S. Junnarkar

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.

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.

A Simultaneous PET/MRI scanner based on RatCAP in small animals

The ability to acquire high resolution anatomical data as well as quantitative functional information in vivo is becoming an increasingly important factor in the diagnosis of disease. Simultaneous acquisition of PET and MRI data would provide essentially perfect co-registration between the two images which is particularly important for tissues whose position and shape can change between sequential scans. RatCAP is a complete 3D tomograph that is designed to image the brain of an awake rat. A special MRI coil composed of 2 saddle elements working in quadrature mode was mounted on a Delrin cylinder specifically designed to fit inside the RatCAP but allowing the rats head to be placed inside as well. Simultaneous PET/MRI images of the rat brain have been acquired in a 4 T MRI scanner using the RatCAP detector, with minimal effect on MRI images.

FPGA based self calibrating 40 picosecond resolution, wide range Time to Digital Converter

We present FPGA-based self calibrating Time to Digital Converter (TDC) architecture specific to applications where one is interested in measuring absolute time of occurrence of an event since t0 by combining coarse (simple binary counter at system clock speed) and fine TDC. The architecture relies on an accurate crystal oscillator to provide stable system clock reference to generate calibration test pulse. It uses two controllable ring oscillators with very small difference in frequencies, where the frequency difference determines the achievable resolution. The scheme was implemented on Altera Stratix II device and we have measured a resolution of 40 ps.

An FPGA-based, 12-channel TDC and digital signal processing module for the RatCAP scanner

Front end digital signal processing and VME based DAQ electronics for the RatCAP (Rat Conscious Animal PET) is discussed. All digital approach to front end signal processing for the mobile animal PET scanner is presented. Altera Cyclone family FPGA based realization of the 12 channel TDC (time to digital converter), address serial decoder and VME based DAQ system development is discussed in detail. Routing delays between logic array blocks combined with propagation delay of logic cells were used to generate different clock phases, to achieve subclock speed resolution. Altera LogicLock/spl trade/ toolsets were used for replicable and tighter placements of the supporting logic to achieve the required timing performance. TDC realized using controlled placements of the logic elements to specific logic cells within a specific LAB (logic array block) has the maximum DNL of 0.7 ns. VME based custom designed board with FIFO memory constituted the DAQ electronics. Test results with full 12 blocks, RatCAP front end electronics are presented. TDC realization and characterization is discussed in details. Timing spectrum obtained for 12 blocks, 384 channels of full RatCAP scanner is also presented.

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.

FPGA-Based Self-Calibrating Time-to-Digital Converter for Time-of-Flight Experiments

We describe the architecture of a FPGA-based self-calibrating Time to Digital Converter (TDC), specifically intended to measure the width of an input pulse. The configuration consists of two controllable ring oscillators with a very small difference in their frequencies, wherein this difference determines the achievable resolution. The calibration scheme relies on an accurate pulse-generator or external crystal-oscillator to provide a stable calibration pulse for the system. We implemented the TDC on an Altera Stratix II device where we measured a Least Significant Bit of 41 ps (an RMS resolution of 11.8 ps). We present details of the methods used to calibrate the TDC, the characterization process, and discuss the effects of variations in temperature and voltage.

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.

The design and performance of the 2 nd -generation RatCAP awake rat brain PET system

The original prototype RatCAP PET scanner for conscious rat brain imaging has undergone a redesign of most major components resulting in a distinct 2nd -generation instrument. While maintaining the same field of view (38 mm diameter, 18 mm axial) and similar overall architecture, the new design allows for longer crystals to provide approximately a factor of 2 increase in coincidence sensitivity with a minimal increase in size and weight. The front-end electronics ASIC has been significantly upgraded, featuring programmable amplifier gains, lower noise, differential digital communication (LVDS), and selectable energy window modes and analog outputs for debugging. The rigid-flex circuit interconnecting the 12 blocks is now more mechanically stable and draws less power which minimizes APD gain shifts. The downstream time-stamp and signal processing module (TSPM) has been modified to be compatible with the new ASICs and further includes DACs for threshold control, twice as many inputs, and a doubling of data throughput capacity. The user interface and data acquisition software is in Labview, and data processing and image reconstruction software is being further developed to maximize imaging accuracy for quantitative neuroscience studies. Finally, a new mechanical support system has been constructed to improve the rats tolerance of the scanner. Preliminary data indicate improved energy and time resolution compared to the 1st-generation prototype and first images of the rat brain while conscious have been obtained.