Rocio Ramirez

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.

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.

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).

Engineering and performance (NEMA and animal) of a lower-cost higher-resolution animal PET/CT scanner using photomultiplier-quadrant-sharing detectors.

The dedicated murine PET (MuPET) scanner is a high-resolution, high-sensitivity, and low-cost preclinical PET camera designed and manufactured at our laboratory. In this article, we report its performance according to the NU 4-2008 standards of the National Electrical Manufacturers Association (NEMA). We also report the results of additional phantom and mouse studies. Methods: The MuPET scanner, which is integrated with a CT camera, is based on the photomultiplier-quadrant-sharing concept and comprises 180 blocks of 13 × 13 lutetium yttrium oxyorthosilicate crystals (1.24 × 1.4 × 9.5 mm3) and 210 low-cost 19-mm photomultipliers. The camera has 78 detector rings, with an 11.6-cm axial field of view and a ring diameter of 16.6 cm. We measured the energy resolution, scatter fraction, sensitivity, spatial resolution, and counting rate performance of the scanner. In addition, we scanned the NEMA image-quality phantom, Micro Deluxe and Ultra-Micro Hot Spot phantoms, and 2 healthy mice. Results: The system average energy resolution was 14% at 511 keV. The average spatial resolution at the center of the field of view was about 1.2 mm, improving to 0.8 mm and remaining below 1.2 mm in the central 6-cm field of view when a resolution-recovery method was used. The absolute sensitivity of the camera was 6.38% for an energy window of 350–650 keV and a coincidence timing window of 3.4 ns. The system scatter fraction was 11.9% for the NEMA mouselike phantom and 28% for the ratlike phantom. The maximum noise-equivalent counting rate was 1,100 at 57 MBq for the mouselike phantom and 352 kcps at 65 MBq for the ratlike phantom. The 1-mm fillable rod was clearly observable using the NEMA image-quality phantom. The images of the Ultra-Micro Hot Spot phantom also showed the 1-mm hot rods. In the mouse studies, both the left and right ventricle walls were clearly observable, as were the Harderian glands. Conclusion: The MuPET camera has excellent resolution, sensitivity, counting rate, and imaging performance. The data show it is a powerful scanner for preclinical animal study and pharmaceutical development.

Monte Carlo Simulation Study on the Time Resolution of a PMT-Quadrant-Sharing LSO Detector Block for Time-of-Flight PET

We developed a detailed Monte Carlo simulation method to study the time resolution of detectors for time-of-flight positron emission tomography (TOF PET). The process of gamma ray interaction in detectors, scintillation light emission and transport inside the detectors, the photoelectron generation and anode signal generation in the photomultiplier tube (PMT), and the electronics process of discriminator are simulated. We tested this simulation method using published experimental data, and found that it can generate reliable results. Using this method, we simulated the time resolution for a 13 times 13 detector block of 4 times 4 times 20 mm3 lutetium orthosilicate (LSO) crystals coupled to four 2-inch PMTs using PMT-quadrant-sharing (PQS) technology. We analyzed the effects of several factors, including the number of photoelectrons, light transport, transit time spread (TTS), and the depth of interaction (DOI). The simulation results indicated that system time resolution of 360 ps should be possible with currently available fast PMTs. This simulation method can also be used to simulate the time resolution of other detector design method.

GATE Monte Carlo Simulation of a High-Sensitivity and High-Resolution LSO-Based Small Animal PET Camera

In this paper, we describe a Monte Carlo simulation of the performance of a high-sensitivity and high-resolution small animal positron emission tomography (PET) scanner with a large axial fleld-of-view (AFOV). The simulated camera is based on the photomultiplier-quadrant-sharing (PQS) concept and composed of 180 blocks of 14 times 14 lutetium oxyorthosilicate (LSO) crystals each measuring 1.16 mm transaxially, 1.27 mm transaxially, and 9.4 mm radially. The camera has 84 detector rings with an 11.6 cm AFOV and a ring diameter of 16.6 cm. For the simulation, we used the Geant4 Application for Tomographic Emission (GATE) simulation package. We validated GATE by comparing its predictions for spatial resolution, absolute sensitivity, and count rate with measured data obtained using an existing bismuth germanate (BGO) based dedicated animal PET scanner that had a similar AFOV and ring diameter and was based on the PQS technique. Simulated and experimental images of the Data Spectrum Micro Deluxe phantom were also compared. The simulation data suggested that new LSO-based scanner could have reconstructed radial (tangential) spatial resolutions of 1.14 mm (1.14 mm), 1.31 mm (1.32 mm), 1.54 mm (1.52 mm), 2.01 mm (1.8 mm), and 2.4 mm (2.1 mm) at the center and 1 cm, 2 cm, 3 cm, and 4 cm off center, respectively. The simulation data also suggested that 1.2-mm hot rods in the Micro Deluxe phantom will be distinguishable. Simulation predicted an absolute sensitivity of about 7.3% for a point source at the center of the camera assuming an energy window of 300 keV to 750 keV, a coincidence time window of 8 ns, and a system dead time of 60 ns.

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.

Monte Carlo simulation study on the time resolution of a PMT-quadrant-sharing LSO detector block for time-of-flight PET

We developed a detailed Monte Carlo simulation method to study the time resolution of detector for time-of-flight positron emission tomography (TOF PET). The process of gamma ray reaction in detector, scintillation light emission and transport inside the detector, the photoelectron generation and anode signal generation in the photomultiplier tube (PMT), and the electronics process of discriminator are simulated. We tested this simulation method using published experimental data, and found that it can generate reliable results. Using this method, we simulated the time resolution for a 13×13 detector block of 4×4×20 mm3 lutetium orthosilicate (LSO) crystals coupled to four 2-inch PMTs using PMT-quadrant-sharing (PQS) technology. We analyzed the effects of several factors, including the number of photoelectrons, light transport, transit time spread (TTS), and the depth of interaction (DOI). The simulation results indicated that system time resolution of 300–350ps should be possible with currently available fast PMTs. This simulation method can also be used to simulate the time resolution of other detector design method.

A Lower-Cost High-Resolution LYSO Detector Development for Positron Emission Mammography (PEM)

In photomultiplier-quadrant-sharing (PQS) geometry for positron emission tomography applications, each PMT is shared by four blocks and each detector block is optically coupled to four round PMTs. Although this 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 PMTs sensitive window remains unused at the detector panel edge. Our goal was to develop a LYSO detector panel which minimizes the unused portion of the PMTs for a low-cost, high-resolution, and high-sensitivity positron emission mammography (PEM) camera. We modified the 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 packing fraction was 96.3% for asymmetric block and 95.5% for symmetric block. Both of the blocks have excellent decoding capability with all crystals clearly identified, 156 for asymmetric and 144 for symmetric and peak-to-valley ratio of 3.0 and 2.3 respectively. The average energy resolution was 14.2% for the asymmetric block and 13.1% for the symmetric block. Using a modified PQS geometry and asymmetric block design, we reduced the unused PMT region at detector panel edges, thereby increased the field-of-view and the overall detection sensitivity and minimized the undetected breast region near the chest wall. This detector design and using regular round PMT allowed building a lower-cost, high-resolution and high-sensitivity PEM camera.