Improved temperature stability for a digital positron emission tomography (pet) detector

0001The present application generally relates to nuclear imaging. The present application has applications in relation to the temperature stability of digital positron emission tomography (PET) detectors and is described with particular reference to this. However, of course, the present application is also applied to other usage scenarios and is not necessarily limited to the above applications.

0002When a digital PET scanner is placed near a computed tomography (CT) scanner, as in a hybrid PET / CT system, the PET scanner receives Compton-scattered gamma photons from the CT scanner. While the gamma photon is being received by the PET scanner, the PET detector scintillator emits light and the digital PET detector cell is discharged. For each scintillation event, multiple cells are discharged and subsequently recharged approximately simultaneously. This recharging results in an increase in the amount of current drawn, usually a substantial increase, which can overload the power supply.

<p num="0003"> The present application provides a new and improved system and method for solving these problems and the like.</p>

<p num="0004"> According to one aspect, a gamma detector having thermal stability is provided. The gamma detector includes at least one controller that sets the detector's detection sensitivity to a level that disables the detection of gamma photons. The controller also controls the heat generator to maintain the temperature of the detector at a predetermined temperature. The predetermined temperature is the steady-state temperature of the detector when the detection sensitivity of the detector is set to a level that enables the detection of gamma photons.</p><p num="0005"> According to another aspect, a method of maintaining the thermal stability of the gamma detector is provided. It is determined whether the detection sensitivity of the detector is set to a level that disables the detection of gamma photons. Heat is generated to maintain the current temperature of the detector at a predetermined temperature in response to determining that the detector's detection sensitivity is set to a level that disables the detection of gamma photons. The predetermined temperature is the steady-state temperature of the detector when the detection sensitivity of the detector is set to a level that enables the detection of gamma photons.</p><p num="0006"> According to another aspect, a nuclear imaging system is provided. The system includes a detector that includes a quiet mode and a data acquisition mode. The detector includes at least one controller that determines whether the detector is in quiet mode or data acquisition mode. In response to the determination that the detector is in quiet mode, the controller generates heat to keep the detector's current temperature at the detector's steady-state temperature in data acquisition mode.</p><p num="0007"> One advantage is improved temperature stability of digital positron emission tomography (PET) detectors.</p><p num="0008"> Another advantage is that PET imaging quality and quantization are improved.</p><p num="0009"> Further advantages of the present invention will be appreciated by those skilled in the art after reading and understanding the detailed description below.</p>

0010The present invention takes the form of various components and arrangements of components, as well as various steps and arrangements of steps. The drawings are for purposes of illustration only to illustrate preferred embodiments and should not be construed as limiting the invention.

0011<figref num="1">FIG. 1 shows a diagnostic imaging system using a first nuclear imaging modality and a second imaging modality according to aspects of the present application.</figref><figref num="2">Figure 2 shows one tile of the gamma detector in the diagnostic imaging system of Figure 1.</figref><figref num="3">FIG. 3 shows a block diagram of the electrical components of the detector of FIG.</figref><figref num="4A">FIG. 4A shows the roots of the hierarchical tree structure of the controller of the first scanner that implements the first imaging modality of FIG.</figref><figref num="4B">FIG. 4B shows a subtree of the hierarchical tree structure of FIG. 4A.</figref><figref num="5">FIG. 5 shows a temperature vs. time graph for the four different temperature sensors of the detector after returning the detector bias voltage to the normal bias voltage.</figref><figref num="6">Figure 6 shows the temperature profiles of four different tiles collected during the calibration scan.</figref><figref num="7">FIG. 7 shows a method of maintaining the thermal stability of the gamma detector according to the aspects of the present disclosure.</figref><figref num="8">Figure 8 shows how heat is generated by generating false events.</figref>

0012Prevents PET detector cells from triggering the bias voltage of a positron emission tomography (PET) detector during a CT scan to avoid the effects of gamma photons from a computed tomography (CT) scanner. It can be lowered to a level suitable for. After the CT scan is complete, the bias voltage can be returned to the proper calibration level of the PET scanner. One of the drawbacks of mitigating the effects of CT scanners by changing the bias voltage is that under low bias conditions no background events are detected, resulting in a temperature drop. When the bias voltage is returned to the normal bias voltage, the temperature rises.

0013Although temperature fluctuations due to changes in the bias voltage can be controlled, it is preferable to avoid them. In particular, repeated temperature changes can shorten the mean time between failures (MTBF) of PET detectors. In addition, temperature fluctuations reduce the image quality of PET images. This is because the behavior of the scintillator (for example, conversion efficiency, energy center of gravity, pixel sensitivity, etc.) fluctuates based on temperature.

0014To avoid temperature fluctuations, the PET detector may allow the heat generator to maintain the steady-state temperature that is typical of gamma detectors when in "data collection" mode when in "quiet" mode. it can. Quiet mode usually reduces the bias voltage of the detector's silicon photomultiplier tube (SiPM), which prevents visible light pulses from being detected. The heat generator may be the controller of the detector, a resistor heater, or any other device that generates heat. The controller generates heat by performing busy work. The heat generator is controlled based on parameters determined in advance to achieve the required amount of heat or based on feedback from the temperature sensor.

0015Referring to FIG. 1, a diagnostic imaging system 10 is provided that uses first and second imaging modality to image a subject. The first imaging modality is a nuclear imaging modality that uses radiation, such as gamma photons, received from a subject's target volume for imaging. Examples of such nuclear imaging modality include PET and single photon emission computed tomography (SPECT). The second imaging modality is computed tomography (CT), or any other imaging modality that produces the type of radiation used by the first imaging modality for imaging. As shown, the system is a hybrid PET / CT diagnostic imaging system.

0016The first nuclear scanner 12, represented as the PET scanner in System 10, produces raw scan data for the first nuclear imaging modality. The scanner 12 includes a fixed gantry 14 that houses a plurality of gamma detectors 16 (eg, 90 detectors) that are arranged around the bore 18 of the scanner 12. The bore 18 defines a test volume 20 that accepts the target volume of the subject to be imaged, such as the brain, torso, etc. The detector 16 is typically located within one or more fixing rings over the length of the inspection volume 20. However, a rotary head is also conceivable. The detector 16 detects gamma photons from the inspection volume 20 and generates raw scan data.

0017With reference to FIG. 2, each detector 16 includes one or more scintillators 22 arranged in a grid. The scintillator 22 emits light and generates a visible light pulse in response to the energy application of the gamma photon. As shown, the gamma photon 24 imparts energy within the scintillator 26, which results in a visible light pulse 28. The magnitude of the visible light pulse is proportional to the magnitude of the corresponding energy application. Examples of scintillators 22 are thallium-doped sodium iodide (NaI (Tl)), cerium-doped lutetium-iturium orthosilicate (LYSO) and cerium-doped lutetium oxyorthosilicate (LSO). Can be mentioned.

0018In addition to the scintillator 22, each detector 16 includes a sensor 30 that detects visible light pulses in the scintillator 22. Each sensor 30 includes a plurality of tiles 31 arranged in a grid of the same size as the grid of the scintillator 22. Typically, the sensor 30 includes four tiles 31, but only one tile 31 is shown. The typical power usage of one tile 31 is about 1.0 watt (W) under idle conditions (ie, the background event rate from the internal collapse of the scintillator 22) and at about 1.1 W at the typical tumor patient level. Yes, it is about 1.5 W at the typical rubidium (Rb) 82 heart disease patient level. The tile 31 contains a plurality of SiPM 32s (ie, pixels) that are optically coupled to the corresponding scintillator 22. In most cases, there is a one-to-one correspondence between the scintillator 22 and SiPM32, as shown, but other correspondences are possible. Suitably, the SiPM32 is configured to operate in Geiger mode to generate a series of unit pulses to operate in digital mode. Alternatively, the SiPM32 may be configured to operate in analog mode. Each SiPM32 includes a photodiode array (eg, a Geiger mode avalanche photodiode array), and each photodiode corresponds to one cell in the photodiode array.

0019The circuit board 34 of tile 31 supports not only the corresponding SiPM32, but also the other electronic components of tile 31. With reference to FIG. 3, these other electronic components are a tile controller 36 (eg, a field programmable gate array (FPGA)) and multiple compartments of SiPM32, as shown by N&gt; = 1 controller 38. Includes a die controller 38 (eg, a non-programmable complementary metal oxide semiconductor (CMOS) controller) for each of the above. The compartments correspond to equally sized SiPM (ie, die) arrays forming a grid of SiPM32. For example, tile 31 includes a SiPM32 8x8 grid divided into SiPM32 4x4 arrays, as shown. The die controller 38 is typically located on the same side of the circuit board 34 as the SiPM 32 (eg, near the inner diameter of the ring formed by the detector 16), and the tile controller 36 is typically on the circuit board 34, with respect to the SiPM 32. Placed on the opposite side (eg, near the outer diameter of the ring formed by the detector 16).

0020The die controller 38 produces event data (ie, time and energy) for the event detected by the corresponding SiPM32. As shown, each die controller 38 generates event data for M&gt; = 1 SiPM32. The tile controller 36 aggregates the data from the die controller 38 and transmits the aggregated data downstream. Aggregated event data is typically transmitted downstream in fixed time frames, such as 327.68 microseconds (μsec). At this time, the event data is buffered in the memory 40 of the tile 31 for a certain period of time. Buffered event data that expires over a period of time is transmitted downstream with an "end of frame" marker. If no event data is stored, only the "end of frame" marker is sent. The "End of Frame" marker is used to track the timestamp alignment between tiles. The tile controller 36 can further perform basic processing such as saturation and skew correction on the event data before transmitting the event data downstream.

0021With reference to FIGS. 4A and 4B, the first nuclear scanner 12 includes a plurality of aggregation controllers 36, 42, 44, 46 (eg FPGAs) arranged in a tree structure. These aggregate controllers 36, 42, 44, 46 include a tile controller 36 (indicated as L&gt; = 1 for each parent controller) and a central detector unit (CDU) controller 42. The CDU controller 42 represents the root of the tree structure and the tile controller 36 represents the leaves of the tree structure. Although not required, as shown, the main controller 44 (shown as N&gt; = 1) and / or the sensor controller 46 (shown as M&gt; = 1 for each main controller) are the tile controllers 36. And the CDU controller 42 may be placed in between. Each sensor controller 46 corresponds to, for example, a different sensor 30. FIG. 4A shows the roots of the tree structure, and FIG. 4B shows the subtree structure connected to each main controller 44.

0022In the tree structure, each aggregation controller 36, 42, 44, 46 aggregates event data. The tile controller 36 aggregates the event data from the die controller 38, and the other aggregation controllers 42, 44, 46 aggregate the event data from the child aggregation controllers. Further, with the exception of the CDU controller 42, each of the controllers 36, 44, 46 passes the aggregated data to the parent aggregate controller. For example, the CDU controller 42 receives aggregated event data from a plurality of main controllers 44, such as 18 main controllers 44. Each main controller 44 receives aggregated event data from a plurality of sensor controllers 46, such as 28 sensor controllers 46. Each sensor controller 46 receives aggregated event data from a plurality of tile controllers 36, such as four tile controllers 36. Each tile controller 36 receives event data from a plurality of die controllers 38, such as 16 die controllers 38.

0023Referring again to FIG. 1, a radiopharmaceutical or radionuclide is injected into the target volume of the subject while scanning the subject using the first imaging modality. Radiopharmaceuticals or radionuclides emit gamma photons or emit gamma photons from the target volume. The target volume is then placed in the test volume 20 using the subject support 48 corresponding to the first scanner 12. When the target volume is placed in the inspection volume 20, the first scanner 12 is controlled to scan the target volume, and event data is usually obtained from the CDU controller 42. The acquired event data describes the time, place and energy of each scintillation event detected by the detector 16 and is appropriately stored in the first data buffer 50, as indicated by the PET data buffer.

0024Following or at the same time as the acquisition, the event verification processor 52 filters the buffered event data. Filtering involves comparing the energy of each scintillation event (total in digital mode) with the energy window. The energy window defines the allowable energy range for scintillation events. Scintillation events outside the range of the energy window are excluded. Typically, the energy window is centered on the known energy of the gamma photon received from the test volume 20 (eg, 511 kiloelectronvolts (keV)) and uses the full width at half maximum (FWHM) of the energy spectrum generated from the calibration phantom. It is determined. In PET imaging, the event verification processor 52 can also generate a response line (LOR) from the filtered event data.

0025The first reconstruction processor 54, designated as a PET reconstruction processor, reconstructs the filtered event data or LOR into a core image of the target volume, depending on the imaging modality. Any number of well-known algorithms for reconstructing LOR into PET images can be considered. Similarly, any number of well-known algorithms for reconstructing filtered event data into SPECT images can be considered. The nuclear image is properly stored in a first image memory 56, designated as PET image memory.

0026A second scanner 58, shown as a CT scanner in System 10, produces raw scan data for the second imaging modality. The second scanner 58 includes a fixed gantry 60 whose bore 62 defines a test volume 64 that accepts the target volume of the subject to be imaged, such as the brain and torso. As shown, an array of X-ray detectors 66 is housed in a rotary gantry 68. The X-ray detector 66 receives transmitted radiation from an X-ray source 70 (eg, an X-ray tube) located on the rotary gantry 68 on the opposite side of the detector 66. Alternatively, if the detector ring is housed in a fixed gantry 60 around the rotary gantry 68 and receives transmitted radiation from an X-ray source 70 (eg, an X-ray tube) located on the rotary gantry 68. Good. The detector 66 produces data indicating X-ray absorption integrated along the corresponding beam between the X-ray source and the detector.

0027Usually, the first and second scanners 12, 58 are separate scanners that are separated from each other. However, it is conceivable that the first and second scanners 12 and 58 are integrated so as to be a common scanner. If the first and second scanners 12 and 58 are separate scanners, the scanners 12 and 58 are placed adjacent to each other so that the gamma detector 16 of the first nuclear scanner 12 is scanning, The radiation generated by the second scanner 58 can be detected. As shown, the first and second scanners 12, 58 share a common subject support 48 that translates along the patient support track 72 that extends between the two test volumes 20, 64. Alternatively, the first and second scanners 12, 58 may include individual subject supports.

0028While scanning the subject using the second imaging modality, the target volume of the subject is placed within the test volume 64 using the subject support 48, which corresponds to the second scanner 58. The second scanner 58 is then used to obtain the raw scan data for the target volume. The acquired scan data is stored in a second data buffer 74, which is referred to as the CT data buffer, and is processed by the second reconstruction processor 76, which is designated as the CT reconstruction processor, so as to be an image representation of the target volume. To. The image representation is stored in a second image memory 78, which is referred to as the CT image memory.

0029System 10 further includes a central control system 80, such as a computer, that provides users of system 10 with a graphical user interface (GUI). The GUI allows the user to interact with the control system 80 using the display device 82 and the user input device 84. Through the GUI, the control system 80 can be used to control the scanners 12 and 58 to image the subject. For example, the user may adjust the PET image after the CT image of the target volume of the subject. Further, the images from the image memories 56 and 78 can be confirmed and arbitrarily operated by using the control system 80 via the GUI. For example, the image stored in the image memory 56 of the first scanner 12 is displayed on the display device 82. In some cases, one or more of the data buffers 50, 74, the reconstruction processors 54, 76, the image memories 56, 78 and the event verification processor 52 may be integrated with the central control system. For example, the reconfiguration processors 54, 76 and the event verification processor 52 may share a common processor in the central control system 80.

0030One difficulty with the first scanner 12 being placed near the second scanner 58 is that the first scanner 12 from the second scanner 58, Compton, while scanning with the second scanner 58. The point is that it may be exposed to radiation such as scattered gamma photons. Upon receiving this radiation, the cells of tile 31 are discharged and then recharged. In some cases, this recharging substantially increases the amount of current drawn by the tile 31, which can overload the power supply. In addition, the die controller 38 and tile controller 36 on tile 31 also increase activity. This increase in current draw and controller activity raises the temperature of tile 31. This is a problem because the scintillator 22 is close to the tile 31 and the temperature fluctuation of the tile 31 fluctuates the temperature of the scintillator 22. The temperature fluctuation of the scintillator 22 in turn affects the behavior of the scintillator 22 (for example, conversion efficiency, energy center of gravity, pixel sensitivity, etc.), which affects the image quality. In addition, repeated temperature changes reduce the MTBF of tile 31.

0031To mitigate the impact of the second scanner 58 on the first scanner 12, the first scanner 12 includes two modes of operation: a "data collection" mode and a "quiet" mode. In data collection mode, the detection sensitivity of tile 31 is set to a level that enables gamma photon detection (ie, normal level for imaging), and the first scanner 12 uses tile 31 to provide event data. Is ready to collect. In quiet mode, the detection sensitivity of tile 31 is set to a level that disables gamma photon detection. Usually, the difference between the two modes is the bias voltage of the SiPM 32 on tile 31. In quiet mode, the bias voltage is dropped to a level where the sensitivity to light is zero (ie, a level suitable to prevent the cell on tile 31 from triggering in response to the light pulse).

0032The mode of the first scanner 12 is managed by the data collection / quiet mode controller 85. When the second scanner 58 is used, the data acquisition / quiet mode controller 85 puts the first scanner 12 in quiet mode. Otherwise, the data collection / quiet mode controller 85 puts the first scanner 12 in data collection mode. The data acquisition / quiet mode controller 85 controls the mode of the first scanner 12 based on the signal from the central control system 80, as shown. Alternatively, the data collection / quiet mode controller 85 may be integrated with the central control system 80. For example, the data collection / quiet mode controller 85 may be implemented as a software module by the central control system 80.

0033In a typical workflow of imaging the target volume of a subject using the first and second scanners 12, 58, the first scanner 12 is controlled to enter quiet mode. The target volume of the subject is then scanned using the second scanner 58. When a scan using the second scanner 58 is performed, the first scanner 12 is controlled to enter the data acquisition mode, and the first scanner 12 is used to scan the target volume. Therefore, the target volume is typically scanned using the second scanner 58 and then using the first scanner 12 (ie, the second scanner 58 is used first). .. When two scanners 12, 58 share a common subject support 48, the target volume is usually injected with a radiopharmaceutical or radionuclide before scanning using the second scanner 58. To.

0034As described above, the difficulty of changing the bias voltage is that background noise is not detected either. This reduces the amount of current drawn by recharging. In addition, the die controller 38 and tile controller 36 on tile 31 also reduce activity. Decreasing the amount of current drawn and the amount of controller activity of tile 31 lowers the temperature of tile 31. As mentioned above, the temperature fluctuation of the tile 31 fluctuates the temperature of the scintillator 22, which affects the behavior of the scintillator 22. Although temperature fluctuations are controllable, it is preferable to avoid them.

0035With reference to FIG. 5, temperature vs. time graphs for the four different temperature sensors of detector 16 illustrate the temperature rise. The horizontal axis corresponds to the time in seconds, and the vertical axis corresponds to the temperature in Celsius. Before the temperature measurements were collected, the detector 16 was in a low bias state for 1 hour. The detector 16 was then returned to normal bias voltage and temperature measurements were collected. As can be seen from the figure, when the detector 16 was returned to the normal bias voltage, the temperature began to rise.

0036To cope with the temperature fluctuations of the scintillator 22 due to various modes, the quiet mode activates the heat generator included as part of each tile 31. The heat generator generates heat directly or indirectly in quiet mode so as to maintain the steady temperature of tile 31 in data acquisition mode. The heat generator may use any heat generating means.

0037With reference to FIG. 3 again, the heat generator of tile 31 is properly controlled by the tile controller 36. The tile controller 36 receives a mode signal indicating the mode (that is, quiet mode or data acquisition mode) of the first scanner 12. This signal is properly received directly or indirectly from the Quiet / Data Collection Mode Controller 85. The tile controller 36 monitors the signal and controls the heat generator to maintain the steady temperature of tile 31 in data acquisition mode when quiet mode is indicated.

0038The heat generator for tile 31 may be, for example, the tile controller 36 for tile 31. In this case, the tile controller 36 performs "busy work" to generate heat. The heat generator may be tile 31, for example a resistor heater 86. The tile controller 36 may generate heat according to the method described above, for example using feedback from a temperature sensor 88 (eg, having a 1/16 Celsius temperature resolution) of tile 31. In this case, the tile controller 36 generates heat so as to maintain the current value of the temperature sensor 88 at the steady state value of the temperature sensor 88 in the data acquisition mode. As another example, the tile controller 36 generates heat according to the above method using predetermined control parameters to achieve the heating requirements required to maintain the steady temperature of the tile 31 in the data acquisition mode. For example, it may be predetermined that a current of 0.5 amperes needs to be applied to the resistor heater 86 to achieve the required heating.

0039The steady-state temperature of each tile 31 is determined by performing a calibration scan with the first scanner 12. During the scan, the scanner 12 is placed in data acquisition mode and temperature measurements are taken at a predetermined rate of 11.9209 samples per second for a predetermined period of 15 minutes. The first scanner 12 is emptied and the second scanner 58 is not used. Then, the steady temperature of each tile 31 is determined as the average of the temperature measurement results of the tile 31 over the predetermined period. In some cases, the first "warming-up" period may precede the calibration scan.

0040With reference to FIG. 6, the temperature profiles of four different tiles 31 are graphically shown for the calibration scan. The vertical axis corresponds to the temperature in degrees Celsius, and the horizontal axis corresponds to the time in seconds. The temperature value was obtained using the temperature sensor 88 on tile 31. During the scan, the bias voltage was fixed at the data collection level and the 200 MHz (MHz) reference clock was turned on to warm up over the first two hours. Tile 31 was temperatureed at an internal rate of 3051.8 samples per second. The measurements were downsampled to 11.9209 samples per second and an output file representing the temperature profile was stored. As can be seen from the figure, tile 31 exhibits a high degree of temperature stability.

0041Instead of the above heat generation method, the heat generator may be a die controller 38. In this case, the tile controller 36 can control the synchronization line of the die controller 38 to create a false event, as shown by the die controller 90. Knowing that the event data received from the die controller 38 is a "junk", the tile controller 36 only discards the event data received. Alternatively, this data may be discarded higher in the hierarchy of FIGS. 4A and 4B. As mentioned above, the tile controller 36 generates heat according to this method, for example using feedback from the temperature sensor 88. At this time, the tile controller 36 generates an artificial event at a ratio necessary to maintain the current value of the temperature sensor 88 at the steady state value of the temperature sensor 88 in the data acquisition mode. As another example, the tile controller 36 may generate an artificial event at a predetermined rate to maintain the steady temperature of the tile 31 in the data acquisition mode. As another example, the tile controller 36 may generate an artificial event at a steady-state rate of tile 31 in data acquisition mode. The steady state rate is the rate at which background events are detected.

0042The steady-state event rate for each die controller 38 is typically per frame period, such as 327.68 μsec, and can be determined by performing a calibration scan using the first scanner 12. It is important that the calibration scan is performed using the scanner 12 in the same condition as when imaging the subject. During the scan, the scanner 12 is placed in data acquisition mode and the event rate is measured at a predetermined rate of 11.9209 samples per second for a predetermined period of 15 minutes. The first scanner 12 is emptied and the second scanner 58 is not used. Then, the steady-state ratio of each die controller 38 is determined as the average of the steady-state ratios of the die controller 38 over the predetermined period. In some cases, the first "warming-up" period may precede the calibration scan.

0043Instead of having the tile controller 36 control the die controller 38 to generate heat, the die controller 38 may generate heat independently of the control from the tile controller 36. In this case, the die controller 38 typically gives mode instructions for the first scanner 12 from the tile controller 36, as shown for the die controller 92, or from a higher controller, as shown for the die controller 94. Receive. When in quiet mode, each die controller 38 can generate an artificial junk event at a steady-state percentage of the die controller 38 in data acquisition mode. Alternatively, the die controller 38 can perform "busy work" at a preset rate to generate sufficient heat.

0044With reference to FIG. 7, the above description is summarized by method 100 for maintaining thermal stability of the gamma detector 16 of the first scanner 12. Method 100 includes step 102 to determine if the detection sensitivity of the detector 16 is set to a level that disables the detection of gamma photons. As described above, the detection sensitivity of the detector 16 is set by adjusting the bias voltage of the SiPM32. For example, the detection sensitivity of the detector 16 is set to a low bias state before the CT scan so that the power supply of the detector 16 is not overloaded.

0045Typically, the determination 102 of whether the detection sensitivity of the detector 16 is set to a level that disables the detection of gamma photons is a mode signal indicating the mode of the first scanner 12 (ie, quiet mode or data acquisition mode). Is based on. The quiet mode corresponds to the detection sensitivity level that disables the detection of gamma photons, and the data acquisition mode corresponds to the detection sensitivity level that enables the detection of gamma photons. The detection sensitivity level that enables the detection of gamma photons is usually determined during the calibration of the detector 16. The mode signal is properly received directly or indirectly from the central control system 80 that coordinates the imaging.

0046The detection sensitivity of the detector 16 in response to determining that the detection sensitivity of the detector 16 is set to a level that disables the detection of gamma photons (ie, the detector 16 is in quiet mode). To maintain the current temperature of the detector 16 at the steady temperature of the detector 16 when is set to a level that enables detection of gamma photons (ie, the detector 16 is in data acquisition mode). Heat is generated in (step 104). Therefore, heating keeps the temperature of the detector 16 stable between the two modes. Heating may be controlled, for example, based on feedback from the temperature sensor 88, or may be controlled based on heating parameters predetermined to achieve the required heating amount. For example, the amount of heat generated is dynamically generated based on the measured temperature of the detector 16.

0047The heat can be generated by any method including the resistor heater 86 or the controllers 36, 38 of the detector 16. For example, the tile controller 36 excludes event data describing false events received from the die controller 38, either by pulsing the synchronization lines of the die controller 38, at the rate required to achieve the required heating. By doing so, a false event can be generated. As another example, the die controller 38 generates false events at the rate required to achieve the required heating, and the tile controller 36 excludes event data describing the false events received from the die controller 38. be able to. The proportion of the die controller 38 usually corresponds to the steady state proportion (ie, background factor) of event detection by the corresponding SiPM32.

0048With reference to FIG. 8, a method 150 of generating heat by generating false events is shown. Method 150 is properly performed by the tile controller 36 when in quiet mode. According to Method 150, false events are generated at a rate where the tile controller 36 uses one or more die controllers 38 to receive event data from it (step 152). The proportions may be predetermined or dynamically adjusted based on feedback from, for example, the temperature sensor 88. False events are generated, for example, by generating a pulse on the sync line of the die controller 38. Event data describing a false event is received (step 152) and then discarded (step 156). If the proportion is predetermined, the preceding step starts at generation step 152 and repeats. However, if the proportions are dynamically adjusted, additional steps are taken.

0049As explained, the proportions are dynamically adjusted. The temperature measurement result is received from, for example, the temperature sensor 88 (step 158). The measured temperature is then compared to the steady-state temperature of the tile (step 160). Based on the comparison, it is determined if there is a temperature difference (step 162). If there is a temperature difference, the proportion is increased or decreased so that the temperature of the tile is increased or decreased to a steady temperature (step 164). If there is no temperature difference, the preceding step starts at generation step 152 and repeats.

0050As used herein, memory includes any device or system that stores data, such as random access memory (RAM) or read-only memory (ROM). Further, as used herein, the processor is any input processing device or device that produces output data, such as a microprocessor, microcontroller, graphics processing unit (GPU), application specific integrated circuit (ASIC), FPGA, etc. Including devices. A controller includes any device or system that controls another device or system, and typically includes at least one processor. User input devices include any device that enables a user of a user input device, such as a mouse or keyboard, to provide input to another device or system. Display devices include any device that displays data, such as a liquid crystal display (LCD) or light emitting diode (LED) display.

0051The present invention has been described with reference to preferred embodiments. Modifications and modifications may be conceived by others by reading and understanding the above detailed description. The present invention is intended to be construed as including all such modifications and modifications, as long as they fall within the scope of the appended claims and their equivalents.