J.-F. Pratte

Front-end electronics for the RatCAP mobile animal PET scanner

We report on the development of the front-end electronics for rat conscious animal positron emission tomography (RatCAP), a portable and miniature positron emission tomography scanner. The application-specific integrated circuit (ASIC) is realized in a complementary metal-oxide-semiconductor 0.18 /spl mu/m technology and is composed of 32 channels of charge sensitive preamplifier, third-order semi-Gaussian bipolar shaper, timing discriminator with independent channel adjustable threshold, and a 32-line address serial encoder to minimize the number of interconnections between the camera and the data acquisition system. Each chip has a maximum power dissipation of 125 mW. A mathematical model of the timing resolution as a function of the noise and slope at the discrimination point as well as the photoelectron statistics was developed and validated. So far, three ASIC prototypes implementing part of the electronics were sent to fabrication. Results from the characterization of the first two prototypes are presented and discussed.

Design and performance of 0.18-/spl mu/m CMOS charge preamplifiers for APD-based PET scanners

The CMOS 0.18-/spl mu/m technology was investigated for two analog front-end projects: the low-power budget rat-head mounted miniature rat conscious animal PET (RatCAP) scanner, and the high-performance, low-noise, high-rate PET/CT application. The first VLSI prototypes consisted of 1- and 5-mW charge sensitive preamplifiers (CSP) based on a modified cascode telescopic architecture. Characterization of the rise time, linearity, dynamic range, equivalent noise charge (ENC), timing resolution and energy resolution are reported and discussed. When connected to an APD-LSO detector, time resolutions of 2.49 and 1.56 ns full-width half-maximum (FWHM) were achieved by the 1- and 5-mW CSPs, respectively. Both CSPs make it possible to achieve performance characteristics that are adequate for PET imaging. Experimental results indicate that the CMOS 0.18-/spl mu/m technology is suitable for both the low-power and the high-performance PET front-end applications.

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.

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.

A prototype CZT-based PET scanner for high resolution mouse brain imaging

One of the most challenging and potentially rewarding research applications of PET is imaging of the mouse brain. Although very high spatial resolution is required (< ~1 mm), there is a much wider variety of transgenic models in mouse compared to the rat. The solid state material CdZnTe (CZT) has long held promise for high resolution PET. Compared to scintillators, its limitations in time resolution and sensitivity can in some ways be compensated by its extremely high spatial and energy resolution, its compact geometry, and by sophisticated data processing techniques. Using such techniques, a time resolution of ~10 ns has been demonstrated for ~1 cm thick CZT pixel detectors, and this may be sufficient for mouse studies. The depth-of-interaction capability and high energy resolution can improve sensitivity by allowing detectors to be placed very close to the subject and by enabling both reconstruction of detector-scattered events and rejection of object-scattered events. A full-ring prototype scanner has been designed to demonstrate feasibility of the concept, consisting of 6 CZT pixel detectors in a novel geometry. The design of the detector, front-end electronics components, and data acquisition are presented, along with performance characterization of the custom-manufactured CZT detectors.

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.

The RatCAP conscious small animal PET tomography

The RatCAP is a small, head mounted PET tomograph designed and built to image the brain of an awake rat. It allows PET imaging studies to be carried out on laboratory rats without the use of anesthesia, which severely suppresses brain functions and affects many of the neurological activities that one would like to study using PET. The tomograph consists of a 4 cm diameter ring containing 12 block detectors, each of which is comprised of a 4 times 8 array of 2.2 times 2.2 times 5 mm3 LSO crystals read out with a matching APD array. The APDs are read out using a custom designed ASIC and VME readout system. We have successfully performed a system integration test with a partially instrumented tomograph ring. We present the recent progress towards a fully integrated system

Studies of CZT for PET applications

CdZnTe (CZT) has been investigated by several researchers as a detector material for positron emission tomography (PET) applications. CZT detectors can be manufactured into 1 cm3 or larger detectors with pixelated anodes, providing high spatial resolution at the 1 mm level or smaller. Indeed the spatial and energy resolution of CZT can be far superior to those of current state-of-the-art PET detectors, most of which are scintillator-based. On the other hand, at 511 keV its timing performance and photopeak detection efficiency are generally inferior, which pose challenges that must be surmounted. In order to obtain sufficient efficiency with a practical number of electronics channels and interconnections in a realistic full-scale system, the focus is on thick detectors (~10 mm). However, the timing becomes more challenging with increasing thickness due to the low charge mobility. We evaluated planar and coplanar grid CZT detectors, as well as position-sensitive pixel-anode detectors to assess methods of improving timing performance. For a 7.5-mm thick coplanar grid detector at 1000 V bias, we obtained a preliminary time resolution vs. BaF2 detector of 21 ns FWHM by fitting the digitally sampled rising edge of the cathode signal. Optimization of the front-end electronics and data-processing methods is expected to further improve these results. We are currently characterizing the performance of a pixel cube detector of ~1 cm3 in size with a single cathode and a 4times4 array of anodes with 2.5-mm pitch on the opposing side, for which a data acquisition system has been designed and fabricated

Preliminary Studies of a Simultaneous PET/MRI Scanner Based on the RatCAP Small Animal Tomograph

We are developing a scanner that will allow the simultaneous acquisition of high resolution anatomical data using magnetic resonance imaging (MRI) and quantitative physiological data using positron emission tomography (PET). The approach is based on the technology used for the RatCAP conscious small animal PET tomograph which utilizes block detectors consisting of pixelated arrays of LSO crystals read out with matching arrays of avalanche photodiodes (APDs) and a custom-designed ASIC. A version of the detector is being developed that will be constructed out of all nonmagnetic materials that can be operated inside the MRI field. We have demonstrated that the PET detector works inside the MRI field using 511 keV gamma rays, and have obtained MRI images with various detector components that show minimal distortion in the MRI image. We plan to improve on the image quality in the future using completely nonmagnetic components and by tuning the MRI pulse sequences. The combined result will be a highly compact, low mass PET scanner that can operate inside an MRI magnet without distorting the MRI image, and can be retrofitted into existing MRI instruments.

Tests of small X-ray Active Matrix Pixel Sensor prototypes at the National Synchrotron Light Source

X-ray Active Matrix Pixel Sensors (XAMPS) were designed and fabricated at Brookhaven National Laboratory. Devices based on J-FET technology were produced on 100 mm high-resistivity silicon, typically 400 μm-thick. The prototypes are square matrices with n rows and n columns with n = 16, 32, 64, 128, 256, 512. Each pixel of the matrix is 90 × 90 μm2 and contains a JFET switch to control the charge readout. The XAMPS is a position sensitive ionization detector made on high resistivity silicon. It consists of a pixel array detector with integrated switches. Pixels are isolated from each other by a potential barrier and the device is fully depleted by applying a high voltage bias to the junction on the entrance window of the sensor. The small features of the design presented some technological challenges fully addressed during this production. The first prototypes were tested at the National Synchrotron Light Source (NSLS) with a monochromatic beam of 8 keV and millisecond readout and exhibit good performances at room temperature.