Hossain Baghaei

Basic imaging performance characteristics of a variable field of view PET camera using quadrant sharing detector design

The basic imaging performance of the prototype PET camera (MDAPET) developed in our laboratory was measured. The MDAPET was developed to test the engineering feasibility of two design concepts: (a) the photomultiplier-quadrant-sharing detector design and (b) a lower cost variable detector ring which forms a densely-packed smaller ring for imaging breast/brain/animals, and a less-packed ring for whole-body imaging. The basic imaging performance was measured for the 32 and 55 cm FOV with tests similar to those described in the SNM/NEMA performance standard. For the 32 cm FOV, the transaxial image resolutions at 0, 5, 10 cm were found to be 2.8, 3.1, 4.0 mm, respectively, while for the 55 cm FOV, the image resolutions were 2.9, 3.1, 3.6 mm. The axial resolution ranged from 2.5 to 4.4 mm as a function of detector-ring position. For a 21.5-cm uniform phantom, the coincidence-detection sensitivity was 82.3 kcps//spl mu/Ci/cc at the smallest 32 cm field of view and 16.3 kcps//spl mu/Ci/cc for the whole body configuration. The scatter fractions were 19% and 30% for the whole-body mode and the brain/breast mode, respectively. The smaller mode has 2.4 times higher noise-equivalent-sensitivity than the larger mode at 1 /spl mu/Ci/cc. High quality, artifact-free brain-phantom images were obtained in both modes.

Electronics for a prototype variable field of view PET camera using the PMT-quadrant-sharing detector array

Electronics for a prototype high-resolution PET camera with eight position-sensitive detector modules has been developed. Each module has 16 BGO (Bi/sub 4/Ge/sub 3/O/sub 12/) blocks (each block is composed of 49 crystals). The design goals are component and space reduction. The electronics is composed of five parts: front-end analog processing, digital position decoding, fast timing, coincidence processing and master data acquisition. The front-end analog circuit is a zone-based structure (each zone has 3/spl times/3 PMTs). Nine ADCs digitize integration signals of an active zone identified by eight trigger clusters; each cluster is composed of six photomultiplier tubes (PMTs). A trigger corresponding to a gamma ray is sent to a fast timing board to obtain a time-mark, and the nine digitized signals are passed to the position decoding board, where a real block (four PMTs) can be picked out from the zone for position decoding. Lookup tables are used for energy discrimination and to identify the gamma-hit crystal location. The coincidence board opens a 70-ns initial timing window, followed by two 20-ns true/accidental time-mark lookup table windows. The data output from the coincidence board can be acquired either in sinogram mode or in list mode with a Motorola/IRONICS VME-based system.

Ultrahigh-Resolution L(Y)SO Detectors Using PMT-Quadrant-Sharing for Human & Animal PET Cameras

The goal of this study is to develop lower-cost ultrahigh resolution detectors for PET systems, using the PMT-quadrant-sharing (PQS) decoding technology on L(Y)SO scintillation crystals. For this work, L(Y)SO PQS block detectors for both animal and human PET cameras were developed and studied. Both simulation and experimental detector design studies were carried out to achieve efficient light distribution and crystal decoding. The effects of crystal finishes and reflector patterns on light distribution, light output and energy resolution were investigated and used to derive the highest resolution PQS-L(YSO) block-detector. The PQS-L(Y)SO detector performance was measured on the best performing blocks. For performance evaluation, list-mode data from the detectors were acquired and analyzed to extract light-collection efficiency, energy-resolution distribution, and pulse height distribution for individual crystals. The potential PET imaging resolution performance was investigated using Monte Carlo simulation studies with the GEANT4/GATE software for both detectors developed for small animal PET and human PET applications. From these studies, we have the following findings: 1) For light distribution studies on the crystal surface finish, 4 mum lapping was found to be the preferred finish for achieving the best position decoding together with good overall light output for all the crystals in both, the human and animal detector arrays; 2) Intricate reflector patterns between crystals can be made from the ESR mirror film (3M Inc.) for optimally controlling the light sharing between crystals and to the four decoding PMTs, with high packing fractions on the PQS-blocks; 3) For the PQS-LYSO detector block for animal PET systems, using 19-mm circular photomultipliers (PMT), we achieved decoding a 14 times 14 arrays with a crystal pitch of 1.27 times 1.27 mm2. This animal detector has a packing fraction of 95.6%, an energy resolution ranging between 12.9%~15.8% for individual crystals (average energy resolution of 14%), the pulse height for the least favorable crystal is 63.5% of the most favorable crystal; 4) For PQS-LSO detector block for human PET systems using very large circular 51-mm PMT, we achieved decoding a 15 times 15 array with a crystal pitch of 3.25 times 3.25 mm2. The human PET detector has packing fraction of 98.2%, an energy resolution range 12.9%~15.8% (average energy resolution 14%). The pulse height of least favorable crystals is 80% of the most favorable crystal. 5) From Monte Carlo simulations for LSO small animal PET, a spatial resolution of 1.1-1.2 mm may potentially be achieved using low cost 19-mm circular PMT. For human PET systems, 3-mm spatial resolution may potentially be achieved using very large 51-mm circular PMT for cost reduction.We developed high resolution L(Y)SO detectors for human and animal PET applications using Photomulti- plier-quadrant-sharing (PQS) technology. The crystal sizes were 1.27 times 1.27 times 10 mm3 for the animal PQS-blocks and 3.25 times 3.25 times 20 mm3 for human ones. Polymer mirror film patterns (PMR) were placed between crystals as reflector. The blocks were assembled together using optical grease and wrapped by Teflon tape. The blocks were coupled to regular round PMTs of 19/51 mm in PQS configuration. List-mode data of Ga-68 source (511 keV) were acquired with our high yield pileup-event recovery (HYPER) electronics and data acquisition software. The high voltage bias was 1100 V. Crystal decoding maps and individual crystal energy resolutions were extracted from the data. To investigate the potential imaging resolution of the PET cameras with these blocks, we used GATE (Geant4 Application for Tomographic Emission) simulation package. GATE is a GEANT4 based software toolkit for realistic simulation of PET and SPECT systems. The packing fractions of these blocks were found to be 95.6% and 98.2%. From the decoding maps, all 196 and 225 crystals were clearly identified. The average energy resolutions were 14.1% and 15.6%. For small animal PET systems, the detector ring diameter was 16.5 cm with an axial field of view (AFOV) of 11.8 cm. The simulation data suggests that a reconstructed radial (tangential) spatial resolution of 1.24 (1.25) mm near the center is potentially achievable. For the whole-body human PET systems, the detector ring diameter was 86 cm. The simulation data suggests that a reconstructed radial (tangential) spatial resolution of 3.09(3.38) mm near the center is potentially achievable. From this study we can conclude that the PQS design could achieve high spatial resolutions and excellent energy resolutions on human and animal PET systems with substantially lower production costs and inexpensive readout devices.

Front end electronics for a variable field PET camera using the PMT-quadrant-sharing detector array design

This is an electronic design for a PET camera with 8 arrays of position sensitive detectors. Each array has 16 BGO blocks (each block is composed of 49 crystals). The design goals are component and space reduction. Each PMT has its fast amplifier and triggered integrator. Nine flash ADC serve an array of 27 PMT and 784 crystals. The fast signals from one or more PMT clusters are summed. The fast summed signals are thresholded and sent to a programmable logic to determines the probable scintillation zone (4 blocks) and to turn on the associated 9 PMT integrators. After integration, the logic also drives a set of ultra-fast analog switches to connect the flash ADC set to the PMT integration channels in the scintillation zone. The 9 ADC output (integrated pulses with good photon statistics) are latched and compared. The comparator result determines which block in the zone is firing and the appropriate 4 ADC are selected and bused into a position decoder. The decoder outputs the image-slice address and the crystal address. The circuit is pipelined and driven by a 250 MHz master clock. The maximum event-processing rate for the circuit and a random radiation source is 2/spl times/10/sup 6//sec (with 10:1 randomness allowance).

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.

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.

Breast cancer imaging studies with a variable field of view PET camera

This is a study of the FDG breast career imaging capability of the prototype high resolution multi-ring variable field of view PET scanner developed at the University of Texas MD Anderson Cancer Center (MDAPET). For the primary-lesion-imaging test, four small phantom lesions embedded into two different size cylindrical breast phantoms were imaged using the breast-mode of the camera. Data for the hot lesion phantoms and the warm background were taken separately. Then, before image reconstruction, two sets of data were selectively combined to generate the data for the desired SUV. The cylindrical lesions have diameter and height of 3-6 mm with standard uptake value (SUV) ranging from 2.3 to 15. For the axillary-lesion imaging test, four small hot lesion phantoms taped to the axillary area of a torso phantom were imaged using a whole-body mode of the camera. To create a realistic background a 68-kg volunteer injected with 3.5 mCi of FDG were also imaged. The two sets of data were selectively combined to generate the sinogram data for the desired SUV. The lesions were 4-11.5 mm with SUV ranging from 2.5 to 17. The primary breast lesion imaging results show that a 3 mm lesion can be detected in both the small (/spl phi/=11.5 cm) and large (/spl phi/=16 cm) breast phantoms when SUV is greater than 2.5. In the whole body mode, axillary lesions of 6 mm can be detected for lesion SUV of 5 or more.

A new statistics-based online baseline restorer (SOBLR) for a high count-rate fully digital system

The goal of this work is to develop a novel, accurate, real-time digital baseline restorer using online statistical processing for a high count-rate digital system such as positron emission tomography (PET). In high count-rate nuclear instrumentation applications, analog signals are DC-coupled for better performance. However, the detectors, pre-amplifiers and other front-end electronics would cause a signal baseline drift in a DC-coupling system, which will degrade the performance of energy resolution and positioning accuracy. Event pileups normally exist in a high-count rate system and the baseline drift will create errors in the event pileup-correction. Hence, a baseline restorer (BLR) is required in a high count-rate system to remove the DC drift ahead of the pileup correction. Many methods have been reported for BLR from classic analog methods to digital filter solutions. However a single channel BLR with analog method can only work under 500 kcps count-rate, and normally an analog front-end application-specific integrated circuits (ASIC) is required for the application involved hundreds BLR such as a PET camera. We have developed a simple statistics-based online baseline restorer (SOBLR) for a high count-rate fully digital system. In this method, we acquire additional samples, excluding the real gamma pulses, from the existing free-running ADC in the digital system, and perform online statistical processing to generate a baseline value. This baseline value will be subtracted from the digitized waveform to retrieve its original pulse with zero-baseline drift. This method can self-track the baseline without a micro-controller involved. The circuit consists of two digital counter/timers, one comparator, one register and one subtraction unit. Simulation shows a single channel works at 30 Mcps count-rate with pileup condition. 336 baseline restorer circuits have been implemented into 12 field-programmable-gate-arrays (FPGA) for our new fully digital PET system.

The initial design and feasibility study of an affordable high-resolution 100-cm long PET

This is a design and feasibility study of an affordable high-resolution 100 cm long PET covering the entire body (EB-PET) for imaging head-&-torso in one fixed bed position. Our design studies show that EB-PET may image the entire body in 2-4 minutes with a low 2.5 mCi FDG dose. The high patient throughput may lower the cost of wholebody imaging and the low dose would allow more frequent cancer-management monitoring. EB-PET can capture dynamic wholebody time-activity images and arterial (cardiac) input function concurrently to yield quantitative metabolic images for the wholebody to improve diagnosis and to measure wholebody systemic side effects of therapy. Dynamic imaging using EB-PET may also unshackle wholebody PET imaging from the static FDG-type of tracers required by current PET to new classes of more dynamic tracers. The EB-PET detection system is based on the latest generation of the low-cost BGO detector prototypes developed in our laboratory which can decode 121 BGO crystals per PMT (39 mm diameter), thereby enabling this very large system to use only 1768 PMT for its 205,700 high resolution crystals (3.5 x 3.5 x 20 mm). The system resolution and NES characteristics were also calculated with Monte Carlo (MC) simulations (GATE/GEANT) for point sources, NEMA NES phantom and wholebody Turkington phantoms. Prototype detectors achieved a 15% energy resolution and clearly decoded 3.5 x 3.5 mm detectors. With such data, MC simulations show that the central transaxial image resolution is 3.2 mm (4.4 mm) for 5 cross-ring coincidences (274 cross-ring coincidences), while at 10 cm transaxial radius, the image resolution is 4.2 mm (5.1 mm).