Abstract:
The invention is directed to a method and apparatus for timing calibration in a PET scanner. According to one embodiment, the invention relates to a method for timing calibration in a PET scanner having a plurality of scintillator blocks. The method comprises: detecting, in a first scintillator block, a first radiation event, wherein the first scintillator block time-stamps the first radiation event; detecting, in a second scintillator block that is adjacent to the first scintillator block, a second radiation event that corresponds to the first radiation event, wherein the second scintillator block time-stamps the second radiation event; and determining a timing characteristic of the first scintillator block with respect to the second scintillator block based on a comparison between the time-stamps of the first radiation event and the second radiation event.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to the field of medical imaging and more particularly to a method and apparatus for timing calibration in a PET scanner.  
         [0002]     Positron emission tomography (PET) is a technique commonly used in clinical medicine and biomedical research to create images of a living body in its active state. PET scanners can produce images that illustrate various biological process and functions. In a PET scan, the patient is initially injected with a radioactive substance known as a PET isotope. The injected PET isotope can act as a tracer when it becomes involved in certain physiological processes in the patient&#39;s body. Typical positron-emitting PET isotopes include  11 C,  13 N,  15 O and  18 F. When positrons are emitted within the body, they combine with electrons in the neighboring tissues and become annihilated. The annihilation event mostly result in a pair of gamma photons being released in opposite directions. The gamma photons are then detected by a detector ring like the one shown in  FIG. 1 . The detector ring  100  may comprise a number of detectors or detector channels (e.g., 11, 12, 13, and 14 etc.) each having a scintillator block and a photomultiplier tube (PMT). For example, detector  11  comprises a scintillator block  112  and a PMT  114 , detector  12  comprises a scintillator block  122  and a PMT  124 , and so on.  
         [0003]     In operation, a patient  102 , who has been injected with a PET isotope, may be positioned in the detector ring  100 . One pair of gamma photons from a body part  104  may be detected along a line of response (LOR)  116  on opposite sides of the patient, for example. Another pair of gamma photons from the body part  104  may be detected along another LOR  136 . Along the LOR  116 , the gamma photons may cause substantially simultaneous scintillations in the scintillator blocks  112  and  122 . These scintillations may then be amplified and converted into electrical signals by the PMTs  114  and  124  respectively. Subsequent electronic circuitry may determine whether these substantially simultaneous scintillations are coincidence events, that is, radiation events originating from the same annihilation event in the patient  102 &#39;s body. Data associated with coincidence events along a number of LORs may be collected and further processed to reconstruct two-dimensional (2-D) tomographyic images. Some modern PET scanners can operate in a three-dimensional (3-D) mode, where coincidence events from different detector rings positioned along the axial direction are counted to obtain 3-D tomographic images. An exemplary PET scanner with multiple detector rings is shown in  FIG. 2 , where the PMTs are not shown. As shown, the PET scanner  200  comprises three detector rings  22 ,  24  and  26 .  
         [0004]     One aspect of PET detection methods is Time-Of-Flight PET (TOF PET), where the arrival time of a pair of coincident photons is measured. In non-TOF PET, the arrival time is ignored and the annihilation is equally probable to have occurred along the full extension of the LOR. In TOF PET, upon detection of a radiation event (e.g., a gamma photon), the scintillator block at the detection locale time-stamps the detected radiation event. Incorporation of the arrival time gives more weight to the more probable locations of the emission point for each event, thereby reducing statistical uncertainty in the reconstructed images. For the TOF PET technique to be successful, the PET scanner has to maintain a high timing resolution (e.g., within a fraction of a nanosecond). The timing resolution greatly depends on how two scintillator blocks on opposite sides of the detector ring time-stamp their respectively detected radiation events. If the two opposite blocks have different timing characteristics, radiation events detected along the LOR connecting the two blocks may exhibit a timing drift, therefore causing difficulty in reconstructing PET images or causing errors in the reconstructed images. Timing drifts are typically caused by changes in time delay inside the PMTs, change in thresholds in the time-stamping circuits, master clock skew, or transmission delays in the cables. Any of these changes can be caused by either actual component change or thermal effects.  
         [0005]     Due to the timing resolution requirements, it is typically necessary to monitor and calibrate the timing drift for a PET scanner on a frequent basis. This type of timing calibration is traditionally performed by placing a radiation source at the center of the detector rings and adjusting each detector channel until the coincidence events registered by each channel is maximized and relatively uniform around the entire ring. The use of an external radiation source requires interruptions to normal operation of the PET scanner. As a result, the traditional timing calibration cannot be performed concurrently with data acquisition from patients.  
         [0006]     In view of the foregoing, it would be desirable to provide a more efficient solution for timing calibration in a PET scanner.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     The present invention is directed to a method and apparatus for timing calibration in a PET scanner that overcomes drawbacks of known systems and methods.  
         [0008]     According to one embodiment, the invention relates to a method for timing calibration in a PET scanner having a plurality of scintillator blocks. The method comprises: detecting, in a first scintillator block, a first radiation event, wherein the first scintillator block time-stamps the first radiation event; detecting, in a second scintillator block that is adjacent to the first scintillator block, a second radiation event that corresponds to the first radiation event, wherein the second scintillator block time-stamps the second radiation event; and determining a timing characteristic of the first scintillator block with respect to the second scintillator block based on a comparison between the time-stamps of the first radiation event and the second radiation event.  
         [0009]     According to another embodiment, the invention relates to a method for timing calibration in a PET scanner having a plurality of scintillator blocks. The method comprises: detecting a plurality of radiation events, wherein each of the plurality of radiation events is associated with one of the plurality of scintillator blocks and locally time-stamped; identifying at least one event pair each comprising a first radiation event and a second radiation event, wherein the first radiation event is detected in a first scintillator block, the second radiation event is detected in a second scintillator block that is adjacent to the first scintillator block, and the second radiation event substantially coincides with the first radiation event; and determining a timing characteristic associated with the first scintillator block based on a comparison between the time-stamps of the first radiation event and the second radiation event.  
         [0010]     According to yet another embodiment, the invention relates to a method for timing calibration in a PET scanner having a plurality of scintillator blocks. The method comprises: performing an initial timing calibration of the PET scanner based on an external radiation source; measuring a first timing characteristic a scintillator block based on an intrinsic radiation; measuring, at a later time, a second timing characteristic for the scintillator block based on the intrinsic radiation; and detecting a timing drift for the scintillator block based on a comparison of the first timing characteristic and the second timing characteristic. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     In order to facilitate a fuller understanding of the present invention, reference is now made to the appended drawings. These drawings should not be construed as limiting the present invention, but are intended to be exemplary only.  
         [0012]      FIG. 1  is a concept view of a PET system.  
         [0013]      FIG. 2  illustrates an exemplary PET scanner with multiple detector rings.  
         [0014]      FIG. 3  is a diagram illustrating the energy states involved in a lutetium beta decay.  
         [0015]      FIG. 4  illustrates an exemplary detection of intrinsic radiation in two neighboring scintillator blocks according to an embodiment of the present invention.  
         [0016]      FIG. 5  is flow chart illustrating an exemplary method for timing calibration in a PET scanner according to an embodiment of the invention.  
         [0017]      FIG. 6  shows an exemplary histogram for timing calibration in a PET scanner according to an embodiment of the invention.  
         [0018]      FIG. 7  shows another exemplary histogram for timing calibration in a PET scanner according to an embodiment of the invention.  
         [0019]      FIG. 8  shows yet another exemplary histogram for timing calibration in a PET scanner according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     Reference will now be made in detail to exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings.  
         [0021]     One technical effect of the invention is to provide a method and apparatus for timing calibration in a PET scanner, as set forth in the Brief Summary of the Invention, above and as more fully described here in the Detailed  
       DESCRIPTION OF THE INVENTION  
       [0022]     Embodiments of the present invention may achieve timing calibration in a PET scanner without using any external radiation sources. Instead, timing drifts may be measured based on intrinsic radiation events in the scintillator blocks. For example, many high-speed scintillators for TOF PET contain lutetium (Lu), a naturally radioactive substance.  FIG. 3  is a diagram illustrating the energy states involved in a lutetium beta decay. As shown, multiple gamma rays may be emitted when lutetium ( 176 Lu) decays to hafnium ( 176 Hf). The total energy of these gamma rays is 597 keV. According to embodiments of the present invention, these gamma rays may be detected for timing calibration purposes. Though only lutetium is described here as a source of internal radiation, other radioactive elements or isotopes may also be incorporated in a scintillator material to produce desired internal radiation that is useful for timing calibration. For example, uranium (U), thorium (Th), hafnium (Hf), samarium (Sm) and potassium (K) may be candidates for radioactive scintillators since they are also capable of decays that produce at least two concurrent photons.  
         [0023]      FIG. 4  illustrates an exemplary detection of an intrinsic radiation event in two neighboring scintillator blocks according to an embodiment of the present invention. In  FIG. 4 , there are shown a first scintillator block  402 , a second scintillator block  404 , and their respective detection electronics (e.g., PMTs)  406  and  408 . The block  402  and the block  404  are adjacent to each other and share an edge  410 . Apart from the neighboring block  404 , the block  402  may have other neighboring blocks not shown here. In a 3-D PET scanner like the one shown in  FIG. 2 , each scintillator block may have 3-4 neighboring blocks with which it shares an edge. For example, in  FIG. 2 , the block  202  has three neighboring blocks  204 ,  206 , and  208 . The block  210  has four neighboring blocks  212 ,  214 ,  216 , and  218 .  
         [0024]     When an intrinsic radiation event (e.g., Lu beta decay) occurs in the block  402 , there is a reasonable probability that some of the resulting gamma rays will escape to the neighboring blocks. For example, an intrinsic radiation event  412  may take place relatively close to the edge  410 . Some of the resulting gamma photons may be detected in the block  402  as a radiation event  414 , while some gamma photons may escape into the block  404  and be detected therein as a radiation event  416 . The detected events  414  and  416  may be further processed by the detection electronics  406  and  408  respectively, where the event  414  may be assigned a location ID L 1  and a time-stamp T 1 , and the event  416  may be assigned a location ID L 2  and a time-stamp T 2 . The location IDs indicate in which block the events are detected, and the time-stamps indicate the detection time for the events. Since the events  414  and  416  originate from the same intrinsic radiation event  412 , they are coincidence events. Absent any difference between the timing characteristics of the blocks  402  and  404 , the time-stamps T 1  and T 2  should be substantially the same. However, if the block  402  becomes slow, for example, the event  414  may appear to have arrived earlier than the event  416 . Therefore, by comparing the time-stamps of coincidence events detected in two neighboring blocks, a timing difference between the blocks may be determined.  
         [0025]      FIG. 5  is a flow chart illustrating an exemplary method for timing calibration in a PET scanner according to an embodiment of the invention. The timing calibration may be performed during time periods when the PET scanner is not in use (e.g., overnight or in between patients).  
         [0026]     In step  502 , radiation events may be detected in the various scintillator blocks in the PET detector ring. Based on the specific type of intrinsic radiation, different energy windows may be set up to accept the resulting gamma photons. That is, the detectors may be configured to accept only those radiation events that fall in certain energy ranges. Lu beta decay, for example, has a total decay energy of 597 keV. In order to detect a same event in two blocks, it is required that one portion of the 597 keV be detected in one block and the remainder portion of the 597 keV be detected in the other block. The possible energy splits between the two blocks may be roughly: 300 to 300 keV, 500 to 100 keV, 400 to 200 keV. Because of these multiple energies, it may be desirable to have multiple and concurrent energy windows to accept the gamma photons. Such a multiple energy window setup is not typically provided in conventional PET systems. The gamma photons from the intrinsic radiation may cause scintillations which are then converted by PMTs to electric signals proportionate to the incident energy of the photons. The front-end detectors may select only those events that fall within the predetermined energy windows. The multiple energy windows may not only allow events to be selected more closely, but may also facilitate semi-independent measurements at the different energies.  
         [0027]     In step  504 , each single event detected/selected may be assigned a location ID and a time-stamp. Then data associated with the single events may be passed on to a coincidence processing module. In each clock cycle (typically 250 ns), the coincidence processing module may receive many events from the scintillator blocks.  
         [0028]     In step  506 , each single event may be compared with other single events to identify coincidence events. Two single events that are detected within a short time interval may have originated from a shared intrinsic radiation event, therefore they may be considered coincidence events.  
         [0029]     In step  508 , coincidence events may be identified for each block edge. That is, for each block edge adjoining two neighboring blocks, coincidence events detected in either block may be identified. According to one embodiment of the present invention, a location map may be loaded in the coincidence processing module, so that only those events from two neighboring blocks are considered.  
         [0030]     In step  510 , the coincidence events from every two neighboring blocks may be histogrammed based on two timing difference channels. One channel may be set up to accept events that are close to but earlier than the same time, and the other channel may be set up to accept events that are close to but later than the same time. For example, if coincidence events E 1  (time-stamped t 1 ) and E 2  (time-stamped t 2 ) are detected in two neighboring blocks B 1  and B 2  respectively, it may be determined, based on detector energy windows, that E 1  and E 2  originated from an intrinsic radiation event in the block B 1 . A timing difference (Δt=t 2 −t 1 ) may be calculated. If Δt equals zero, the two neighboring blocks may be deemed to have substantially the same timing characteristics. If Δt is less than zero (e.g., −1 time unit), the same intrinsic radiation event observed in the block B 1  appears to be later than observed in the block B 2 , which indicates the block B 1  is 1 time unit faster in timing than the block B 2 . On the other hand, if Δt is greater than zero (e.g., +2 time units), the block B 1  may be 2 time units slower in timing than the block B 2 . By histogramming coincidence event pairs in neighboring blocks based on time-stamp differences, a number of individual timing comparisons between a particular block and its neighbor(s) may be accumulated and analyzed. For example, a distribution of the timing comparisons may be indicative of the particular block&#39;s timing characteristics. Or the distribution of the timing comparisons relative to the above-mentioned timing difference channels may be show the particular block&#39;s timing with respect to its neighbor(s). That is, one timing difference channel may be called a “NEAR but LATER” channel, which accepts coincidence event pairs whose timing differences (Δt) are less than zero. And another timing difference channel may be called a “NEAR but EARLIER” channel, which accepts coincidence event pairs whose timing differences (Δt) are greater than zero.  
         [0031]     One example of event histogramming in the timing difference channels is shown in  FIG. 6 . As shown, coincidence events are histogrammed based on their perceived timing difference with respect to the expected zero. The vertical axis represents event counts for the coincidence events recorded. If the timing characteristic of a block is consistent with the expected zero, the histogram will not lean toward either the “NEAR but LATER” or the “NEAR but EARLIER” timing difference channels. Instead, the histogram will be centered at the zero time difference point.  
         [0032]     On the other hand, if a block becomes one time unit slower than its neighbors, for example, the histogram will be shifted from the zero time difference point.  FIG. 7  shows a histogram of coincidence events as observed in a slower block according to an embodiment of the present invention. Since this block is slower than its neighbors, radiation events that originate from the neighboring blocks are seen in this block as if they arrived earlier than the time at which they actually arrived. Therefore, the histogram in  FIG. 7  is shifted to the right by one time unit.  FIG. 8  shows a histogram of the same events as observed in a neighboring block next to the slower block. When seen from the perspective of the neighboring block, these same events appear as if they have arrived later than the time at which they actually arrived. As a result, the histogram in  FIG. 8  is shifted to the left by one time unit.  
         [0033]     Based on the observation of the histograms, timing characteristics for every two neighboring blocks may be determined in step  512 . The histograms may be displayed for visual inspection by an operator of the PET scanner. Or the histogram data may be directly processed by a computer to determine the existence and amount of timing drifts. It should be noted that, although the distribution of timing comparisons in  FIGS. 6-8  are shown as continuous curves, an actual distribution may or may not be continuous.  
         [0034]     In step  514 , the timing characteristic for each block may be adjusted if necessary. The adjustment may be achieved by changing the circuit delay in the detection electronics associated with the block, for example. Both manual or automated adjustments are possible.  
         [0035]     According to embodiments of the invention, a retrospective analysis may be performed to select the above-described timing comparisons. Depending on the geometry and location of a scintillator block, histogrammed timing comparisons for all its neighboring blocks may not be equally indicative of its actual timing characteristics. Therefore, it may be desirable to select timing comparisons from accumulated histograms in calculating the block-level timing changes. For example, one block&#39;s timing comparisons with its various neighbors may be weighted in the timing calculation.  
         [0036]     According to further embodiments of the present invention, there may be at least two types of timing calibration, one with an external radiation source and one without. As described above, without any external radiation source, the timing calibration may be done by measuring and eliminating the timing difference between each pair of neighboring blocks. Alternatively, an external radiation source may be used for an initial timing calibration of the PET system. This external-source-based calibration may be one of those methods generally known in the PET industry. This initial calibration may place the PET system in a desired state in terms of timing. Upon this initial calibration, the initial timing characteristics of the individual blocks may be measured based on intrinsic radiation. The initial timing characteristics may then be used as a reference for subsequent timing drifts. When it is time to calibrate the timing again, the present timing characteristics of the individual blocks may be measured based on the intrinsic radiation. And the present timing characteristics may be compared with the initial timing characteristics. If the change is significant, the PET system may be calibrated again using the external radiation source. Otherwise, the PET system may be deemed adequately calibrated.  
         [0037]     The above-described timing calibration method can be implemented without much change to existing hardware in a PET scanner. When the PET scanner goes into time-calibration mode, the existing coincidence processing module can simply be reprogrammed to process coincidence events in neighboring blocks instead of those in opposite blocks. And the detector electronics may be switched to appropriate energy acceptance windows. Further, the timing calibration may be performed in a flexible timeframe, either over a long period of time (e.g., overnight) or in a short period of time (e.g., in between patients). The overnight long-term data collection can improve statistical accuracy of the timing calibration. And the in-between-patients type of data collection can provide more real-time timing check.  
         [0038]     While the foregoing description includes many details and specificities, it is to be understood that these have been included for purposes of explanation only, and are not to be interpreted as limitations of the present invention. It will be apparent to those skilled in the art that other modifications to the embodiments described above can be made without departing from the spirit and scope of the invention. Accordingly, such modifications are considered within the scope of the invention as intended to be encompassed by the following claims and their legal equivalents.