Greg Stortz

Design and Performance of a Resistor Multiplexing Readout Circuit for a SiPM Detector

A silicon photomultiplier (SiPM)-based positron emission tomography (PET) detector was developed using a resistor network charge division multiplexing circuit for detector readout. The detector consists of a lutetium-yttrium oxy-orthosilicate (LYSO) scintillation crystal array, an SiPM array detector (SPMArray 4, SensL Inc., Cork, Ireland) and the resistor multiplexing network implemented in a through-hole package to facilitate changing of resistor values. For purposes of optimizing the readout circuit, the LYSO array used was a 4 4 crystal array with crystal size mm on a pitch of 3.37 mm, matched to the SiPM pixel size. Flood image, energy resolution, photopeak amplitude, timing resolution, and signal time-pickoff measurements were performed using standard NIM electronics. The resistor network values were optimized through an iterative process. The performance of the detector was evaluated over a range of temperatures from 23°C to 60°C by heating the detector. The ability of the detector to resolve crystals smaller than the SiPM pixel pitch was evaluated using a dual-layer LYSO array with crystals of 1.67-mm pitch. The optimal resistor network values were found to be 100 Ω along the rows connecting the SiPM pixels and 56 Ω for the columns. For these resistor value settings, the average energy resolution for the central four crystals in the array at 23.5°C was 13.3% ± 0.3% and degraded to 16.3% ± 0.3% at 60°C. The photopeak amplitude decreased by 2%/°C, and the timing resolution degraded from 3.43 ± 0.22 ns to 4.64 ± 0.25 ns for a 350-750-keV energy window over this temperature range. The signal time-pick-off point shifted earlier by 2.7 ns as the temperature increased, an effect likely due to changes in the signal shape with temperature. The detector was able to resolve all 113 crystals in the dual-layer LYSO array. These results demonstrate that the resistor multiplexing readout circuit functions well for reading out SiPM array based detectors, which use scintillator crystal arrays much smaller than the SiPM pixel pitch. The reduced number of output signals achieved through this signal multiplexing greatly reduces the number of signal cables required. In addition, the ability of this detector to function over a wide range of temperatures offers significant flexibility in defining the system operating temperature set point.

Evaluation of High Density Pixellated Crystal Blocks With SiPM Readout as Candidates for PET/MR Detectors in a Small Animal PET Insert

Arrays of silicon photo-multipliers (SiPMs) are good candidates for the readout of detectors in PET/MR inserts due to their high packing density, efficiency, low bias voltage and insensitivity to magnetic fields. We tested two dual-layer blocks of pixellated lutetium oxy-orthosilicate (LYSO) coupled to SensL 4 × 4 SiPM arrays in terms of their ability to resolve all elements using resolvability index (RI) defined by the FWHM of the crystal response function divided by the separation. Our crystal blocks had 49 1.67 × 1.67 × 6.0 mm3 crystals on the bottom layer and 36 1.67 × 1.67 × 4.0 mm3 crystals in the top layer (offset by 1/2 of the crystal pitch). All 85 crystals were well resolved: compared with for a conventional pre-clinical PET scanners block with 40% lower crystal density. A pair of crystal blocks mounted on translation stages scanned a 0.25 mm 22Na source in 60 0.25 mm steps with the detectors angulated as if there were 16 blocks in a ring. The FWHM of the coincidence response function near the centre of the field of view was 1.31 mm and FWTM was 2.7 mm with the detectors separated by 200 mm.

Development and evaluation of a LOR-based image reconstruction with 3D system response modeling for a PET insert with dual-layer offset crystal design

In this study we present a method of 3D system response calculation for analytical computer simulation and statistical image reconstruction for a magnetic resonance imaging (MRI) compatible positron emission tomography (PET) insert system that uses a dual-layer offset (DLO) crystal design. The general analytical system response functions (SRFs) for detector geometric and inter-crystal penetration of coincident crystal pairs are derived first. We implemented a 3D ray-tracing algorithm with 4π sampling for calculating the SRFs of coincident pairs of individual DLO crystals. The determination of which detector blocks are intersected by a gamma ray is made by calculating the intersection of the ray with virtual cylinders with radii just inside the inner surface and just outside the outer-edge of each crystal layer of the detector ring. For efficient ray-tracing computation, the detector block and ray to be traced are then rotated so that the crystals are aligned along the X-axis, facilitating calculation of ray/crystal boundary intersection points. This algorithm can be applied to any system geometry using either single-layer (SL) or multi-layer array design with or without offset crystals. For effective data organization, a direct lines of response (LOR)-based indexed histogram-mode method is also presented in this work. SRF calculation is performed on-the-fly in both forward and back projection procedures during each iteration of image reconstruction, with acceleration through use of eight-fold geometric symmetry and multi-threaded parallel computation. To validate the proposed methods, we performed a series of analytical and Monte Carlo computer simulations for different system geometry and detector designs. The full-width-at-half-maximum of the numerical SRFs in both radial and tangential directions are calculated and compared for various system designs. By inspecting the sinograms obtained for different detector geometries, it can be seen that the DLO crystal design can provide better sampling density than SL or dual-layer no-offset system designs with the same total crystal length. The results of the image reconstruction with SRFs modeling for phantom studies exhibit promising image recovery capability for crystal widths of 1.27-1.43 mm and top/bottom layer lengths of 4/6 mm. In conclusion, we have developed efficient algorithms for system response modeling of our proposed PET insert with DLO crystal arrays. This provides an effective method for both 3D computer simulation and quantitative image reconstruction, and will aid in the optimization of our PET insert system with various crystal designs.

Development of a PET Scanner for Simultaneously Imaging Small Animals with MRI and PET

Recently, positron emission tomography (PET) is playing an increasingly important role in the diagnosis and staging of cancer. Combined PET and X-ray computed tomography (PET-CT) scanners are now the modality of choice in cancer treatment planning. More recently, the combination of PET and magnetic resonance imaging (MRI) is being explored in many sites. Combining PET and MRI has presented many challenges since the photo-multiplier tubes (PMT) in PET do not function in high magnetic fields, and conventional PET detectors distort MRI images. Solid state light sensors like avalanche photo-diodes (APDs) and more recently silicon photo-multipliers (SiPMs) are much less sensitive to magnetic fields thus easing the compatibility issues. This paper presents the results of a group of Canadian scientists who are developing a PET detector ring which fits inside a high field small animal MRI scanner with the goal of providing simultaneous PET and MRI images of small rodents used in pre-clinical medical research. We discuss the evolution of both the crystal blocks (which detect annihilation photons from positron decay) and the SiPM array performance in the last four years which together combine to deliver significant system performance in terms of speed, energy and timing resolution.

Performance evaluation of SensL SiPM arrays for high-resolution PET

Silicon photomultipliers (SiPMs) have high gain, excellent timing performance, and are well suited to PET/MRI applications due, in part, to their MR-compatibility and small form factor. Within the constraints of a resistor-based multiplexing circuit, it is useful to evaluate the four generations of SiPM arrays manufactured by SensL: the SPMArray4, ArraySL-4, ArraySM-4, and ArraySB-4. Breakdown voltage and dark current were measured as a function of temperature in two each of the four generations of SensL SiPM arrays. Flood histograms were created with a 68Ge-irradiated 9×9 LYSO crystal array at temperatures of 5 °C to 45 °C in 5 °C increments and overvoltages of 2 to 4 V in 0.5 V increments. Measurements of dark current vs. bias voltage increased as temperature increased, with a corresponding increase in the breakdown voltage, Vb. The temperature dependence of Vb is similar between all four generations of SiPM arrays with slopes ranging from 17.0 to 23.8 mV/°C. Notably, the ArraySB-4 has lower values for the breakdown voltage, with Vb = 24 V at 0 °C. Mean energy resolution for individual LYSO crystals showed improvements in each successive generation. The average energy resolution of the ArraySB-4 was 11.9% after correcting for non-linearity in the SiPM pixels. The linearity of the SensL SiPM arrays as a function of temperature and breakdown voltage makes them a suitable choice for a high-resolution, small animal PET/MRI system. Based on its improved resolvability and energy resolution, lower sensitivity to temperature and higher PDE, the ArraySB-4 will be used in our PET system.

Characterization of a New MR Compatible Small Animal PET Scanner Using Monte-Carlo Simulations

We are currently designing a small animal PET insert for use in an MRI with a bore size that constrains the insert inner diameter to be no larger than 66 mm while leaving 25 mm for ring thickness. The insert will be made from 10 mm thick DOI-capable Dual Layer Offset LYSO blocks coupled to MR-compatible SiPMs. The block is made from a 9 × 9 array of 1.345 × 4 mm3 crystals in the front layer and a 10 × 10 array of 1.345 × 1.345 × 6 mm3 crystals in the back layer (crystal pitch = 1.422 mm). A ring of blocks is made by repeating a block around a ring with inner diameter of 64.776 mm 16 times. Here, GATE simulations have been made to estimate mousenoise-equivalent count rate (NECR), mouse-scatter fraction (SF), peak sensitivity (Sp) resolution, and resolution uniformity to evaluate the design of our PET insert. These simulations make use of hardware performance estimates measured from a prototype block. For the one, three, and six ring tomographs, NECR curves, SF, and fígures were produced for the best and worst expected hardware performance. Simulations of a point source in a one-ring tomograph were made to estimate resolution across the field of view (FOV). For a six-ring tomograph with a 250-750 keV energy window and best expected hardware performance, the peak NECR, peak NECR activity, and Sp were 1273 kcps, 96 MBq and 10.0%. With three rings, these figures were 389 kcps at 95 MBq, and 5.9%. And with one-ring, they were 43 kcps, 85 MBq, and 2.0%. SF was ~ 16% in these three cases. Spatial resolution in the radial direction was found to change from 1.0 to 1.9 mm FWHM moving from the center of the FOV to a 15 mm offset. These results indicate that our scanner design is highly suited for high-resolution preclinical mouse imaging.

First Results From a High-Resolution Small Animal SiPM PET Insert for PET/MR Imaging at 7T

We present the initial results from a small animal PET insert designed to be operated inside a 7T MRI. The insert fits within the 114 mm inner diameter of the Bruker BGA-12S gradient coil while accommodating the Bruker 35 mm volume RF coil (outer diameter 60 mm), both used in the Bruker 70/20 MRI systems. The PET insert is a ring comprising 16 detectors. Each detector has a dual-layer offset (DLO) lutetium-yttrium oxyorthsilicate (LYSO) scintillator array read out by two SensL SPMArray4B SiPM arrays. The DLO scintillator has bottom (top) layers of: 22 × 10 (21 × 9) crystals of size 1.2 × 1.2 × 6 (4) mm3 for a total of 409 crystals per block, providing an axial extent of 28.17 mm. The detector outputs are multiplexed to four signals using a custom readout board and digitized using the OpenPET data acquisition platform. The detector flood images successfully resolve over 99% of the crystals, with average energy resolution of 12.5 ± 2.0% at 511 keV. Testing of the PET system inside the MRI showed that the PET insert had no effect on MRI image homogeneity and only a small effect on echo planar images (EPI) signal to noise ratio (SNR) (-9%), with neither PET nor MRI images showing obvious artefacts. These acquisitions used the OpenPET operating in “oscilloscope mode” with USB2.0 interface, allowing a maximum total singles event rate of 280 kcps, strongly limiting the count rate capabilities of the system. The PET radial spatial resolution (as measured with a 22Na point source and FBP-3DRP reconstruction) is 1.17 mm at the centre, degrading to 1.86 mm at a 15 mm radial offset. Simultaneous phantom and mouse PET/MR imaging produced good quality images that were free of any obvious artefacts.

A PET detector interface board and slow control system based on the Raspberry Pi

Construction of a full PET system requires scaling up from a few detectors on the bench top to dozens or even hundreds of detectors all operating simultaneously and connected to high channel count electronics. Our collaboration is building a MRI compatible PET insert system that will contain 16 detector modules in the prototype phase and 64 detector modules in the final phase. This number of detectors makes it difficult, if not impossible, to manually configure and monitor each detector in the system. In order to make possible the scaling of the system for up to 64 detectors, we require a scalable slow control system to manage the low level functions of the PET system, such as controlling detector bias and monitoring temperature and power consumption, that can be interfaced to and controlled by a host computer. We have implemented this slow control system in conjunction with a detector interface board (DIB) that is an intermediary between the detectors and the OpenPET digitizer system. Each DIB is connected to four detector modules and is controlled by a Raspberry Pi® computer directly attached to it. The Raspberry Pi® computers report to a host PC software program developed in National Instruments LabWindows™/CVI to provide the capability of central monitoring and control.

Measurement of energy and timing resolution of very highly pixellated LYSO crystal blocks with multiplexed SiPM readout for use in a small animal PET/MR insert

Arrays of silicon photo-multipliers (SiPMs) are good candidates for the readout of detectors in PET MR inserts due to their high packing density, efficiency, low bias voltage and insensitivity to magnetic fields. In this study we report the readout performance of SensL SiPM arrays in terms of their ability to resolve all elements of pixellated lutetium oxy-orthosilicate (LYSO) crystals, and their energy and timing resolution. A SensL SB4-300-35-CER SiPM array consisting of sixteen 3 × 3 mm elements were used as light sensor. An LYSO crystal block consisting of 10×10 1.2 mm × 1.2 mm × 6.0 mm crystals on the bottom layer and 9×9 1.2 mm × 1.2 mm × 4.0 mm crystals on the top layer (which is offset by ½ the crystal width) was mounted on the SensL array, and covers over 95% of its area. A DPC encoding multiplexor with HDMI cable readout made by the Triumf Instrumentation group with a footprint suitable for allowing 16 modules to form a ring inside a Brooker 7T MR rodent imager was used as a readout. The HDMI cable supplies power, and provides readout of four channels and the device temperature. The average energy resolution for all 100 crystals in the lower layer was 11.3±1.8% and 10.8±1.2% for the 81 crystals in the upper layer. The average timing resolution for all 100 crystals in the lower layer was 2.52±0.23 nsec. and 2.55±0.23 nsec. for the 81 crystals in the upper layer.

MR-compatibility of a high-resolution small animal PET insert operating inside a 7 T MRI.

A full-ring PET insert consisting of 16 PET detector modules was designed and constructed to fit within the 114 mm diameter gradient bore of a Bruker 7 T MRI. The individual detector modules contain two silicon photomultiplier (SiPM) arrays, dual-layer offset LYSO crystal arrays, and high-definition multimedia interface (HDMI) cables for both signal and power transmission. Several different RF shielding configurations were assessed prior to construction of a fully assembled PET insert using a combination of carbon fibre and copper foil for RF shielding. MR-compatibility measurements included field mapping of the static magnetic field (B 0) and the time-varying excitation field (B 1) as well as acquisitions with multiple pulse sequences: spin echo (SE), rapid imaging with refocused echoes (RARE), fast low angle shot (FLASH) gradient echo, and echo planar imaging (EPI). B 0 field maps revealed a small degradation in the mean homogeneity (+0.1 ppm) when the PET insert was installed and operating. No significant change was observed in the B 1 field maps or the image homogeneity of various MR images, with a 9% decrease in the signal-to-noise ratio (SNR) observed only in EPI images acquired with the PET insert installed and operating. PET detector flood histograms, photopeak amplitudes, and energy resolutions were unchanged in individual PET detector modules when acquired during MRI operation. There was a small baseline shift on the PET detector signals due to the switching amplifiers used to power MRI gradient pulses. This baseline shift was observable when measured with an oscilloscope and varied as a function of the gradient duty cycle, but had no noticeable effect on the performance of the PET detector modules. Compact front-end electronics and effective RF shielding led to minimal cross-interference between the PET and MRI systems. Both PET detector and MRI performance was excellent, whether operating as a standalone system or a hybrid PET/MRI.