Jean-Francois Pratte

Small animal simultaneous PET/MRI: initial experiences in a 9.4 T microMRI

We developed a non-magnetic positron-emission tomography (PET) device based on the rat conscious animal PET that operates in a small-animal magnetic resonance imaging (MRI) scanner, thereby enabling us to carry out simultaneous PET/MRI studies. The PET detector comprises 12 detector blocks, each being a 4 × 8 array of lutetium oxyorthosilicate crystals (2.22 × 2.22 × 5 mm(3)) coupled to a matching non-magnetic avalanche photodiode array. The detector blocks, housed in a plastic case, form a 38 mm inner diameter ring with an 18 mm axial extent. Custom-built MRI coils fit inside the positron-emission tomography (PET) device, operating in transceiver mode. The PET insert is integrated with a Bruker 9.4 T 210 mm clear-bore diameter MRI scanner. We acquired simultaneous PET/MR images of phantoms, of in vivo rat brain, and of cardiac-gated mouse heart using [(11)C]raclopride and 2-deoxy-2-[(18)F]fluoro-D-glucose PET radiotracers. There was minor interference between the PET electronics and the MRI during simultaneous operation, and small effects on the signal-to-noise ratio in the MR images in the presence of the PET, but no noticeable visual artifacts. Gradient echo and high-duty-cycle spin echo radio frequency (RF) pulses resulted in a 7% and a 28% loss in PET counts, respectively, due to high PET counts during the RF pulses that had to be gated out. The calibration of the activity concentration of PET data during MR pulsing is reproducible within less than 6%. Our initial results demonstrate the feasibility of performing simultaneous PET and MRI studies in adult rats and mice using the same PET insert in a small-bore 9.4 T MRI.

Architecture of a dual-modality, high-resolution, fully digital positron emission tomography/computed tomography (PET/CT) scanner for small animal imaging

Contemporary positron emission tomography (PET) scanners are commonly implemented with very large scale integration analog front-end electronics to reduce power consumption, space, noise, and cost. Analog processing yields excellent results in dedicated applications, but offers little flexibility for sophisticated signal processing or for more accurate measurements with newer, fast scintillation crystals. Design goals of the new Sherbrooke PET/computed tomography (CT) scanner are: 1) to achieve 1 mm resolution in both emission (PET) and transmission (CT) imaging using the same detector channels; 2) to be able to count and discriminate individual X-ray photons in CT mode. These requirements can be better met by sampling the analog signal from each individual detector channel as early as possible, using off-the-shelf, 8-b, 100-MHz, high-speed analog-to-digital converters (ADC) and digital processing in field programmable gate arrays (FPGAs). The core of the processing units consists of Xilinx SpartanIIe that can hold up to 16 individual channels. The initial architecture is designed for 1024 channels, but modularity allows extending the system up to 10 K channels or more. This parallel architecture supports count rates in excess of a million hits/s/scintillator in CT mode and up to 100 K events/s/scintillator in PET mode, with a coincidence time window of less than 10 ns full-width at half-maximum.

Digital Coincidence Processing for the RatCAP Conscious Rat Brain PET Scanner

The RatCAP has been designed and constructed to image the awake rat brain. In order to maximize system performance, offline digital coincidence data processing algorithms including offset delay correction and prompt and delayed coincidence detection have been developed and validated. With offset delay correction using a singular value decomposition (SVD) technique, overall time resolution was improved from 32.6 to 17.6 ns FWHM. The experimental results confirm that the ratio of prompts to randoms was improved because a narrower timing window could be used. 18F-fluoride rat bone scan data were reconstructed using our fully 3-D ML-EM algorithm with a highly accurate detector response model created from Monte Carlo simulation.

Results from prototype II of the BNL simultaneous PET-MRI dedicated breast scanner

At Brookhaven National Laboratory, we are developing a simultaneous PET-MRI breast imaging system. A prototype II version of the PET system has been constructed. This device consists of 24 detector blocks where each block consists of a 4 × 8 array of 2.2 × 2.2 × 15 mm3 LYSO crystal directly coupled to a 4 × 8 non-magnetic APD array. The scanner has an inner diameter of 100mm and an axial extent of 18mm. Resolution measurements were carried out for the prototype system to evaluate the depth of interaction effects. Average resolution less than 2mm FWHM was maintained throughout the field of view. The prototype PET system was operated unshielded inside the RF coil of the Aurora 1.5 T dedicated breast MRI machine. Artifact free MRI images with good SNR were obtained.

The RatCAP front-end ASIC

We report on the design and characterization of a new ASIC for the RatCAP, a head-mounted miniature PET scanner intended for neurological and behavioral studies of an awake rat. The ASIC is composed of 32 channels, each consisting of a charge sensitive preamplifier, a 5-bit programmable gain in the pole-zero network, a 3rd order bipolar semi-Gaussian shaper (peaking time of 80 ns), and a timing and energy discriminator. The energy discriminator in each channel is used to arm the zero-crossing discriminator and can be programmed to use either a low energy threshold or an energy gating window. A 32-to-1 serial encoder is embedded to multiplex into a single output the timing information and channel address of every event. Finally, LVDS I/O were integrated on chip to minimize the digital noise on the read-out PCB. The ASIC was realized in the TSMC 0.18 mum technology, has a size of 3.3 mm times 4.5 mm and a power consumption of 117 mW. The gate length of the N-channel MOSFET input device of the charge sensitive preamplifier was increased to minimize 1/f noise. This led to a factor 1.5 improvement of the ENC with respect to the first version of the ASIC. An ENC of 650 e-rms was measured with the APD biased at the input. In order to predict the achievable timing resolution, a model was derived to estimate the photon noise contribution to the timing resolution. Measurements were performed to validate the model, which agreed within 12%. The coincidence timing resolution between two typical LSO-APD-ASIC modules was measured using a 68Ge source. Applying a threshold at 420 keV, a timing resolution of 6.7 ns FWHM was measured. An energy resolution of 18.7% FWHM at 511 keV was measured for a 68Ge source.

Development of a single photon avalanche diode (SPAD) array in high voltage CMOS 0.8 µm dedicated to a 3D integrated circuit (3DIC)

We present the realization of Single Photon Avalanche Diode (SPA D) arrays for Positron Emission Tomography (PET). These SPAD arrays are designed in Teledyne BALSA High Voltage (HV) CMOS technology targeted for a 3 dimensional (3D) heterogeneous integration with deep-submicron CMOS readout electronics to realize a 3D Single Photon Counting Module (3DSPCM). We are developing a post-process Through Silicon Vias (TSV) technology at Universite de Sherbrooke to implement the 3D heterogeneous bonding. The 3D integration of SPAD and electronics reduces the SPAD interconnect parasitic capacitance, greatly increases the photosensitive area and improves overall performances. Also, SPAD are known for their excellent timing performance which enables Time of Flight capabilities in PET, significantly improving image contrast and spatial resolution. For this purpose, we designed, fabricated and characterized SPAD arrays with active quenching circuit using the HV CMOS 0.8 μm technology. The chosen structure is a p+ anode in an n-well using a p-well isolation layer to achieve 54 % of fill factor and takes full advantage of the 3D integration scheme. SPAD evaluation results show a Photodetection Efficiency (PDE) up to 49 % at 480 nm, Dark Count Rate (DCR) of 50 kcps, afterpulsing probability <;2 %, crosstalk probability <;1 %, and timing jitter of 100 ps at room temperature.

Next Generation of Real Time Data Acquisition, Calibration and Control System for the RatCAP Scanner

RatCAP (Rat Conscious Animal PET) is miniature positron emission tomography scanner intended for neurological and behavioral study of small awake animal. The RatCAP system comprises of three distinct modules: rigid-flex technology based Printed Circuit Board (PCB) which houses the detector components and front end Application Specific Integrated Circuit (ASIC), Time to Digital Converter and Signal Processing module (TSPM) which receives and processes ASIC signals and transmits processed data over two Giga bit fiber optic links to PCI based data acquisition and control PCB (PACRAT). TSPM-3 is redesigned from previous versions to accommodate second generation front end ASIC and possible future two scanner expansion. ASICs programmable features are exploited using new additional TSPM electronics for scanner calibration and test. Designs of these three modules and corresponding firmware and software upgrades are complete. Results from fully integrated next generation RatCAP on the bench are presented.

Implementation Study of Single Photon Avalanche Diodes (SPAD) in

Single Photon Avalanche Diodes (SPAD) are known for their excellent timing performance which enables Time of Flight capabilities in positron emission tomography (PET). However, current array architectures juxtapose the SPAD with its ancillary electronics at the expense of a poor fill factor of the SPAD array. The 3D vertical integration of SPADs and readout electronics represents a solution to the aforementioned problem. Compared to systems with external electronics readout, 3D vertical integration reduces the SPAD interconnect parasitic capacitance while greatly increasing the photosensitive area and improving overall performances. This paper presents the implementation of two SPAD structures designed for PET. The SPAD structures are designed using Teledyne DALSA high voltage (HV) CMOS technology targeted for a 3-dimensional single photon counting module (3DSPCM). SPAD with two types of guard ring (diffusion-based and virtual guard ring) are designed, fabricated and characterized. All structures are based on a p + anode in an n-well cathode and are implemented along with active quenching circuits for proper characterization. The results show that the contact distribution and the anode-cathode spacing impact the dark count rate (DCR). The design of SPADs with a diffusion guard ring have a DCR down to 3 s- 1μm-2 at room temperature, afterpulsing probability of , timing resolution of 27 ps FWHM and PDE of 49% at 480 nm.

0.8~\mu\hbox{m}

We propose to develop a high resolution scanner for simultaneous PET and MRI breast imaging that is capable of providing highly sensitive and specific breast cancer examinations. The addition of high resolution positron emission tomography capability to an existing dedicated MRI system will give a device in which each of the modalities contributes its strengths to compensate for the weaknesses of the other. In this combined modality scanner, we have the anatomical information from the MRI to compensate for the somewhat poorer resolution in PET, and we have the predictive power of PET in identifying the type of lesion to overcome the high false positive rate of MRI. This device is based on the technical approach used in the RatCAP scanner with the innovation of detecting coincident events in separate rings of the RatCAP scanner. We are presenting the design and GATE simulations of the full breast imaging system and preliminary PET and MRI results from the prototype system.

HV CMOS Technology

The RatCAP (Rat Conscious Animal PET) is a miniature PET scanner intended for neurological and behavioral studies of small awake animals. The RatCAP detectors are based on Hamamatsu 4 times 8 APD arrays (S8550) coupled to lutetium oxyorthosilicate (LSO) scintillators of 2 times 2 times 5 mm3 in size. These detectors are coupled to a custom front-end Application-Specific Integrated Circuit (ASIC) realized in a CMOS 0.18 mum technology. From this point on in the data stream, the data acquisition hardware has been completely redesigned, replacing previous obsolete VME based system with an optical system which can withstand the environment within an MRI scanner for the intended application of simultaneous PET and MR imaging. Agilent HDMP-1022-1024 G-Link, SDX-19-4-1-S Stratos Lightwave optical transceivers and PCI technology based high speed data acquisition hardware is expected to support a transfer rate up to 70 MB/sec. This hardware system mainly comprises two electronics modules which are read out using Windows/Linux based data acquisition software.