Wai-Hoi Wong

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.

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).

A high speed position-decoding electronics for BGO block detectors in PET

A high-speed prototype electronics for positioning gamma event at EGO block detectors in PET has been developed. It adapts the high-yield pileup-event recovery method (HYPER) proposed by the authors for nonimaging probes and gamma camera. HYPER uses recurring residual subtractions for multiple/continuous pileup conditions. The regular Anger positioning algorithm was modified by the HYPER method. The HYPER-Anger positioning circuit was designed for individual EGO blocks or block arrays and tested on a EGO block (2/spl times/2 cm with 7/spl times/7 elements on 4 PMT). The block was well decoded even at 800 kcps, which is 6-7/spl times/ higher than the typical maximum rate of 130 kcps for EGO blocks. The position-decoding electronics may be used to increase the maximum imaging-rate of regular EGO PET or to compensate for the lower imaging rate of the PMT-quadrant-sharing detector design, which doubles spatial resolution in both transaxial and axial directions.

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.

A lower-cost high-resolution LYSO detector development for positron emission mammography (PEM)

We have developed several positron emission tomography (PET) cameras using photomultiplier-quadrant-sharing (PQS) geometry; in which each detector block is optically coupled to four round PMTs, and each PMT is shared by four blocks. Although PQS design reduces the cost of high-resolution PET systems, when the camera consists of detector panels that are made up of square blocks, half of the PMT’s sensitive window remains unused at the detector panel edge. Our goal was to develop a LYSO detector panel, which minimize the unused portion of the PMTs while maintaining the low cost, high resolution, and high sensitivity of positron emission mammography (PEM) camera. Our plan was to modify PQS design by using elongated blocks at panel edges and square blocks in the inner area. For elongated blocks, symmetric and asymmetrical reflector patterns were developed, and PQS and PMT-half-sharing (PHS) arrangements were implemented in order to obtain a suitable decoding. The performance of our blocks was good, producing good crystal-decoding and average energy resolution. Using a modified PQS geometry and asymmetric block design, we reclaimed the unused PMT region at detector panel edges, thereby increasing field-of-view and overall detection sensitivity and minimizing undetected breast region near the chest wall. This lower cost design using regular round PMT allowed us to use larger detector panels and hence to build a lower-cost, high-resolution, high-sensitivity PEM camera.

Compensation of missing projection data for MDAPET camera

The MDAPET camera is a prototype multiring PET scanner with variable transaxial field of view (FOV) that allows for the resizing of the patient opening for optimizing the detection sensitivity for different body cross-sections. This variable FOV feature introduces gaps between the modules: which cause gaps in the sinograms. To fill the gaps, the camera is rotated during data acquisition. The authors have studied the possibility of not rotating the gantry in the brain mode, where the gaps are smallest, and estimating the missing data. For three-dimensional (3D) data, three methods of compensation were evaluated. Without any compensation, artifacts dominated the images. In the first method, the missing data were estimated using linear interpolation in the angular direction of the sinogram. In the second method, the missing data for each pixel was estimated using the average value of several pixels in the angular direction. In the third method, the missing data were first estimated using the averaging or linear interpolation method to construct preliminary images. These images were then used to create new projections and by appropriate weighting these new data were used to fill the gaps in the original data. The authors also evaluated the effect of rebinning the 3D sinograms into two-dimensional (2D) sinograms and then performing a 2D image reconstruction. All the methods produced almost artifact-free images. But none were able to match the quality of the images reconstructed from the 3D data that were acquired with gantry rotation.

An improved quadrant-sharing BGO detector for a low cost rodent-research PET (RRPET)

This is a study to improve the detector engineering of a low-cost high-sensitivity rodent-research PET camera (RRPET). The detector system is a solid ring of BGO made of tapered-pentagon blocks, each with 8/spl times/8 crystals and an average crystal pitch of 2.0 mm. The detector system used a variation of the photomultiplier-quadrant-sharing design (PQS). The first version of the RRPET blocks used white-paint reflectors of different shapes/sizes to control light distribution to the 4 decoding PMT. In this improved version, the white paint was replaced by a new ESR-mirror film. The film is 0.06 mm thick and mechanically robust. The construction provides a very high crystal-packing fraction (96% linearly) with a gap of only 0.08 mm between crystals. The film construction is more suitable for the slab-sandwich-slice construction we developed to make PQS blocks as the film reduces construction time. Average light output increased by 32% compared to the painted version. The light-output ratio (signal uniformity) between the worst crystal group (in the gap between 4 round PMTs) and the best crystal group (near the middle of a PMT) improved from 53% from 63%. The crystal-decoding resolution has also improved. The detection-efficiency uniformity has improved as well. The energy resolution for individual crystals has improved from 24% to 19% for the corner crystals and from 30% to 22% for the central crystals. The film also has better mechanical precision than paint, thereby positioning the small crystals more accurately relative to each other.

A simulation study on optically decoding reflecting windows for PMT quadrant sharing scintillation detector block

A large number of decodable crystals per photomultiplier tube (PMT) can be achieved by using the PMT-quadrant-sharing (PQS) technique with proper optically reflecting windows to channel light distribution in scintillation detector block. However, to develop brand new optically decoding reflecting windows for a detector block with different crystal material, PMT size or decoding resolution, is still very time-consuming and also requires much experience. This study is to develop a computer software tool that can simulate an expected two-dimensional (2-D) crystal decoding map before implementing a real detector block with a new set of decoding reflectors. After comparing the experimental decoding data to the simulated results with the same reflector set on a block, data are feed to adjust the software parameters. More accurate decoding reflectors will then be created by the adjusted parameters. Our result shows the decoding simulation of a 10times10 BGO block can be finished within a few minutes, which is much faster than using the Monte Carlo simulation with DETECT. This software has been evaluated by five different PQS blocks. Our preliminary study shows this simulation tool is very promising which can significantly reduce a new product developing time; only about two-three development cycles are needed to get to the final optimized decoding reflectors

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 comparison of BGO, GSO, MLS, LGSO, LYSO and LSO scintillation materials for high-spatial-resolution animal PET detectors

We are developing very-high resolution detector blocks for animal PET applications. We studied different scintillating crystals materials, BGO, GSO, LSO, MLS, LGSO, and LYSO, to determine the most suitable material for a low cost and high-resolution detector. In this study, we measured and evaluated two different light-output data from individual crystal samples (two different sizes of needles): the scintillation light output received by the PMT as used in PET (LOPET) and the intrinsic light output (ILO). The ILO data, were measured with the largest side of crystal needles coupled to the PMT (crystal needles lying down onto the PMT), the LOPET data, were measured with the smallest crystal end coupled to the PMT as used in a PET detector (the crystal is standing up on the PMT). The pulse-height spectra for both ILO and LOPET were acquired for all the individual crystal samples, for deducing the percentages of light loss from self-absorption (LLSA) in the PET detector configuration, and energy resolution in both positions (ILO-ER and LOPET-ER). With these crystals, four detectors blocks were also developed, and we measured the overall light output and the position-decoding maps to gauge the decoding capability (DC) of the crystals. We also compared the visual color and clarity of individual crystals and detector blocks. For the two different sizes of the crystal samples that we received, we have the following finding: (a) For the 1.3times1.3times10 mm3 crystals standing up as used in a PET camera, GSO, MLS and LYSO, lost almost half of the light while LSO lost more than 2/3 thereby ended up with the same light output as GSO. The energy resolution of LSO and GSO are quite similar (16% and 15%); but the light absorption is significant higher in LSO (71%) in comparison to GSO (54%), MLS (52%) and LYSO (48%). (b) For the 2times2times10 mm3, the light loss from self-absorption (LLSA) was less than the 1.3 mm samples. MLS has the lowest self-absorption (26%), while GSO and LSO have the highest light loss. LGSO and LSO have the lowest LOPET for all the lutetium crystals tested. (c) From the position-decoding results from the detector blocks, LYSO, MLS and GSO provided better position-decoding resolution than the LSO. (d) For the visually comparison, LSO were visually much darker than the other lutetium crystals