Tao Xing

A new pileup-prevention front-end electronic design for high-resolution PET and gamma cameras

A new method for processing signals from Anger position-sensitive detectors used in gamma cameras and positron emission tomography (PET) is proposed for very high count-rate imaging. It has a same concept as high yield pileup-event recover (HYPER) method we introduced before by using 1) dynamically integrating a present event, the integrating will stop immediately before the next event is detected; 2) estimating a weighted-value to indicate the total energy inside the scintillation detector; and 3) remnant correction to remove the residual energy of all the previous events from the weighted-value. This paper introduces two improved practical techniques to get a better weighted-value with low noise sensitivity in order to improve the final pileup-free energy resolution. One applies a low-pass filter combined with multiple sampling to a weight-sum of the instantaneous signal and integrated signal. The other one is weighting the integration value of the income signal; the weighting also includes exponential distortion compensation. This paper also describes the application of the HYPER electronics in a high resolution low cost PET camera with 12 photomultipliers (PMTs)-quadrant-sharing (PQS) detector modules that can decode 38 016 bismuth-germinate (BGO) crystal elements using 924 PMTs. Each detector module has four Anger-HYPER circuits to further increase the count-rate. To use the HYPER circuit in coincidence imaging applications, there is a serious synchronization problem between the arrival time of an event and the end time of integration that is variable from event to event. This synchronization problem is solved by a field programmable gate array (FPGA) circuit with real time remnant correction and a high-resolution trigger delay unit with a small dead-time for recovering the synchronization of data and the event-trigger.

A HOTLink/networked PC data acquisition and image reconstruction system for a high-resolution whole-body PET with respiratory or ECG-gated performance

An ultra high resolution PET camera in whole-body scanning or gated imaging study needs super computer-processing power for creating a huge sinogram as well as doing image reconstruction. A real-time HOTLink attached to networked cluster personal computers (PC) has been developed for this special purpose. In general, the coincidence data from a PET camera is unidirectional; therefore an additional daisy-chain bus using high speed HOTLink (400Mbit/s, Cypress Semiconductor, inc.) transmitters and receivers is designed to carry the coincidence data to networked (LAN) computers (PCs). In whole-body scanning, each PC will acquire sinogram data for one bed position, the data from HOTLink is interfaced to a PC through a fast PCI I/O board (80Mbyte/s); and after completion of data acquisition the PC begins to reconstruct the image meanwhile another PC will start data acquisition for the next bed position. The overall architecture for the image acquisition and reconstruction computing system is a pipeline design. The image result from one PC will be sent to a master computer for final tabulation and storage through the standard network, and this PC will be free for processing a new bed position. In gated respiratory or gated ECG imaging study, each PC will be reconfigured for processing a specified time-section image of a respiratory or ECG cycle. We are developing a high resolution PET camera with 38,016 BGO crystal elements which needs 1 to 2 gigabytes sinogram memory; the HOTLink/networked structure design allows us to split the huge sinogram into several PCs in real-time and the image reconstruction can be done in parallel.

Signal characteristics of individual crystals in a high resolution BGO detector design using PMT-quadrant sharing

The PMT-quadrant sharing (PQS) detector design allows very high resolution detectors to be built with 70% fewer PMTs and lower cost. A common concern for the design is that there is a big gap (photo-insensitive area) between four circular PMTs and the photoelectron signal (pulse height) may be much lower for the central crystals. The concern increases with the use of smaller PMTs for high-resolution designs because small PMTs have relatively thicker walls and relatively larger tolerance spaces between them. The authors measured the pulse heights and energy resolution for each crystal in three different types of PQS blocks for 19 mm PMT. For a square 7 /spl times/ 7 block detector (2.66 mm /spl times/ 2.66 mm /spl times/ 18 mm BGO needles), the maximum photopeak signals occurred at the corner crystal of the block. The signals for the worst central five crystals (sitting on space with no PMT connection) had pulse heights 0.87 as high as that of the corner crystals. The 12 crystals (outside the central five) with coupling only to the glass wall but not to the photocathode had a relative pulse height of 0.92. The eight crystals with partial exposure to photocathodes had a 0.94 relative pulse height. The energy resolution for individual crystals was 22% - 30% with an average of 26%. Asymmetric photopeaks, especially for the corner crystals, were observed, and these were found to be the result of the depth-of-interaction effect. In the latest PQS design, extended blocks with asymmetric light distributions were used on the four edges and four corners of a large detector module so that the previously unused (wasted) half-row of peripheral PMT could be covered by crystals. An asymmetric block, single-extended (7 /spl times/ 8 crystals) was also tested. The pulse-height ratio between the worst and best group of crystals in the single-extended block was 0.72 and that of the double-extended block was also 0.72. In a more demanding, higher spatial resolution 8 /spl times/ 8 array (2.3 mm /spl times/ 2.3 mm /spl times/ 10 mm BGO) for mouse PET with shallower crystals, the pulse-height ratio was 0.73 with an average energy resolution of 20%. This study demonstrated that pulse height uniformity for the PQS design using circular PMT was excellent, better than the typical 3/1 pulse-height ratio in conventional block detectors.

Evaluation of the effect of filter apodization for volume PET imaging using the 3-D RP algorithm

We investigated the influence of filter apodization and cutoff frequency on the image quality of volume positron emission tomography (PET) imaging using the three-dimensional reprojection (3-D RP) algorithm. An important parameter in 3-D RP and other filtered backprojection algorithms is the choice of the filter window function. In this study, the Hann, Hamming, and Butterworth low-pass window functions were investigated. For each window, a range of cutoff frequencies was considered. Projection data were acquired by scanning a uniform cylindrical phantom, a cylindrical phantom containing four small lesion phantoms having diameters of 3, 4, 5, and 6 mm and the 3-D Hoffman brain phantom. All measurements were performed using the high-resolution PET camera developed at the M.D. Anderson Cancer Center (MDAPET), University of Texas, Houston, TX. This prototype camera, which is a multiring scanner with no septa, has an intrinsic transaxial resolution of 2.8 mm. The evaluation was performed by computing the noise level in the reconstructed images of the uniform phantom and the contrast recovery of the 6-mm hot lesion in a warm background and also by visually inspecting images, especially those of the Hoffman brain phantom. For this work, we mainly studied the central slices which are less affected by the incompleteness of the 3-D data. Overall, the Butterworth window offered a better contrast-noise performance over the Hann and Hamming windows. For our high statistics data, for the Hann and Hamming apodization functions a cutoff frequency of 0.6-0.8 of the Nyquist frequency resulted in a reasonable compromise between the contrast recovery and noise level and for the Butterworth window a cutoff frequency of 0.4-0.6 of the Nyquist frequency was a reasonable choice. For the low statistics data, use of lower cutoff frequencies was more appropriate.

A modular low dead-time coincidence system for high-resolution PET cameras

A high-resolution-oncologic-transformable PET (HOTPET) is currently under development. The detector ring of the camera consists of 12 detector modules. Because the High-Yield-Pile-Up-Event-Recovery (HYPER) front-end electronics is applied in this new generation PET camera, a low dead-time coincidence circuitry is needed to take advantage of the efficiency improvement from this new technology. The coincidence matching of events coming from different detector modules is performed by an AND-logic on the arriving edges of the module output timing-pulses. A flexible modular architecture has been adopted to facilitate the use of the coincidence circuitry for different detector module configurations as well as different electronic and mechanical implementation. The application of backplane/plug-in architecture and programmable devices (EPLD/FPGA) and DSP (Digital Signal Processor) provide enough reprogrammable flexibility and expandability, ranging from animal and breast PET to whole-body PET. There are 42 possible pair combinations of modules derived from maximum 12 detector modules in coincidence with 7 opposite modules. Both the total (true + accidental) and accidental coincidences are simultaneously collected in real time; the accidental timing shift is /spl ges/200ns relative to true events. The timing-gate window for the coincidence AND-logic can be dynamically digitally adjusted during data acquisition between 6.5-16ns to optimize signal/noise in the data. The prototype circuit showed that the timing accuracy is far better than 0.5ns and the coincidence dead-time is less than 21ns.

Gantry design with accurate crystal positioning for a high-resolution transformable PET camera

A positron emission tomography (PET) camera capable of transforming its geometric configuration is being developed. This high-resolution oncologic transformable PET (HOTPET) can be modified from a large detector ring of 83 cm to a small diameter ring of 54 cm. The system consists of 12 rectangular detector modules arranged in a polygon. The detector gap between modules remains constant in both configurations because each module is rotated around its own axis and displaced radially, bringing together adjacent modules. HOTPETs detectors are highly pixilated (crystal pitch 2.6 mm), requiring accurate placement of the modules relative to each other to ensure alignment of crystals within the same detector ring. We have designed a precise detector bank holder with keyways and complementary keys built onto its sides to allow interlocking with each other to form a polygon and maintain crystal coplanarity. Consequently, we were able to design the gantry supporting the modules using wider tolerances and so reduce its construction cost. The module provides support to 77 photomultiplier tubes (PMTs), the analog front-end electronics, and an automated PMT-gain control, all enclosed within a controlled environment. Potential development of light leaks was minimized with only two parting surfaces throughout the modules box, and tortuous-path air ducts inside the walls. Internal airflow allows temperature control. Simple removal of a back cover and a motherboard gives access to any part of the electronic components or a PMT with minimal disturbance to other components.

A gain-programmable transit-time-stable and temperature-stable PMT Voltage divider

A gain-programmable, transit-time-stable, temperature-stable photomultiplier (PMT) voltage divider design is described in this paper. The signal-to-noise ratio can be increased by changing a PMT gain directly instead of adjusting the gain of the pre-amplifier. PMT gain can be changed only by adjusting the voltages for the dynodes instead of changing the total high voltage between the anode and the photo-cathode, which can cause a significant signal transit-time variation that cannot be accepted by an application with a critical timing requirement, such as positron emission tomography (PET) or time-of-flight (TOF) detection/PET. The dynode voltage can be controlled by a digital analog converter (DAC) isolated with a linear optocoupler. The optocoupler consists of an infra-red light emission diode (LED) optically coupled with two phototransistors, and one is used in a servo feedback circuit to control the LED drive current for compensating temperature characteristics. The results showed that a 6 times gain range could be achieved; the gain drift was < 0.5% over a 20/spl deg/C temperature range; 250 ps transit-time variation was measured over the entire gain range. A compact print circuit board (PCB) for the voltage divider integrated with a fixed-gain pre-amplifier has been designed and constructed. It can save about

Brain lesion detectability studies with a high resolution PET operating in no-septa and partial-septa configurations

30 per PMT channel compared with a commercial PMT voltage divider along with a variable gain amplifier. The pre-amplifier can be totally disabled, therefore in a system with large amount of PMTs, only one channel can be enabled for calibrating the PMT gain. This new PMT voltage divider design is being applied to our animal PET camera and time-of-flight/PET research.

High resolution GSO block detectors using PMT-quadrant-sharing design for small animal PET

We investigated the effect of partial-septa on the noise equivalent sensitivity and lesion detectability for a high resolution PET camera. For this purpose we used the MDAPET camera to detect small lesions in brain images obtained from the scan of the Hoffman brain phantom. The three-dimensional (3-D) positron emission tomography (PET) acquisition in comparison to two-dimensional (2-D) PET acquisition improves the sensitivity of the system at the cost of higher scatter and accidental coincidence contributions. A partial-septa allowing 3D-acquisition may provide a better alternative. For this work, three small lesion phantoms with diameters of 3, 5 and 8.6 mm were embedded into the Hoffman brain phantom. The activity concentration ratio of the lesions to the surrounding brain gray matter was ranging from 1.5 to 10. For this study, the eight detectors modules of the prototype MDAPET scanner were modified axially to extend from 38.5 mm to 131 mm in order to simulate more closely the performance of a clinical size camera. Data for the hot lesion phantoms and the normal Hoffman brain phantom were taken separately. Then, the two sets of the sinograms data were selectively combined to generate the sinograms data for the desired SUVs. Visual inspection of the lesion images show that we could clearly see the 8.6 mm lesion, with or without septa, at even the lowest activity ratio that we measured. The 5 mm and 3 min lesions were observable at activity ratio of 2.2 and 5.4, respectively. We found that even though the use of septa could increase the noise equivalent count rate and lower the image noise, it does not necessary translate into improvement of the lesion detectability. For partial-septa configurations the white matter regions of brain have less count and the brain images visually looked better; however, images from no-septa data had slightly higher contrast.

Front-end electronics based on high-yield-pileup-event-recovery method for a high resolution PET camera with PMT-quadrant-sharing detector modules

GSO position-sensitive block detectors have been developed using the PMT-Quadrant -Sharing (PQS) technology for animal PET imaging. Two prototype block detectors with dimensions of 1.51 mm/spl times/1.51 mm/spl times/10 mm (12/spl times/12 array) and 1.66 mm/spl times/1.66 mm/spl times/10 mm (11/spl times/11 array) were built using 19 mm round PMTs. The Enhanced Specular Reflector mirror-film (98% reflectance, 0.065 mm thickness, 3M Co.) was used to control the light collection and distribution in the PQS design. The detector pitches are 1.59 (12/spl times/12) and 1.74 mm (11/spl times/11) and the crystal-packing fractions 95.0 and 95.4%, respectively. List-mode measurements with Cs-137 sources were carried out to investigate the crystal decoding and analyzed to extract the individual crystal spectra. All the crystals in both the detectors were clearly identified and well isolated. The light-collection efficiency is lowest in the central crystals and highest in the crystals of the four corners, with a ratio of 0.76 for the 11/spl times/11 array. The overall energy resolution of the 11/spl times/11 array is 13.5% (15.4% at 511 keV) with a standard deviation of 1.2%, which indicates that the individual energy resolutions of the detector are high and uniformly distributed. From this study, we achieved a high decodable crystals/PMT ratio of 144:1 for the 19 mm PQS GSO detector technology with crystals 10 mm deep. It is believed that these detectors would be useful for high resolution animal PET imaging.