Combined optical fiber and RF coil array system for magnetic resonance data acquisition

FIELD OF THE INVENTION

The invention relates to a nuclear magnetic resonance radio frequency coil arrangement, a nuclear magnetic resonance examination system, a method of performing a nuclear magnetic resonance scan and an optical examination, as well as a computer program product.

BACKGROUND OF THE INVENTION

X-ray computer tomography, ultrasonic computer tomography and MRI, as well as nuclear magnetic resonance spectroscopy are well known methods for localizing objects in a turbid medium, especially for the localization of breast cancer and tumors. Optical computer tomography uses the fact, that near infrared light exhibits a high transmissivity with respect to biological tissues and growth of tumors or cancer can be monitored by a characteristic absorption of light in breast tissue.

Magnetic resonance imaging (MRI) is a state of the art imaging technology which allows cross-sectional viewing of objects like the human body with unprecedented tissue contrast. MRI is based on the principles of nuclear magnetic resonance (NMR), a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules. The basis of both NMR and MRI is the fact, that atomic nuclei with non-zero spin have a magnetic moment. In medical imaging usually nuclei of hydrogen atoms (i.e. protons ¹ H) are studied since they are present in the body in high concentrations like for example as water. Radio frequency waves are directed to nuclei in strong external magnetic fields, which leads to an excitation of the protons and a relaxing of the protons. Due to the relaxation of the protons, radio signals are emitted which can be detected and computer processed to form an image. The magnetic resonance (MR) radio frequency (RF) receive coils are necessary parts to receive said RF signals transmitted in particular MR experiments. The best RF coil arrangement location is close to the human body being scanned and therefore most of the MR receive coils are positioned on the patient by a scanner operator.

WO 2005/074797 Al discloses a system which allows combining of anatomical imaging technologies like MR with optical imaging technologies, wherein a light wire for the purpose of optical imaging is located at the centre of an MR coil.

SUMMARY OF THE INVENTION

The present invention provides a nuclear magnetic resonance radio frequency (RF) coil arrangement configured for receiving magnetic resonance RF signals from within the volume circumscribed by the RF coil arrangement, wherein the RF coil arrangement further comprises at set of optical couplers, wherein the optical couplers are adapted for receiving optical signals from the volume circumscribed by the RF coil arrangement. In accordance with a further embodiment of the invention the optical couplers are movable with respect to the RF coil arrangement, for example radially towards the isocentre of the RF coil arrangement.

The MR coil arrangement according to the invention has the advantage, that a parallel MR coil system with integrated optical fiber transmit and receiver system for optical imaging can be provided. The invention achieves, that there is no spatial mismatch in the acquired MRI data and the optically acquired data, like for example in diffused optical tomography (DOT). The reason why there is no spatial mismatch in the acquired MRI data and the optically acquired data is because the object which is imaged is located at the same spatial position for both acquisitions. Further, on the basis of the known relative positions of the MRI acquisition system (RF coils) and the DOT acquisition system (optical fibers) image information of both acquisitions can be combined in one image. In accordance with an embodiment of the invention, simultaneous MR and optical data acquisition of the same area of interest can be performed which allows combining and overlying MR and optical images of said area of interest in a highly accurate manner without the need for complicated and intricate image registration.

In general, the RF coil arrangement can be single- or multi-resonant RF coil arrangements. A multi-resonant RF coil arrangement has the advantage, that an additional detection of optical and/or therapheutical contrast agents, for example resonating at ¹⁹ F or ¹³ C resonance frequency, can be used to enhance the detection capabilities of the system. Further, with a multi-resonant RF coil arrangement, molecular imaging is possible.

In accordance with an embodiment of the invention, the RF coil arrangement further comprises a positioner, the positioner being adapted for moving the optical couplers relative to the object. Such positioners have the advantage, that it is possible to adjust the

position of the optical couplers relative to the object and also relative to the RF coil arrangement itself: for example in case of an MRI head coil, the coil itself has no direct contact with head tissue of an imaged head such that extra positioning means for the optical couplers enable the system to move the optical couplers into an optimal position, for example directly onto the skin surface of the head. In case the optical couplers would be integrated into the head coil and not being movable, the distance between the optical couplers and the skin surface of the head may be too large in order to obtain reliable and good optical examination results.

For example the positioner is given by a Piezo electric motor which is directly mechanically connected to the optical couplers. In an alternative, a transmission system can be used to move the optical couplers. In this case, the optical couplers are positioned by for example a rotary movement of a flexible shaft mechanically connected to the optical couplers and an external motor.

In accordance with an embodiment of the invention, the RF coil arrangement further comprises a digitization unit, the digitization unit being adapted to digitize and receive a magnetic resonance imaging RF signal and/or to digitize an optical signal received in response to the optical examination of the object by means of the optical couplers. For example, for this purpose either two digitizers may be used, one for the RF signal and one for the optical signal. Alternatively a single digitizer is comprised in the RF coil arrangement. The single digitizer is arranged to alternatingly digitize the received magnetic resonance RF signals and the received optical signals. This can be accomplished by multiplexing the single digitizer.

In accordance with a further embodiment of the invention, said RF coil further comprises a first combiner, wherein the first combiner is adapted to combine the digitized magnetic resonance RF signal and digital optical signal. The RF coil arrangement further comprises for example a communication interface, the communication interface being adapted to provide the combined digitized signals in one common signal.

For example, the digital signals of the optical couplers and the MR antenna are digitally multiplexed and sent by means of the communication interface via one optical channel to a spectrometer adapted for MR and optical data acquisition. This has the advantage, that due to the usage of a digital transmission technique a spatial and electrical separation of the MR receive chains in the examination room and the control system in a separate technical room can be achieved in an easy manner. This is helpful since MR receive coils or in general MR receive chains comprising the coils and various electronic components

like amplifiers, switches, etc are highly sensitive to disturbances by external radio frequency waves. For this reason it is preferred that MR receive chains are strongly electromagnetically shielded which requires said spatial separation of the MR receive chain and the control system. Also, current state of the art MR receive chains feature a receive parallel analogue solution with many expensive analogue design elements such as RF switches, RF amplifiers, RF power supplies, RF cables and RF connectors. All these components are typically strewn over a distance of 10-20 meters between the receive chain in the exam room and the analogue to digital converters in the technical room, which makes it one of the most complex and challenging aspects to cost effectively design and produce an MR scanner due to the component spread and unwanted interactions between the many galvanic parts. All these issues can be addressed using a digitization unit for both the optical signals and the magnetic resonance RF signals.

In accordance with an embodiment of the invention, the RF coil arrangement is comprised in a holder, the holder comprising the optical couplers and the positioners. Such a holder has the advantage, that it is possible to combine MR receive coils with a set of optical couplers, such that various kinds of MR receive coils can be used with one holder comprising the optical couplers and the positioners. For example many state of the art MR head coils already have nowadays a spatial coil structure comprising openings which allows to place around a respective holder comprising optical couplers which can be moved through the cavities of the RF coil arrangement onto the skin of a head to be imaged.

In accordance with an embodiment of the invention, the RF coil arrangement further comprises a preamplifier for each optical coupler. Such a preamplifier ensures that even low power optical signals can be detected without the risk of loosing valuable information due to noisy measurements.

In accordance with an embodiment of the invention, the preamplifier is located in close proximity to the distal end of each optical coupler, the distal end pointing towards the object. This further ensures that optical signal loss due to long signal propagation delays is prevented. In accordance with an embodiment of the invention, the RF coil arrangement further comprises a sensor for each optical coupler, the sensor being adapted to detect a spatial position of the optical coupler with respect to the object. Such a sensor allows for an automatic positioning of the optical couplers with respect to the object. For example an ultrasound or infrared detector can be used in combination with an automization module

which moves the optical couplers onto the object to be scanned until a predetermined distance between the optical coupler and the surface of the object is obtained. It is further possible to adapt the sensors for a mechanical spatial position detection, for example by using a simple pressure switch as sensor which sends a certain signal to a control unit as soon as the sensor registers a pressure change due to a mechanical contact with the object surface to be imaged.

In accordance with an embodiment of the invention, the distal end of each optical coupler comprises a resonant marker, the resonant marker being adapted to provide a magnetic resonance RF signal upon magnetic resonance excitation of the object. In other words, by using the resonant marker it is possible to use normal MRI scanning, for example during the pre-scan procedure, in order to obtain a spatial information on the position of the optical coupler with respect to the object.

A resonant or other marker according to the present invention to obtain spatial information on the position of an optical coupler may, in an alternative embodiment, be used in relation to an optical coupler adapted for receiving optical signals from a volume outside the volume circumscribed by the RF coil arrangement. For instance the position of the optical coupler described in US 2006/0100529 Al may be indicated by a resonant or other marker as described in the present invention.

Regarding a resonant marker, different possibilities exist which can be applied to carry out the invention. For example, passive markers can be used which may comprise protons or any other kinds of nuclei investigated during an MR-scan. Also possible is the usage of 'fiducials' which is a certain nuclei mix, surrounded by a local resonant coil, wherein the local resonant coil is coupled with external RF coil arrangement, e.g. a surface coil or body coil or a body coil element. The marker can be actively (e.g. optical switch) or passively (crossed diodes) detuned (SAR reduction). This has the advantage of causing an increase of the MR-signal detected with the RF coil arrangement. A third possibility is the usage of a separate coil, which is connected by for example a coaxial or optical cable to a separate receiver. This separate coil is a resonant marker, connected to a coax cable via impedance match (local RF micro preamplifier ) or via an optical cable (local ADC or modulation of laser diode). The main advantage of a resonant marker connected to a separate receiver is its tremendous higher SNR signal-to noise ratio, but requires a separate receiver. During transmit this coil is detuned (local SAR, for correct flip angle). A detuning is performed via a mechanical or electronic switch.

It has to be further mentioned, that it might be useful to use several markers along an optical fiber.

In accordance with an embodiment of the invention, the distal end of each optical coupler further comprises a micro -coil, the micro -coil being adapted to receive the RF signal provided by the resonant marker. Using such a micro-coil has the advantage, that due to the small size of the micro-coil an MR signal is only detected from the micro-coil in case the respective gradient fields of the magnet are switched in such a manner that exactly the spatial position of the micro -coil with the resonating marker is excited. Combining the spatial information of the excitation area defined by the gradient control and the signal received from a given micro -coil, an exact spatial position of the micro -coil with respect to the MR receive coil and/or the object can be obtained. This allows obtaining an exact positioning of the optical coupler on an object using for example an MR preparation sequence.

In accordance with an embodiment of the invention, the RF coil arrangement further comprises a second combiner, the second combiner being adapted to combine the magnetic resonance signals received from the micro -coils. This again has the same advantage as already mentioned above for combining the MR signal and the optical signal since only one digital connection is required to the spectrometer which reduces cable cluttering, ensures a highly effective galvanic separation of the MR receive chain and the control and data acquisition system etc. In accordance with an embodiment of the invention, the RF coil arrangement further comprises a position controller, the position controller controlling the positioner, the position controller being adapted for an automatic positioning of the optical couplers relative to the object. For example, the position controller can use the information received from sensors of each optical coupler and/or the micro coils in order to automatically position the optical couplers in optimal spatial position with respect to the object to be imaged. In case for example the head of a person has to be imaged, the optical couplers may be automatically moved towards the head of said person until the couplers are in contact with the skin of the head. Using no sensors or micro coils for such a positioning procedure could cause severe injury of the skin. In accordance with an embodiment of the invention, the optical examination is a diffuse optical imaging and/or a spectroscopy method. For example, the diffused optical imaging could be a diffuse optical tomography imaging method and the spectroscopy method could for example be Raman spectroscopy or a special fluorescence spectroscopy technique.

In another aspect, embodiments of the invention relate to a holder, the holder being adapted to receive an RF coil arrangement, the RF coil arrangement being configured for receiving magnetic resonance RF signals from within the volume circumscribed by the RF coil arrangement , wherein the holder further comprises at set of optical couplers, wherein the optical couplers are adapted for receiving optical signals from the volume circumscribed by the RF coil arrangement, wherein the holder further comprises a set of openings for receiving the optical couplers, wherein the RF coil arrangement comprises a set of openings for receiving the optical couplers, wherein the openings of the holder and the openings of the RF coil arrangement are aligned. In accordance with a further embodiment of the invention, the optical couplers are radially moveable with respect to the holder and the RF coil arrangement.

This has, as already mentioned above, the advantage, that state of the art RF coil arrangements can be used for simultaneous MR data acquisition and optical imaging. For this purpose, either complete receive coil units, single or multiple RF coil arrangements can be combined with the holder according to the invention. In case of such a holder, preferably the holder comprises the positioner, the first and/or second digitization units, the first and second combiner, preamplifiers, sensors, the position controller etc. and any other mechanical or electrical components.

In another aspect, the invention relates to a nuclear magnetic resonance examination system, the system comprising an RF coil arrangement according to the invention. A control and data acquisition which may be further comprised in the nuclear magnetic resonance examination system may be adapted to control a positioning of an examined object with respect to the RF coil arrangement and to control a positioning of the optical couplers with respect to the object. In another aspect, the invention relates to a method of performing a nuclear magnetic resonance scan and an optical examination, the method being performed using an RF coil according to the invention, the method comprising the steps of positioning the object with respect to the RF coil arrangement and positioning the optical couplers relative to the object. Preferably, the positioning of the optical couplers relative to the object is performed automatically. Also preferably, the method of performing nuclear magnetic resonance data acquisition and the optical examination is a method which performs the MRI scan and the optical examination in a parallel manner.

In accordance with an embodiment of the invention, the positioning of the optical couplers relative to the object is performed by a control and data acquisition system,

wherein for positioning of the optical couplers the control and data acquisition system analyzes the magnetic resonance signals received from the micro-coils and/or the magnetic resonance signals received from the RF coil arrangement comprised in the RF coil arrangement and/or the signal provided by the sensor of each optical coupler. In another aspect, the invention relates to a computer program product comprising computer executable instructions to perform any of the method steps of the method of performing a nuclear magnetic resonance scan and an optical examination according to the invention.

EP 0 808 124 Bl describes (Fig. 8) a hairbrush optical coupler constructed for in vivo examination of tissue using simultaneously magnetic resonance imaging (MRI) and medical optical imaging (MOI). The coupler includes a styrofoam cap with four rows of eight fibres extending from frontal to occipital region of a patient's head located inside an MRI magnet. Each fibre is coupled at its optical coupling port to a fibre junction box. The fibre junction box, located outside the magnet, as appropriate electromechanical or electro-optical switches to time sequence the switching of a fibre conduit to any one of the fibres coupled to the head. The system employs any one or more fibres for transmission and any other fibres for detection. An MRI/MOI control centre includes an imaging centre and a computer system, which is constructed to create and overlay the optical and magnetic images. Coordination of the optical and MRI images is achieved by MRI/optical markers. The setup described in Fig. 8 in EP 0 808 124 Bl, however, does not have several advantages provided by the invention, which is subject of the present application. First, the signal to noise ratio of an MR coil very close to the tissue to be studied is significantly better than the signal to noise ratio of an MR coil placed farther away from the tissue. The current invention can be implemented with a surface coil, which may be comprised in, for instance a thin flexible unit, like a blanket-like structure, that can be placed into contact with the tissue (for instance a patient) to be studied. The optical couplers are then integrated into the blanket-like structure.

Moreover, an MR coil according to the invention comprises a set of optical couplers. The MR and optical parts are thus fully integrated in a single device. This is not the case in EP 0 808 124 Bl . There, the optical hairbrush and the MR full body tube are separate devices, with the optical hairbrush being inserted into the MR tube. With the present invention, the MR coil and the optical couplers are integrated into a single device, for instance, a blanket-like structure has explained previously. The fact that according to the invention there is a single device, provides better workflow, because, for instance, only a

single device has to be handled and there is no need to insert a separate device into the MR full body coil as described in the prior art. Consequently, handling problems like cables connected to the optical hairbrush jamming the bed rails in the MR a full body tube are avoided. The single integrated device according to the invention is also tested as an integrated unit, increasing safety.

Moreover, the present invention provides full MR electromagnetic compatibility. The invention allows an optical interface by lasers both for data (MR and optical) and power supply. Consequently, there are no cable currents/resonances with the MR systems. There are also no radio frequency hotspots that might cause interference. Moreover, the relative geometry of an individual MR coil and an optical coupler is fixed. If, for instance, a patient is placed in a full body MR tube, different parts of the tube may be used to image the patient. Consequently, a bed on which the patient is lying inside the tube must be moved inside the tube. At the present state of the art this means that the optical hairbrush must be moved as the patient is moved, thus requiring complex handling. With the present invention, MR coils and optical couplers are integrated into a single device which moves with the patient. Consequently, the relative geometry of the MR coils and optical couplers is fixed.

Moreover, in the present invention it is proposed to digitise the MR and optical signals, thus providing for local data-processing (data reduction, processing and compression) allowing optical and MR data on a single optical fibre.

Moreover, the present invention allows wireless transmission of digital optical and MR data.

Moreover, the present invention allows wireless power supply, for instance by using RF power or an integrated battery/charging unit.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following preferred embodiments of the invention are described in greater detail by way of example only making reference to the drawings in which:

Fig. 1 is a schematic illustrating an MRI system according to the invention, Fig. 2 is a schematic illustrating an RF coil according to the invention,

Fig. 3 illustrates a combination of an MRI coil combined with an optical fiber system,

Fig. 4 illustrates different arrangements of optical fibers and coil elements,

Fig. 5 shows an MRI coil system with an optical fiber system,

Fig. 6 is a flowchart illustrating a method of performing a nuclear magnetic resonance scan and in parallel an optical examination.

DETAILED DESCRIPTION OF THE EMBODIMENTS Fig. 1 is a schematic illustrating an MRI system. Only major components of a preferred MRI system which incorporate the present invention are shown in Fig. 1. The magnetic resonance imaging apparatus comprises a data processing system 100, wherein the data processing system 100 typically comprises a computer screen 102 and an input device 104. Such an input device could be for example a keyboard or a mouse. The MRI system in Fig. 1 further comprises a memory 106 and an interface 108. The interface 108 is adapted for communication and data exchange with typical hardware MRI components. Further, the interface 108 is also adapted for communication to optical tomography imaging components which are also used in the magnetic resonance imaging apparatus in parallel to state of the art imaging. Typical hardware MRI components are for example a main field control unit

130 adapted for controlling the main field of the magnet 122. The interface 108 is also adapted to communicate with the gradient control unit 132, wherein respective gradient coils 124 are preferably self- shielded gradient coils for producing gradients along three mutual axes x, y and z. The MRI system further comprises an RF coil 128 electrically connected to an RF coil control unit 134. In general, the RF coil 128 is adapted for transmission of an RF pulse. However, also possible is the adaption of the RF coil arrangement 128 for RF signal reception, which is important in case a reference scan for SENSE (sensitivity encoding) is used.

The transmit coil 128 can also be a multi element transmit/receive coil with independent channels. Each transmit channel can be exited by a different or arbitrary RF pulse, required for the MR experiment.

In the RF generator 138, an RF pulse sequence is generated under the control of the data processing system 100 and for example protons in the body 126 of a person are excited in a predefined manner. The resulting magnetic resonance imaging signal is then detected by for example a surface coil 142, digitized already within the surface coil 142 and amplified by means of the amplifier 136. This is followed by processing of an acquired RF signal by special hardware components like detectors, mixers, etc, well known in the art and not shown here. Such hardware components can be adapted as additional external hardware units or be implemented into the data processing system 100.

The interface 108 is further connected to optical couplers 150 which are adapted to perform diffused optical imaging on the body 126 in parallel to performing the MRI scan. For this purpose, the surface coil 142 comprises optical couplers 150 and each optical coupler comprises an analogue digital converter 152. The analogue digital converter 152 is adapted to convert the optical analogue signals detected by the optical couplers 150 into digital signals, which are input into the combiner 156. As already mentioned above the surface coil 142 is preferably a digital RF surface coil. For this purpose, the surface coil 142 further comprises a further analogue digital converter 152 which is adapted to convert the detected analogue magnetic resonance RF signals into digital signals, which are also preferably input into the combiner 156.

The combined digital signals of the optical couplers 150 and the surface coil 142 are then transmitted to the interface 108 of the data processing system 100.

Alternatively it is possible to have a separate connection of the output of the optical couplers 150 to the interface 108 and a separate connection of the output of the surface coil 142 to the interface 108. Such connections may be directly or indirectly including various other components like amplifiers, filters etc.

The data processing system 100 further comprises a processor 110 which is adapted to execute computer executable instructions of the computer program product 112. In the present embodiment, the data processing system 100 comprises a computer program product 112 by means of a data acquisition module which is adapted to control the hardware units 122-156. Data acquisition is performed and the acquired data is analyzed via the data analysis module 116 for preferably both image construction of the MR image and the optically acquired image.

The computer program product 112 further comprises various modules 118. These modules can be for example adapted for determining for a given optical coupler 150 the optimal position relative to the body 126. For example, the optical coupler may comprise special position detection means which allow a motor which is controlled by means of the modules 118 to be positioned in an optical position with respect to the skin surface of the body 126. Fig 2 is a schematic illustrating an RF coil according to the invention. The RF coil arrangement 200 comprises an RF reception coil 142 adapted to receive RF signals resulting from an excitation of for example proton nuclei in an object to be examined. Preferably, the RF coil arrangement is a digital RF coil arrangement 200, which means that a

digitization within the RF coil arrangement is necessary. This digitization is performed by the analogue digital converter 202.

The RF coil arrangement 200 also comprises various components which are adapted to perform diffused optical imaging. The most important components are the optical couplers 150 through which light is coupled by means of the light source (not shown here) onto for example a skin surface of a person, and in transmission or reflection detected by means of one of the optical couplers 150 through a photo detector (also not shown here). The opening 206 is adapted to transmit light to a surface and receive light reflected or transmitted through said surface. Regarding a light detection, after having detected in an analogue manner the light, the detected signals are pre-amplified by means of a preamplifier 212 and digitized by means of an analogue digital converter 214. The digitized signals are then merged together with the digitized signals received from the RF coil 142 and the respective analogue digital converter 202. This merging is performed by means of the combiner 226. The merged signals are communicated to a communication interface 224 which itself is connected via for example a fiber optics to a control and data acquisition system external to the RF coil arrangement 200.

The optical couplers 150 are adapted as movable couplers. For that purpose, the RF coil arrangement 200 further comprises a positioner 204 which can for example be a non-magnetic Piezo Motor. Using said motor, the optical couplers 150 can be moved to and away from a given object which is located in the direction of the opening 206.

In order to allow the RF coil arrangement 200 to exactly position the optical couplers 150 onto a surface, the optical couplers 150 comprise various position detection elements. The easiest position detection element is a sensor 208 which is connected electrically to a controller unit 210. Upon for example ultrasonic detection of a certain distance of the opening 206 from a surface to be optically examined, the control unit 210 sends a respective signal to the positioner 204 which thereupon stops a movement of the optical couplers onto the surface of the object to be optically examined.

Another possibility to detect the spatial position of the optical couplers 150 is to use the resonant marker 216 which is located close to the tip of the optical coupler 150. By means of the RF coil 142 a usual MR scan sequence can be performed which allows detecting visually within the MR image the spatial location of the resonant markers 216. However, typically the resolution especially for example in a preparation scan sequence is rather low such that it is preferred to have other techniques which allow more precise

detection of the spatial location of the optical couplers 150 or the resonant markers 216, respectively.

For this reason the optical couplers 150 further comprise a micro-coil 218 which is placed around the resonant marker 216. The micro-coils 218 are connected to a preamplifier 220 which itself is connected to a combiner 222. This combiner serves the purpose of merging the signals acquired by a multitude of micro-coils 218 to one signal and input this one signal to the combiner 226. The signals can then be communicated to the communication interface 224 and from the communication interface 224 to a control and data acquisition system. In the control and data acquisition system, the information acquired by means of the micro -coils 218 is analyzed. A micro -coil will only yield an RF signal as a response of the excitation of the resonant marker 216 in case the gradient fields are adjusted in such a manner, that exactly the voxel (three-dimensional pixel) is excited in which the resonant marker is spatially located. Since the control and data acquisition system knows exactly which voxel is excited at a certain point in time, a respective signal response of a micro-coil at exactly this moment tells the control and data acquisition system that exactly at the position where the excitation is currently performed the micro-coil and therewith the optical coupler 150 is located. This gives the highly accurate position information of the optical coupler 150 with respect to the RF coil arrangement 200 itself and/or the patient, respectively. Upon detection of the actual position of the optical couplers 150 with respect to an object to be imaged, the positioner 204 can be controlled by means of a communication with the communication interface 224.

Preferably, the controller 210 and the positioner 204 are adapted in such a manner, that an automatic positioning of the optical coupler 150 on a surface to be imaged can be attained.

Fig. 3 illustrates a combination of an MRI coil combined with an optical fiber system. Around the object 300 the MR receive coil 142 is located. In the embodiment of Fig. 3, the MR receive coil 142 has various openings such that optical couplers can be directed to the object to be imaged by diffused optical imaging in parallel to MR imaging. For example, diffused optical imaging is sensitive to functional tissue information, the high sensitivity is required for molecular imaging and the speed enables dynamic and functional imaging. Additionally, the administration of contrast agent reveals new imaging possibilities, like information on specific cells, diseases, abnormalities etc.

In Fig. 3 the optical couplers are adapted for either an optical input 304 or for an optical output 302 with respective preamplifiers and analogue digital converters as already described above. The optical couplers are mounted with respect to the RF coil 142 on a holder 306 which is in the embodiment of Fig. 3 placed around the RF coil 142. The holder 306 comprises positioners (not shown here) which allow for a radial movement of the optical couplers to the object 300 and from the object away. In case of a flat RF coil this would correspond to a perpendicular movement of the optical couplers with respect to the coil. The optical couplers may be in general adapted as 2D or 3D fiber arrays, wherein the optical sensor is built on the surface of the MR receive coil. The MR receive coil may be itself adapted as for example a coil array. For example the MR receive coil may be adapted as a flexible coil mattress with optical sensors directly located on the lower surface of the flexible coil mattress. This allows for a realization of a hybrid optical MR coil array.

Fig. 4 illustrates different arrangements of optical fibers and coil elements. In Fig. 4a a transmit fiber is used to transmit light to an optical coupler. The light which is detected with a second optical coupler is converted by means of an analogue digital converter 214 and merged together with a digitized magnetic resonance signal in a merging unit 226. This means that for communication of detected optical and magnetic resonance signal only one optical fiber is used. Of course, besides usage of an optical transmission it is also possible to use any other kind of digital transmission known in the art. For example, it is possible to use a wireless communication or an electrical (coaxial) digital communication of signals detected by means of the MR receive coil 142 and the optical couplers to a control and data acquisition system.

In Fig. 4b, only one optical fiber is used for transmission and reception of signals for diffused optical imaging. For this purpose, a special control unit is used which allows for example upon reception of a certain digital command to generate a certain light pulse which is then transmitted by means of the optical couplers onto an object to be imaged, reflected or transmitted through said object and detected again with further optical couplers. Preferably as already mentioned above, the optical couplers comprise a preamplifier, as well as an analogue digital converter such that digital signals are communicated to said special control unit in order to be transmitted to a control and data acquisition system. For the MR receive coil 142, it is possible to perform a communication of MR signals detected by means of the coil 142 in an analogue or digital manner. For example in Fig. 4 the analogue version is shown which means that by means of a power amplifier a high power RF pulse is transmitted from the control and data acquisition system to the coil and due to the excitation

of nuclei a respective RF signal is detected and amplified by means of a preamplifier and transmitted back to the control and data acquisition system, where image reconstruction is performed.

Independent of the embodiments shown in Fig. 4, any combinations of analogue and digital transmission techniques can be used to transmit the data acquired by means of the coil 142 and the optical couplers to a control and data acquisition system.

Fig. 5 shows an MRI coil system with an optical fiber system. Shown on the left side of Fig. 5 is an RF coil 506 like for example a tubular small diameter RF coil which is used for magnetic resonance imaging of animals. In order to perform simultaneous optical imaging a cylindrical holder 500 is provided which has a diameter similarly to the outer diameter of the RF coil 506. The holder 500 has openings or holes 502 through which optical couplers or optical fibers (not shown here) are inserted and targeted towards the object like for example the animal to be imaged within the RF coil 506. Preferably, an automatic positioning of the optical couplers through the holes 502 and respective openings 508 of the RF coil 506 is performed. For this purpose, the holder 500 further comprises positioner like for example piezoelectric motors which are adapted for a radial movement of optical couplers, for example optical fibers, through the holes 502 and 508 onto the object to be optically imaged.

Optionally or additionally, in Fig. 5 fixing means 504 are shown which can be for example screws. Such screws can be used to manually fix optical couplers in a predetermined position relative to the holder 500 and the RF coil 506. By using such fixing means 504, an automatic positioning of optical couplers on an object to be imaged is not possible. Nevertheless, this allows using state of the art RF coils 506 in combination with a low-cost holder 500. Fig. 6 is a flowchart illustrating a method of performing a nuclear magnetic resonance imaging scan and an optical examination. In step 600, an object is positioned into for example a cylindrical RF coil arrangement. Optionally, it is also possible to use for example a flexible coil mattress comprising MR coils and optical sensors, wherein the optical sensors are directly located on the lower surface of the flexible coil mattress for the realization of the hybrid optical MR coil array. In this case, the flexible coil mattress is placed onto the object to be optically imaged and imaged by magnetic resonance imaging. After the correct positioning of the object in step 600, the optical couplers comprised in the RF coil arrangement according to the invention have to be placed onto for example the surface of said object. The positioning itself is performed in step 608. However,

in order to determine a correct positioning, the actual position of the optical couplers with respect to the object to be imaged has to be determined. For this determination, three possibilities are depicted in the flowchart of Fig. 6. One possibility is that after step 600 in step 602 a sensor feedback is received from the optical couplers. In this case, the optical couplers contain a mechanical sensor at a tip which could be for example an ultrasound sensor or an electrical sensor which detects the position of the optical couplers relatively to the surface of the object to be imaged. A second possibility is that after step 600, in step 604 a micro -coil feedback is received from the optical couplers, wherein in this case the optical couplers comprise a resonant marker and a micro-coil. Details regarding this micro -coil implementation are described in Fig. 2. The third possibility to determine an actual position of the optical couplers with respect to the object is that after step 600 no direct feedback (step 606) is obtained from the optical couplers. In this case, a position detection of the optical couplers is performed by means of a regular MR scan which visualizes in the acquired MR image the actual position of for example resonant markers contained in the tips of the optical couplers.

After having positioned the couplers into a predetermined position in step 608, in step 610 a simultaneous MRI scan is performed parallel to a diffused optical imaging scan.

REFERENCE NUMERALS:

100 Data Processing System

102 Screen

104 Input Device

106 Memory

108 Interface

110 Processor

112 Computer Program Product

114 Module

116 Module

120 Module

122 Main Magnets

124 Gradient Coils

126 Body

128 RF Coil

130 Main Field Control Unit

132 Gradient Coils Control Unit

134 RF Coil control unit

136 Amplifier

138 RF Generator

142 Surface coil

150 optical coupler

152 A/D converter

154 A/D converter

156 Combiner

200 RF coil arrangement

202 A/D converter

204 positioner

206 opening

208 sensor

210 controller

212 preamplifier

214 A/D converter

216 resonant marker

218 micro-coil

220 preamplifier

222 combiner

224 communication interface

226 combiner

300 object

302 optical output

304 optical input

306 holder

500 holder

502 holes

504 fixing means

506 RF coil

508 opening