SPECTRAL COMPUTED TOMOGRAPHY USING CORRELATED PHOTON NUMBER AND ENERGY MEASUREMENTS

DESCRIPTION The present application relates to the field of spectral computed tomography

(CT). It also relates to the detection of x-rays and other radiation where it is desirable to obtain information regarding the energy of the detected radiation. It finds particular application in medical imaging, and also has application in non-destructive testing and analysis, security applications, and other applications where energy discrimination capabilities are useful.

While conventional CT systems have provided image data representative of the x-ray attenuation of an object under examination, such systems have been limited in their ability to provide information about the material composition of the object, especially where different materials have had similar radiation attenuations. Improving the material separation capability of a CT system can, however, be useful in a number of applications. In medical applications, for example, it may be desirable to distinguish between various tissue types, to distinguish tissue from contrast agent, and the like. As another example, information on the composition of a sample can simplify the inspection task in security applications. One way to obtain material composition information is to measure the energy of the detected radiation, for example through the use of photon counting detectors. Exemplary photon counting detectors include scintillator-based detectors such as those based on lutetium orthosilicate (Lu ₂ SiOs or LSO), bismuth germanate (BGO) and sodium iodide (NaI) together with a photodetectors such as photodiodes or photomultiplier tubes (PMTs). Still other scintillator materials such as lanthanum bromide (LaBr), LuI ₃ , Gd ₂ SiO ₅ (GSO), LuAlO ₃ (LuAP) and YAlO ₃ (YAP) are also known. Cadmium zinc telluride (CZT) based detectors are an example of direct conversion photon counting detectors.

Unfortunately, however, photon counting techniques are not particularly well-suited for use over the count rates and input dynamic ranges typically encountered in

CT and other x-ray applications. One technique for addressing this issue is described in

Kraft, et ah, Counting and Integrating Readout for Direct Conversion X-ray Imaging

Concept, Realization and First Prototype Measurements, 2005 IEEE Nuclear Science Symposium Conference Record, which discloses a counting and integrating pixel (CIX) structure for use with CZT detectors. A photon counter counts the photons received by a detector pixel during a reading period. An integrator simultaneously integrates the total signal current over the reading period. According to the paper, the described technique extends the available dynamic range beyond the limits of photon counting and integration techniques taken individually, and also yields spectral information in terms of mean photon energy in the region where the operating ranges of the photon counting and integration regimes overlap. Nonetheless, there remains room for improvement. More particularly, it remains desirable to provide improved techniques for obtaining spectral information in CT and other x-ray detection systems.

Aspects of the present application address these matters and others.

According to a first aspect, an apparatus includes an x-ray detector which generates a signal having a magnitude which varies in response to x-ray photons received by the detector, a photon counter which counts x-ray photons received by the detector during a reading period, and a photon energy determiner which generates an output which represents a total of the changes in the magnitude of the detector signal in response to x-ray photons received by the detector during the reading period.

According to another aspect, a method includes detecting an x-ray photon received by an x-ray detector element of an x-ray computed tomography apparatus, counting the detected photon, measuring an energy of the detected photon. The measurement is performed in response to the detection of the photon. The method also includes repeating the steps of detecting, counting, and measuring during a reading period and generating an output indicative of the number and total energy of the photons received by the detector element during the reading period.

According to another aspect, an x-ray computed tomography apparatus includes an object support which supports an object under examination in an examination region, an x-ray source which generates x-rays from a plurality of positions about the

- ? -

examination region, an x-ray sensitive detector element which generates a time varying signal in response to x-ray photons received by the detector element, a photon counter which counts x-ray photons received by the detector element during a reading period, and a photon energy determiner which measures a total energy of the counted x-ray photons. The photon energy determiner measures a change in the detector signal during each of a plurality of sub-periods of the reading period.

Still further aspects of the present application will be appreciated by those of ordinary skill in the art upon reading and understanding the following detailed description.

The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. FIGURE 1 depicts a CT system.

FIGURE 2 is a functional block diagram of a detector channel.

FIGURE 3 depicts a detector signal.

FIGURE 4 is a block diagram of an energy discrimination apparatus.

FIGURE 5 depicts an energy encoding scheme Figure 6 depicts an imaging method.

With reference to FIGURE 1, a CT scanner 10 includes a rotating gantry 18 which rotates about an examination region 14. The gantry 18 supports an x-ray source 12 such as an x-ray tube. The gantry 18 also supports an x-ray sensitive detector 20 which subtends an arc on the opposite side of the examination region 14. X-rays produced by the x-ray source 12 traverse the examination region 14 and are detected by the detector 20. Accordingly, the scanner 10 generates projection data indicative of the radiation attenuation along a plurality of projections or rays through an object disposed in the examination region 14.

Depending on the configuration of the scanner 10 and the detector 20, the x- ray source 12 generates a generally fan, wedge, or cone shaped radiation beam which is approximately coextensive with the coverage of the detector 20. Moreover, so-called fourth generation scanner configurations in which the detector 20 spans an arc of 360 degrees and remains stationary while the x-ray source 12 rotates about the examination region may also be implemented. The use of flat panel, single slice, or other detector configurations is also contemplated.

The detector 20 includes a plurality of detector elements or pixels 10Oi . . . 10O _N which are implemented as CZT, scintillator coupled to a photosensitive device or other photon counting detectors. A plurality of photon counters 24i . . . 24 _N and event- driven energy determiners 26 ₁ . . . 26 _N receive signals generated by the various detector elements 10Oi . . . 100 _N . The various photon counters 24^ _N generate outputs n _meas i-N indicative of the number of photons received the corresponding detector element 100 _I _ _N and which satisfy one or more counting criteria, while the event-driven energy determiners 26 generate outputs E _meas i-N indicative of the measured energy of the counted photons. The outputs n _me asi-N and E _meas i-N are generated for each of a plurality of reading periods corresponding to various projection angles about the examination region 14. As will be described further below, the energy measurement can be viewed as an event driven process which is synchronized with the detection of photons by the respective detector elements 100. More particularly, the energy measurements represent the sum of the rise-signal net magnitudes of the photons detected during a given reading period.

A mean energy calculator 46 uses the various count data n _measl _ _N and E _measl _ N to calculate mean energy E _mean i N for the various detector channels and reading periods, with the data stored in a suitable memory or memories or otherwise provided to the reconstructor 22. A count corrector 44 applies pileup or other desired corrections to the measured count data n _meas i-N generated by the channels during the various reading periods so as to generate corrected counts n _corr i-N.

The reconstructor 22 reconstructs the projection data to generate volumetric data indicative of the interior anatomy of the patient. In addition, the energy information is used (before reconstruction, after reconstruction, or both) to provide information about the material composition of the object under examination. Note that the photon number data

and the total energy data may be reconstructed separately without reference to the mean energy data, with information extracted from the two different image sets used in a post processing operation. In such an implementation, the mean energy calculator 46 may be omitted. An object support 16 such as a couch supports a patient or other object in the examination region 14. The support 16 is preferably movable in coordination with a scan in order to provide a helical, axial, circle and line, or other desired scanning trajectory.

A general purpose computer serves an operator console 44. The console 44 includes a human-readable output device such as a monitor or display and an input device such as a keyboard and/or mouse. Software resident on the console allows the operator to control the operation of the scanner by establishing desired scan protocols, initiating and terminating scans, viewing and otherwise manipulating the volumetric image data, and otherwise interacting with the scanner 10.

FIGURE 2 depicts an exemplary detector channel in greater detail. As will be appreciated, the detector element 100 generates a time varying signal S(t) having a magnitude which varies in response to x-ray photons received by the detector element 100.

A signal conditioner 202 such as a pre-amplifier and signal shaper or filter conditions the detector signal, for example to provide a desired signal level and/or to reduce the effects of noise. A first time delay element 204 delays the detector signal by a first delay time At ₁ so as to generate a time delayed detector signal S(t-δti). A photon detector 206 receives the time-delayed detector signal and detects the various photons P ₁ received by the detectors element 100 and which comply to certain conditions in order to be counted, for example by detecting rises in the signal S(t-δtj) which are indicative of detected photons. A detection threshold may also be used to reduce false counts due to noise. The photon detector 206 also generates a trigger signal in response to the detection of a photon P ₁ . A counter 208 counts the photons P ₁ to generate an output n _meas indicative the number of photons detected during a reading period.

A detector signal change determiner such as a delay-subtractor 210 likewise receives the detector signal S(t) and generates an output signal indicative of the magnitude of the change in the detector signal during a second time period δt ₂ :

Equation 1 δS = S(t) - S(t-δt ₂ )

In response to the trigger signals generated by the photon detector 206, a signal sampler 212 such as a sample and hold unit samples the output of the delay-subtractor 210 to generate sampled signals AS ₁ corresponding to the various photons P ₁ . A summer 214 sums the sampled signals to generate an output E _meas indicative of the energy of the photons P ₁ detected during the reading period:

Equation 2 n tLmeas — χ IλO i

where n is the number of photons detected during the reading period.

Following each reading period, the photon count n _meas and energy E _meas the counter 208 and summer 214 are reset for the next reading.

Returning now to Figure 1, the count corrector 44 applies a pile -up correction to the various measured count values n _meas for each channel to generate corresponding corrected count values n _corr . The correction can be accomplished using a correction transfer function, a look up table, or other known techniques. The mean energy calculator 46 uses the measured counts n _meas and energy E _meas for each channel to generate corresponding mean energy values of the photons received by the various detector elements 100 during the various reading periods: Equation 3 tLmeas tLmean — K ftmeas where k is a calibration factor which can be empirically derived based on the operational characteristics of a given system.

Operation will now be further described in relation to Figure 3, which depicts an exemplary detector signal 302 generated in response to photons Pl, P2, and P3.

While the precise characteristics of the detector signal 302 are a function of the detector elements 100 and the associated electronics, the signal generated in response to a photon is typically characterized by period in which signal magnitude rises relatively rapidly,

followed by a relatively slower decay. Accordingly, the delay times δti and δt ₂ are selected based on the characteristics of the detector elements 100 and the associated electronics. More particularly, second delay time δt ₂ determines the time period over which the detector signal change determiner 210 calculates the signal difference δS. As illustrated, the second delay time δt ₂ is selected to approximate the time period of the rising signal portion of the signal 302 (i.e., the time period over which the magnitude of the signal 302 increases) in response to the receipt of a photon by the detector element 100. For practical photon counting detectors and/or scintillator materials, the delay time δt ₂ is in the range of about 2 to 20 nanoseconds (ns). The first delay δtl is used to synchronize the operation of the sampler 212 with the detection of the photons Pi so that the signal differences AS ₁ are sampled during the time period of the rising signal portions. Thus, the various signal differences AS ₁ correspond to the rise signal net magnitudes of the detected photons P ₁ . Of course, the average accuracy of this correspondence is affected by the input photon count rate. One advantage of such a technique is that the photon count and total energy measurements tend to be affected by pileups and noise thresholding in a similar way. As a consequence, the mean energy calculation is accurate over a relatively wide dynamic range. Moreover, both measurements are insensitive to the relatively longer detector decay time component (sometimes also referred to as 'afterglow' effect), which can cause a residual signal from photons received in a first reading period to contribute to the signal level in subsequent reading periods. As a result, the described technique is relatively unaffected by these residual signals, especially when compared to techniques which integrate the detector signal over the reading period.

Figures 4 and 5 depict a block diagram and energy encoding scheme of one possible hardware implementation. Of course, this configuration is an example only and other configurations are also possible. The values of the various parameters are given in order to demonstrate the orders of magnitude of time periods and rates which might be encountered in a typical system.

As illustrated, the first time delay element 204 provides a delay δti of about 3 nanoseconds (ns), while the photon detector 206 is implemented as a rising signal detector having a forced dead time of 10 ns. The detector signal energy change determiner

210 is implemented using a delay element 402 which provides a delay δt ₂ of about 4 ns and a subtractor 404.

Figure 4 also depicts a suitable technique for summing the various samples AS ₁ . As illustrated, the system includes a voltage to time converter 402, a time to clock cycles converter 404, a first clock 406, a prescaler 408, a second counter 410, and a second clock 412. The voltage to time converter 402 converts the sample signals AS ₁ to pulses having a temporal length which is proportional to the magnitude of the sample signal AS ₁ . The time to clock cycles converter 404 operates in conjunction with the first clock 406 to generate a number of pulses which is proportional to the length of the pulse generated by the voltage to time converter 402. For example, the first clock 406 may be implemented as a 1 Gigahertz (GHz) clock and the voltage to time converter 402 configured so that the pulse length generated in response to the maximum expected magnitude of the sampled signal ASi corresponds to eight (8) cycles of the clock 406.

A second counter 410 operates in conjunction with the second clock 412 to count the pulses generated by the time to clock cycles converter 404. The prescaler 408, which is disposed between the time to clock cycles converter 404 and the input of the second counter 410, serves as a frequency divider and thus allows the use of a relatively lower speed counter 410. Where the prescaler 408 is implemented as a 3-bit (i.e., divide by

8) prescaler, a suitable second clock 412 speed would be on the order of 100 Megahertz (MHz), and the first 208 and second 410 counters could be implemented as a 16-bit synchronous counters. Alternatively, asynchronous counters can be used without the need for clock 412.

Figure 5 depicts the relationship between photon energy and the encoding scheme in greater detail. In the present example, the energy range from about 5 to 120 keV is encoded into eight (8) energy ranges or bins El through E8, with each bin having a width AE. Note that the pulse width generated by the voltage to time converter 402 should be established so that the energy bin El is encoded as one (1) clock cycle, the energy bin E2 is encoded as two (2) clock cycles, and so on.

The count value generated by the second counter 410 can readily be re- scaled to correspond to the desired energy units. According to one technique, the output of the second counter 410 is multiplied by the prescaler 408 dividing factor. In an

implementation where the counts remaining in the prescaler 408 can be retrieved, the remaining counts are added to the result of the multiplication. In an implementation in which the remaining counts are not retrievable, the remaining counts may be approximated by adding a number of counts which corresponds to one-half (1/2) of the prescaler 408 dividing factor to the result of the multiplication. The result is then multiplied by the known energy bin width δE and by a known, pre-calibrated factor which converts the sum of the rise signal net magnitudes to the total photon energy in the desired units.

Variations are contemplated. While a particular advantage of the photon counting and energy measurement techniques described is their relative simplicity, other techniques may also be implemented. For example, while the overall energy measurement accuracy is a function of statistical factors, the accuracy of the overall energy measurement is also influenced by the accuracy of the rise signal net magnitude measurement. The accuracy of the energy measurement may be improved, for by example, by dynamically adjusting the first and second delay times At ₁ and δt ₂ as a function of the count rate and/or signal levels so that the sampling corresponds more closely to the beginning of the signal rise and/or the signal peaks. As yet another example, the energy measurement may be performed using other measurement schemes or with greater or lesser resolution.

Operation will now be described in relation to FIGURE 6. A scan is initiated at 602. Radiation which has traversed the examination region 14 is detected at 604.

Photons detected during a given reading period are counted at 606, and the energy of the detected photons is measured at 608. The energy may be measured by measuring the detector signal in each of a plurality of discrete sub-periods within the reading period, where the sub-periods correspond to the detection of x-rays. As described above, for example, the energy may be measured by measuring the change in the detector signal in response to the various detected photons.

The mean energy of the detected radiation is calculated at step 610. At step 612, a corrected photon count value is generated.

The projection data is reconstructed at step 614 using known techniques to generate volumetric data indicative of the radiation attenuation and material composition of

the object under examination. Note that the material composition information may be generated, alternately or additionally, in a post-reconstruction operation.

The volumetric image data and or the material composition information are presented in a human perceptible form at step 616, for example by way of human readable images displayed on a monitor or display associated with the operator console 44.

While the foregoing steps have been discussed from the perspective of a single detector element 100 and a single reading period, it will of course be appreciated that data is collected for the various detector elements and over a plurality of reading periods. The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.