Daryl Bishop

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.

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.

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.

Performance of a PET Insert for High-Resolution Small-Animal PET/MRI at 7 Tesla

We characterize a compact MR-compatible PET insert for simultaneous preclinical PET/MRI. Although specifically designed with the strict size constraint to fit inside the 114-mm inner diameter of the BGA-12S gradient coil used in the BioSpec 70/20 and 94/20 series of small-animal MRI systems, the insert can easily be installed in any appropriate MRI scanner or used as a stand-alone PET system. Methods: The insert consists of a ring of 16 detector-blocks each made from depth-of-interaction–capable dual-layer-offset arrays of cerium-doped lutetium-yttrium oxyorthosilicate crystals read out by silicon photomultiplier arrays. Scintillator crystal arrays are made from 22 × 10 and 21 × 9 crystals in the bottom and top layers, respectively, with respective layer thicknesses of 6 and 4 mm, arranged with a 1.27-mm pitch, resulting in a useable field of view 28 mm long and about 55 mm wide. Results: Spatial resolution ranged from 1.17 to 1.86 mm full width at half maximum in the radial direction from a radial offset of 0–15 mm. With a 300- to 800-keV energy window, peak sensitivity was 2.2% and noise-equivalent count rate from a mouse-sized phantom at 3.7 MBq was 11.1 kcps and peaked at 20.8 kcps at 14.5 MBq. Phantom imaging showed that features as small as 0.7 mm could be resolved. 18F-FDG PET/MR images of mouse and rat brains showed no signs of intermodality interference and could excellently resolve substructures within the brain. Conclusion: Because of excellent spatial resolvability and lack of intermodality interference, this PET insert will serve as a useful tool for preclinical PET/MR.

Pixelated Geiger-Mode Avalanche Photo-Diode Characterization Through Dark Current Measurement

PIXELATED Geiger-mode avalanche photodiodes (PPDs), often called silicon photomultipliers (SiPMs) are emerging as an excellent replacement for traditional photomultiplier tubes (PMTs) in a variety of detectors, especially those for subatomic physics experiments, which requires extensive test and operation procedures in order to achieve uniform responses from all the devices. In this paper, we show for two PPD brands, Hamamatsu MPPC and SensL SPM, that at room temperature, the dark noise rate, breakdown voltage and rate of correlated avalanches can be inferred from the sole measure of dark current as a function of operating voltage, hence greatly simplifying the characterization procedure. We introduce a custom electronics system that allows measurement for many devices concurrently, hence allowing rapid testing and monitoring of many devices at low cost. Finally, we show that the dark current of Hamamastu Multi-Pixel Photon Counter (MPPC) is rather independent of temperature at constant operating voltage, hence the current measure cannot be used to probe temperature variations. On the other hand, the MPPC current can be used to monitor light source conditions in DC mode without requiring strong temperature stability, as long as the integrated source brightness is comparable to the dark noise rate.

DEAP trigger and readout electronics

The DEAP experiments aim is to detect Weakly Massive Particles through their interaction in liquid argon. It is a ball of liquid Argon surrounded by 255 Photo-multiplier tubes. As such it is a relatively small scale experiment; the challenge for the trigger and data acquisition electronics is to find the few possible interesting events occurring at a rate of up to 1 to 100 per year among a 3.6 kHz background due to the beta decay of 39 Ar isotopes. Electron recoil due to beta decay or gamma interaction can be very efficiently rejected by measuring the fraction of scintillation light emitted in the first 100 to 200 ns over the total scintillation light emitted over several Jls. The electron recoils are expected to be suppressed by a factor of 109 to 1010 with such pulse shape discrimination technique. The core of DEAP electronics is a set of 32 commercial CAEN VI720 digitizers that enable the identification of single photo-electron pulses. The readout speed of the digitiser is limited to about 1500 Hz due to the way the zero suppression is performed in CAEN proprietary firmware. A trigger system was hence implemented to discard 50% to 90% of the events. The trigger decision is made by analyzing 22 waveforms, which are the analog sums of the signals from 12 PMTs. Such a trigger system allows continuous monitoring of the data used to make the trigger decision and it provides a flexible way to alter the trigger condition should a background source happen to be larger than expected. Construction of the DEAP electronics system is nearing completion.

Design of a Radial TPC for Antihydrogen Gravity Measurement with ALPHA-g

The gravitational interaction of antimatter and matter has never been directly probed. ALPHA-g is a novel experiment that aims to perform the first measurement of the antihydrogen gravitational mass. A fundamental requirement for this new apparatus is a position sensitive particle detector around the antihydrogen trap which provides information about antihydrogen annihilation location. The proposed detector is a radial Time Projection Chamber, or \textit{rTPC}, whose concept is being developed at TRIUMF. A simulation of the detector and the development of the reconstruction software, used to determine the antihydrogen annihilation point, is presented alongside with the expected performance of the rTPC.

Data Acquisition for a Preclinical MR Compatible PET Insert Using the OpenPET Platform

OpenPET has recently been developed as a modular, flexible data acquisition (DAQ) platform for nuclear imaging applications. We present a description of the system architecture and DAQ implementation using OpenPET for a small animal positron emission tomography (PET) insert designed for hybrid PET/magnetic resonance imaging (MRI) at 7T. The PET insert consists of 16 silicon photomultiplier-based detector modules, creating a total of 64 analog channels. The OpenPET system used consisted of four 16 channel detector boards connected to one support board (SB), in turn connected to a Host PC via a USB2.0 interface. Both detector and SBs contain field-programmable gate arrays, allowing customized firmware for both. The initial OpenPET firmware release allowed data capture in oscilloscope mode only, limiting data collection to a system single events rate of 18.8 kcps. Customized firmware changes were implemented in four revisions, resulting in a final version using an 8 byte data packet and supporting a system single events rate of > 4.5 Mcps. Noise equivalent count rate (NECR) results are presented for each firmware revision, showing improvement from peak NECR of 1.0 kcps @ 0.6 MBq to 19.5 kcps @ 15.3 MBq. The final version of the firmware enables its routine use in small animal PET/MRI.