [ ⁸⁹ Zr]Zr-DFO-Bn-NCS for noninvasive PET imaging based trafficking of white blood cells (WBCs) and stem cells (SCs) up to 7 days in athymic nude mice. We compared the degree of ⁸⁹ Zr complexation and percentage of cell radiolabeling efficiencies for the synthon. The synthon [ ⁸⁹ Zr]Zr-DFO-Bn-NCS, was successfully prepared, and used for radiolabeling of WBCs and SCs. A relatively high cell radiolabeling yield was found for [ ⁸⁹ Zr]Zr-DFO-Bn-NCS. In terms of biodistribution, WBCs radiolabeled with [ ⁸⁹ Zr]Zr-DFO-Bn-NCS primarily accumulated in liver and spleen. This study offers an appropriate selection of ready to use radiolabeling synthon for a noninvasive trafficking of WBCs, SCs and other cell-based therapies. Introduction to Example 2 [00123] Safety and efficacy are the two main pillars of any therapeutics, cell- based therapies and imaging are no exception, however not much is known to effectively assess the biodistribution, clearance and efficacy of cell-based therapies due to the absence of an appropriate noninvasive imaging tool. In vivo cell tracking could provide information about distribution, localization, and clearance of various cell-based therapies including immune cell (CAR-T cells), stem cells and hepatocytes post-administration in the body. There are various non-invasive molecular imaging modalities that could be employed to track cell based therapies including optical imaging via fluorescence imaging (FLI) [Ref.16-17], bioluminescence imaging (BLI) [Ref.18-19], and ultrasound-guided photoacoustic imaging (PA) [Ref.20-22]. The magnetic resonance imaging (MRI) [Ref.23-25] and radiology imaging such as computed tomography (CT) [Ref.26-28] and nuclear medicine imaging such as positron emission tomography (PET) [Ref.29-33], and single photon emission computed tomography (SPECT) [Ref.34-35] could also be employed to effectively measure the distribution, localization, and clearance of various cell-based therapies overtime and to shed light on safety and efficacy of the cell based therapies. [00124] Among various imaging modalities, optical imaging modality is restricted to small animals due to limited tissue penetration (1-2 mm) in humans. Whereas MRI and CT provide high resolution anatomical information; but have low sensitivity in both animals and humans. Both PET and SPECT are advantageous over other techniques and often integrated with CT and MRI. The PET/CT or SPECT/CT or PET/MRI provides quantitative and temporal distribution of immune and stem cells in animals and patients with no limitation of tissue penetration due to high energy gammas [Ref.36-39]. [00125] Cells can be radiolabeled either directly or indirectly [Ref.40]. The direct cell radiolabeling consists of ex-vivo radiolabeling of cells prior to their administration into body followed by short-term (<7 days) in vivo tracking of these radiolabeled cells. The potential limitation of direct cell labeling approach is the short-term tracking capability due the decay of the radioactivity over time and or efflux of radiotracer or instability of labeled radioactive tag over time. On the other hand, indirect cell radiolabeling method is based on transfection of reporter gene (e.g., sodium iodide symporter (NIS) [Ref.41], green fluorescent protein (GFP), simplex herpes virus type -1 thymidine kinase (HSV1-tk) etc.) in the cells that selectively takes up the respective radioactive or non-radioactive reporter probe in the cells upon exposure to its respective reporter probe. If the administered cells keeps expressing reporter protein after administration, then repeated systemic administration of its reporter probe allows long term-visualization of administered cells. Although indirect cell labeling approach allows long term visualization of administered, genetic modification for cell labeling remains a regulatory hurdle. [00126] Cell radiolabeling using the direct cell radiolabeling approach with various SPECT radiopharmaceuticals such as [ ^99m Tc]Tc-HMPAO (t1/2 = 6.01 hours) [Ref.42-44], and [ ¹¹¹ In]In-oxine (t1/2 = 68.2 hours) [Ref.45-48] have been used to track leukocytes for infection and inflammation imaging over past four decades. SPECT is a powerful clinical imaging tool with lower usage cost than PET due to the lack of need of an onsite cyclotron. However, 2-3-fold higher sensitivity of PET over SPECT, superior spatial resolution of PET in clinical setting and quantitative nature makes it a preferred imaging modality for tracking a single cell or small number of administered radiolabeled cells with more precise quantification and hence, require lower radiation exposure [Ref.49]. Examples of commercial PET probes used to label the cells includes [ ¹⁸ F]-FDG (t _1/2 = 109.7 min, β ⁺ = 97 %) [Ref.50], [ ⁶⁴ Cu]Cu- PTSM (t1/2 = 12.7 hours, β ⁺ = 17.9 % ) [Ref.51] and [ ⁶⁸ Ga]Ga-oxine (t1/2 = 68 min, β ⁺ = 88.8 %) [Ref.52-53]. [00127] Recently, among PET radioisotopes, zirconium-89 (β ⁺ = 22.3 %) is gaining popularity for tracking cells attributed to well established cyclotron-mediated production, longer half-life of 3.27 days and low average positron energy (Eβ+= 0.389 MeV) which enables monitoring of radiolabeled cells either through direct cell labeling (also called non-specific cell labeling agents) [Ref.54-55] or indirect labeling approach mediated through antibodies [Ref.56-57], peptides [Ref.58], proteins [Ref. 59] and nanoparticles [Ref.60-62] at late time points (up to 3-weeks). [00128] Among various chelators used for the radiolabeling of cells with ⁸⁹ Zr are tropolone, malonate, hydroxamates, and oxine (8-hydroxyquinoline). Among these, oxine forms a lipophilic complex with ⁸⁹ Zr and enters in the cells passively. To date, [ ⁸⁹ Zr]Zr-oxine is commonly used radiotracer to label various cells including tumor cell lines [Ref.63-64], bone marrow [Ref.65-66] T cells [Ref.67], NK cells [Ref.68], WBCs [Ref.69], stem cells [Ref.70] and leukocytes [Ref.71]. However, efflux of ⁸⁹ Zr from cells labeled with [ ⁸⁹ Zr]Zr-oxine remains a challenge. Recently, Friberger et al. reported one- step clinically translatable method of synthesis of [ ⁸⁹ Zr]Zr-oxine and cell labeling efficiency of (61-68%) with human decidual stromal cells (hDSCs), bone marrow-derived macrophages (rMac) and human peripheral blood mononuclear cells (hPBMCs), however, 29-38% apparent efflux of ⁸⁹ Zr from the labeled cells raised a further concern of radiotoxicity and non-specificity of signal [Ref.72]. [00129] Besides [ ⁸⁹ Zr]Zr-oxine, the other reported method of cell labeling was covalent attachment of radiolabeled [ ⁸⁹ Zr]Zr-DFO-Bn-NCS complex with the primary amines present on the cell surface proteins to form stable thiourea bond, which has solved the efflux problem observed with [ ⁸⁹ Zr]Zr-oxine [Ref.2, 3, 72, and 73]. The [ ⁸⁹ Zr]Zr-DFO-Bn-NCS has been successfully used to radiolabel mouse melanoma, mouse dendritic cells and human mesenchymal stem cells with insignificant efflux of free ⁸⁹ Zr from [ ⁸⁹ Zr]Zr-DFO-Bn-NCS overtime (7 days-post radiolabeling) [Ref.2]. Additionally, a better version of DFO chelator as DFO* has been developed to further strengthen the stability of ⁸⁹ Zr complexation and showed lower bone uptake over time [Ref.74]. Various other chelators are also being developed to address in vivo stability of ⁸⁹ Zr complex over time [Ref.75-79]. [00130] In Example 2, we have optimized and compared the radiolabeling yields of white blood cells (WBCs) and stem cells (SCs) using a ready to use labeling synthon [ ⁸⁹ Zr]Zr-DFO-Bn-NCS [Ref.2, 3, 72, and 73] (see Fig.13), and evaluated its applications in cell trafficking to better understand the biodistribution/pharmacokinetics of cell based therapies. This approach can be extended to various other cell-based therapies like CAR-T cell therapy. Materials and Methods [00131] General. The ⁸⁹ Zr used in this study was produced on a PETtrace cyclotron (GE Healthcare, Waukesha, WI, USA) using ⁸⁹ Y target foil (0.1 mm; 50 X 50 mm, 99.9 %), which was purchased from Alfa-Aesar, Haverhill, MA, USA. The trace metal grade nitric acid (67-70 %), hydrochloric acid (34-37%) were purchased from Thermo Fisher Scientific, Waltham, MA, USA. Sodium bicarbonate, oxalic acid dehydrate (TraceSELECT® >99.9999% metal basis), sodium carbonate, sodium citrate dihydrate and HPLC grade acetonitrile were purchased from Sigma Aldrich, St. Louis, MO, USA. The silica gel iTLC were purchased from Agilent Technologies, Santa Clara, CA, USA. The chelator p-SCN-Bn-Deferoxamine or DFO-Bn-NCS (>94%) was purchased from Macrocyclics, Plano, TX, USA. The empty Luer-Inlet SPE cartridges (1 mL) with frits (20 µm pore size) were purchased from (Supelco Inc, Bellefonte, PA, USA) and Chromafix ^® 30-PS-HCO ₃ PP cartridge (45 mg) were purchased from Macherey-Nagel, Duren, Germany. The Millex ^® -GV filter (0.2 µm) was purchased from Millipore Sigma, Burlington, MA, USA. The hydroxamate resin used in this study was synthesized as demonstrated by Pandey et al. [Ref.9, 10, 13, 80]. Thermomixer was purchased for Eppendorf, Hamburg, Germany. Production and Purification of [ ⁸⁹ Zr]ZrCl ₄ [00132] The ⁸⁹ Zr was produced using yttrium foil on a solid target through a ⁸⁹ Y(p,n) ⁸⁹ Zr nuclear reaction in PETtrace cyclotron as described previously by Pandey et al. [Ref.80 and Example 1 above]. ⁸⁹ Zr was purified first as [ ⁸⁹ Zr]Zr- oxalate and then converted to [ ⁸⁹ Zr]ZrCl ₄ using activated Chromafix 30-PS-HCO ₃ SPE as demonstrated by Pandey [Ref.9, 10, 13, 80 and Example 1 above] and Larenkov et al. [Ref.14], respectively. The final [ ⁸⁹ Zr]ZrCl ₄ was eluted in ^0.5 mL of 1.0 N HCl and then dried using a steady flow of nitrogen gas in a V-vial at 65°C. Apparent Molar Activity of [ ⁸⁹ Zr]ZrCl ₄ [00133] The apparent molar activity of ⁸⁹ Zr was estimated using a DFO-Bn-NCS titration method. In this method, 10 µL [ ⁸⁹ Zr]Zr-chloride (36.4 MBq) was added to 90 µL de-ionized H2O. To this, 4µL of 0.5 M Na ₂ CO ₃ was added to neutralize and adjust the pH to 7.5-8.0. To the neutralized mixture, 0.01-10 µg of DFO-Bn-NCS in 4µL of DMSO was added and mixed. The complexation mixture was then incubated at 37°C for 1 hour. After 1 hour, the degree of ⁸⁹ Zr complexation was determined with respect to the DFO-Bn-NCS concentration using radio-TLC with 20 mM sodium citrate (pH 4.9-5.1) as a mobile phase. The complexed ⁸⁹ Zr as [ ⁸⁹ Zr]Zr-DFO-Bn-NCS showed at the origin with R _f = 0, whereas free or un-complexed ⁸⁹ Zr had an R _f of 0.99 (solvent front). The half maximal inhibitory concentration (IC ₅₀ ) of DFO-Bn-NCS in mg/mL was calculated using non-linear regression curve fitting analysis with two equations – Equation 1: [Concentration] vs Complexation (three parameter) and Equation 2: log (concentration) vs. complexation (three parameter). The analysis was performed using analysis software - GraphPad Prism 9 (GraphPad Software, San Diego, CA, USA). The minimum ligand concentration for which 100% complexation occurred was estimated by multiplying the IC ₅₀ by 2, and the apparent molar activity (GBq/µmole) and the apparent specific activity (GBq/mg) of ⁸⁹ Zr were calculated by correcting for the total activity divided by µmoles or mg of DFO-Bn-NCS needed for 100% ⁸⁹ Zr complexation. Radiosynthesis of [ ⁸⁹ Zr]Zr-DFO-Bn-NCS, [00134] The radiosynthesis of the different synthons [ ⁸⁹ Zr]Zr-DFO-Bn-NCS, was performed using a modified procedure demonstrated in our previous work [Ref.2]. The purified [ ⁸⁹ Zr]Zr-chloride was resuspended in appropriate volume of 0.1 N HCl and then neutralized to pH ^8.0 with 0.5 M Na ₂ CO ₃ . The neutralized [ ⁸⁹ Zr]Zr-chloride solution (70 -100 µL) containing ~21 MBq of ⁸⁹ Zr was used in the case of DFO-Bn- NCS. To this neutralized [ ⁸⁹ Zr]Zr-chloride solution, 4 nmoles of DFO-Bn-NCS (prepared in DMSO) were added in separate reactions. The resultant reaction mixtures were stirred for 30 minutes at 37°C in a thermomixer at 500 rpm. The radiolabeling efficiency was determined at different time points by silica radio-TLC using 20 mM sodium citrate (pH 4.9-5.1) as a mobile phase. Cell Preparation [00135] The human mesenchymal stem cells (SCs) were from the Department of Cardiovascular Medicine, Mayo Clinic, Rochester, MN, USA, and white blood cells (WBCs) were isolated from the peripheral blood provided by the Division of Transfusion Medicine, Mayo Clinic, Rochester, MN, USA. The isolation of WBCs from the peripheral blood was performed using Lymphoprep ^TM (STEMCELL Technologies Inc., Canada) gradient centrifugation method as per manufacturer instruction. The final WBS solution was washed with Hank’s Balanced Salt Solution. Cell Radiolabeling [00136] The SCs and WBCs cells were radiolabeled with, [ ⁸⁹ Zr]Zr-DFO-Bn- NCS,. The cell radiolabeling mixture was prepared by mixing equal volume of the [ ⁸⁹ Zr]Zr-DFO-Bn-NCS reaction mix and equal volume of phosphate buffer-HEPES. The phosphate buffer-HEPES was prepared by mixing 120 µL of 1.2 M phosphate buffer and 100 µL of 1M HEPES. This cell radiolabeling mix was incubated at room temperature for 30 minutes. After this incubation, the cell radiolabeling mixture (~150-200 µL) was added to cell suspension which was at a concentration of 6 x 10 ⁶ cells in 500 µl HEPES Buffered Hank’s Balanced Salt Solution at pH 7.5-8.0 [Ref.2- 74]. The cell radiolabeling was performed for 30 minutes at room temperature for WBCs and 37°C for SCs in a thermomixer. Post-radiolabeling the cells were washed 3X with complete Dulbecco's Modified Eagle Medium. Trypan Blue Exclusion Assay Cellular Viability Test [00137] The effect of radiolabeling on cellular viability was assessed using trypan blue exclusion assay test within 1 hour of labeling. Animals [00138] 8-10 weeks old Athymic nude mice (male and female, 1:1) were obtained from Charles Rivers Laboratories or Taconic Biosciences, Inc. PET Imaging and ex vivo Biodistribution Studies [00139] After radiolabeling, the radiolabeled WBCs - 0.1-0.6 x 10 ⁶ (0.03-0.11 MBq); and SCs - 0.1-1 x 10 ⁶ (0.1-0.15 MBq) were injected in a group (n=3) of athymic nude mice, via tail vein injection. PET images were acquired at 4 hours, 2 days, 4 days and 7 days post-injection (p.i.) using microPET scanner. The free [ ⁸⁹ Zr]ZrCl ₄ with radioactivity (0.15-0.19 MBq) was also injected intravenously via tail vein injection. The micro PET images were analyzed and scaled to SUV by image analysis software. The animals were euthanized at 7 days p.i., and organs/tissues were collected to measure standardized uptake value (SUV) in major organs. Animals were euthanized via cardiectomy under anesthesia using isoflurane as approved by the Institutional Animal Care and Use Committee (IACUC) of the Mayo Clinic Rochester MN, USA. Statistics [00140] The obtained data were analyzed using Microsoft Excel ^® program and results were compared using unpaired Student’s t-test analysis. Differences were regarded as statistically significant for p<0.05. Results and Discussion Production of [ ⁸⁹ Zr]ZrCl ₄ and Radiosynthesis of [ ⁸⁹ Zr]Zr-DFO-Bn-NCS [00141] The PET isotope 89Zr was produced and purified using cyclotron as described earlier by Pandey et al. [Ref.80 and Example 1 above] in a high apparent molar activity of 17.0 – 23.13 GBq/µmol, as assessed by complexing purified [89Zr]ZrCl ₄ with different amounts of DFO-Bn-NCS. The synthon DFO-Bn-NCS was successfully conjugated with 89Zr at 37°C; pH 7.5-8.0 for 30 minutes in 72-98% radiolabeling yield. The DFO-Bn-NCS showed complexation yield of 97.76 ± 0.31 % (n=3) (see Table 3, Fig.14). Table 3 Percentage of ⁸⁹ Zr complexation with DFO-Bn-NCS, at different reaction times. Radiolabeling of WBCs and SCs [00142] [ ⁸⁹ Zr]Zr-DFO-Bn-NCS synthon was successfully employed to radiolabel WBCs and SCs. For WBCs, the radiolabeling efficiency with [ ⁸⁹ Zr]Zr-DFO-Bn-NCS was 22.77 ± 5.02 % (n=3). (see Fig.15). While radiolabeling efficiency for SCs, with [ ⁸⁹ Zr]Zr-DFO-Bn-NCS was 41.83 ± 5.02 % (n=3) (see Fig.16). The radiolabeled WBCs and SCs showed ~90-95% viability as per trypan blue exclusion cell viability assay. The difference in radiolabeling efficiencies between WBCs and SCs were expected due to the difference in their cell sizes and availability of surface proteins for conjugation and radiolabeling. The average cell size in the cell population was measured by TC10 cell counter (Biorad Laboratories, Inc., Hercules, CA, USA) and found to be 4-10 µm for WBCs and 12-20 µm for SCs. Micro PET Imaging and Biodistribution of ⁸⁹ Zr labeled WBCs [00143] The Micro PET imaging and biodistribution of WBCs were performed independently after radiolabeling of WBCs with [ ⁸⁹ Zr]Zr-DFO-Bn-NCS, to assess any differences in pharmacokinetics of WBCs radiolabeled with different synthons in a healthy athymic mice group (see Figure 17). After intravenous injection of WBCs- labeled with [ ⁸⁹ Zr]Zr-DFO-Bn-NCS, the majority of the radioactivity was observed in the liver and spleen, and remained significantly high at all time points (4 hours, 2 days, 4 days, 7 days). Importantly, no bone uptake was observed at any time points over 7 days indicating a good in vivo stability of [ ⁸⁹ Zr]Zr-WBCs radiolabeled with [ ⁸⁹ Zr]Zr-DFO-Bn-NCS. [00144] Overall, the in vivo stability of [ ⁸⁹ Zr]Zr-DFO-Bn-NCS demonstrated here was promising]. The biodistribution of radiolabeled WBCs in the rest of the major organs are presented in Figures 18-19 and Table 4, indicating mild uptake in lung, heart, muscle, pancreas, and skin at 7 days post injection. Table 4 Uptake (SUV) and biodistribution of white blood cells (WBCs) labeled with [ ⁸⁹ Zr]Zr-DFO-NCS (n=4), in athymic nude mice at day 7 post-injection. White Blood Cells labeled with [ ⁸⁹ Zr]Zr-DFO-Bn- Organ NCS SUV Average ± SD Lung 0.68 ± 0.26 Liver 16.67 ± 1.05 Spleen 24.34 ± 4.30 Kidney 0.19 ± 0.75 Bones 0.83 ± 0.56 Intestine 0.02 ± 0.02 Heart 0.12 ± 0.07 Brain 0.003 ± 0.001 Muscle 0.20 ± 0.27 Urine 0.30 ± 0.0044 Bladder 0.30 ± 0.017 Blood 0.0098 ± 0.0030 Pancreas 0.27 ± 0.23 Skin 0.36 ± 0.015 Micro PET Imaging and Biodistribution of ⁸⁹ Zr labeled SCs [00145] The Micro PET imaging and biodistribution of SCs were performed independently after radiolabeling of SCs with [ ⁸⁹ Zr]Zr-DFO-Bn-NCS, to assess any variation in pharmacokinetics of SCs radiolabeled with different synthons in a healthy athymic mice group (see Figure 20). The SCs radiolabeled with ⁸⁹ Zr using [ ⁸⁹ Zr]Zr- DFO-Bn-NCS showed uptake primarily in lung, liver and spleen at all time points with some early uptake in intestine. The biodistribution of radiolabeled SCs in the rest of the major organs are presented in Figures 21-22 and Table 5, indicating mild uptake in lung, heart, kidney, muscle, pancreas, and skin at 7 days post injection. Table 5 Uptake (SUV) and biodistribution of stem cells labeled with [ ⁸⁹ Zr]Zr-DFO-NCS (n=4) , Stem Cells labeled with O _rgan ^[89 Zr]Zr-DFO-Bn- N _CS SUV Average ± SD Lung 5.44 ± 1.72 Liver 10.88 ± 0.72 Spleen 10.88 2.37 Kidney 0.43 ± 0.15 Bone 0.68 ± 0.27 Intestine 0.22 ± 0.006 Heart 0.09 ± 0.04 Brain 0.002 ± 0.001 Muscle 0.014 ± 0.013 Urine 0.44±0.018 Bladder 0.29 ± 0.01 Blood 0.0072 ± 0.0025 Pancreas 0.29 ± 0.20 Skin 0.47 ± 0.006 Micro PET Imaging of Unchelated [ ⁸⁹ Zr]ZrCl ₄ [00146] The in vivo characteristics of unchelated [ ⁸⁹ Zr]ZrCl ₄ was also investigated (see Fig.23). The micro PET imaging showed a high accumulation of free ⁸⁹ Zr in the bones at 4 hours and did not distribute to the lung, liver, spleen or any other organs at any other time points as it was noticed with radiolabeled WBCS and SCs. The radioactivity increased significantly on day 2 and remain stayed in the bones until day 7, attributing to the entrapment of osteophilic ⁸⁹ Zr and its poor clearance from the bones (see Fig.24 and Table 6). Table 6 Uptake (SUV) and distribution of [ ⁸⁹ Zr]ZrCl ₄ in athymic nude mice (n=3) at day 7 post-injection. F _{ree Zr-89} ^{SUV Average ±} S _D Lung 0.36 ± 0.06 Liver 0.61 ± 0.07 Kidney 0.49 ± 0.07 Spleen 0.39 ± 0.09 Intestine 0.09 ± 0.01 Heart 0.26 ± 0.03 Brain 0.02 ± 0.02 Muscle 0.14 ± 0.05 Urine 0.08 ± 0.03 Bladder 0.32 ± 0.09 Blood 0.12 ± 0.01 Pancreas 0.29 ± 0.02 Skin 0.42 ± 0.14 Bone 9.89 ± 0.4 Conclusion [00147] Both WBCs and SCs were successfully directly radiolabeled with ⁸⁹ Zr using ready to use synthons, [ Overall, PET based cell radiolabeling methodology offers an effective tool to noninvasively track WBCs, SCs, and other cells to understand the safety, efficacy, distribution and clearance of cell-based therapies. [00148] List of abbreviations used herein: Bq: Becquerel DBN: p-isothiocyanato-benzyl-desferrioxamine DFO-Bz-NCS: 1-(4-isothiocyanatophenyl)-3-[6,17-dihydroxy-7,10,18,21- tetraoxo-27-(N-acetylhydroxylamino)- 6,11,17, 22- tetraazaheptaeicosine] thiourea DMSO: Dimethyl sulfoxide DTPA: Diethylene triamine pentaacetic acid GBq: Gigabecquerel, 10 ⁹ Bq HPLC: High-performance liquid chromatography IgG: Immunoglobulin G iTLC: Instant thin layer chromatography MBq: Mega Becquerel, 10 ⁶ Bq PET: Positron Emission Tomography R _f : Retention factor rTLC: Radioactive-thin layer chromatography SCs: Stem cells SD: Standard deviation SPECT: Single-photon emission computerized tomography SUV: Standardized uptake value T1/2: Half-life of radioisotope WBCs: White blood cells [00149] REFERENCES: 1. Yoon JK, Park BN, Ryu EK, An YS, Lee SJ, Current Perspectives on ⁸⁹ Zr- PET Imaging, Int J Mol Sci, 2020 Jun 17;21(12):4309. doi: 10.3390/ijms21124309. PMID: 32560337; PMCID: PMC7352467. 2. 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A new solid target design for the production of ⁸⁹ Zr and radiosynthesis of high molar activity [ ⁸⁹ Zr]Zr-DBN. Am J Nucl Med Mol Imaging. 12, 15-24 (2022). 81. Nguyen et al., Stem cell imaging: from bench to bedside. Cell stem cell, Volume 14, Issue 4, 3 April 2014, pages 431-444 The citation of any document or reference is not to be construed as an admission that it is prior art with respect to the present invention. [00150] Thus, the invention provides methods of labeling of a biological material for medical imaging, methods for preparing a labeling agent, and methods for medical imaging of a subject using a biological material labeled with a labeling agent. [00151] Although the invention has been described in considerable detail with reference to certain embodiments, one skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which have been presented for purposes of illustration and not of limitation. Therefore, the scope of the appended claims should not be limited to the description of the embodiments contained herein.