D. Burdette

Performance evaluation of a very high resolution small animal PET imager using silicon scatter detectors

A very high resolution positron emission tomography (PET) scanner for small animal imaging based on the idea of inserting a ring of high-granularity solid-state detectors into a conventional PET scanner is under investigation. A particularly interesting configuration of this concept, which takes the form of a degenerate Compton camera, is shown capable of providing sub-millimeter resolution with good sensitivity. We present a Compton PET system and estimate its performance using a proof-of-concept prototype. A prototype single-slice imaging instrument was constructed with two silicon detectors 1 mm thick, each having 512 1.4 mm x 1.4 mm pads arranged in a 32 x 16 array. The silicon detectors were located edgewise on opposite sides and flanked by two non-position sensitive BGO detectors. The scanner performance was measured for its sensitivity, energy, timing, spatial resolution and resolution uniformity. Using the experimental scanner, energy resolution for the silicon detectors is 1%. However, system energy resolution is dominated by the 23% FWHM BGO resolution. Timing resolution for silicon is 82.1 ns FWHM due to time-walk in trigger devices. Using the scattered photons, time resolution between the BGO detectors is 19.4 ns FWHM. Image resolution of 980 microm FWHM at the center of the field-of-view (FOV) is obtained from a 1D profile of a 0.254 mm diameter (18)F line source image reconstructed using the conventional 2D filtered back-projection (FBP). The 0.4 mm gap between two line sources is resolved in the image reconstructed with both FBP and the maximum likelihood expectation maximization (ML-EM) algorithm. The experimental instrument demonstrates sub-millimeter resolution. A prototype having sensitivity high enough for initial small animal images can be used for in vivo studies of small animal models of metabolism, molecular mechanism and the development of new radiotracers.

Timing performance of the silicon PET insert probe.

Simulation indicates that PET image could be improved by upgrading a conventional ring with a probe placed close to the imaged object. In this paper, timing issues related to a PET probe using high-resistivity silicon as a detector material are addressed. The final probe will consist of several (four to eight) 1-mm thick layers of silicon detectors, segmented into 1 x 1 mm(2) pads, each pad equivalent to an independent p + nn+ diode. A proper matching of events in silicon with events of the external ring can be achieved with a good timing resolution. To estimate the timing performance, measurements were performed on a simplified model probe, consisting of a single 1-mm thick detector with 256 square pads (1.4 mm side), coupled with two VATAGP7s, application-specific integrated circuits. The detector material and electronics are the same that will be used for the final probe. The model was exposed to 511 keV annihilation photons from an (22)Na source, and a scintillator (LYSO)-PMT assembly was used as a timing reference. Results were compared with the simulation, consisting of four parts: (i) GEANT4 implemented realistic tracking of electrons excited by annihilation photon interactions in silicon, (ii) calculation of propagation of secondary ionisation (electron-hole pairs) in the sensor, (iii) estimation of the shape of the current pulse induced on surface electrodes and (iv) simulation of the first electronics stage. A very good agreement between the simulation and the measurements were found. Both indicate reliable performance of the final probe at timing windows down to 20 ns.

Last Results of a First Compton Probe Demonstrator

A first prostate probe prototype based on the Compton imaging technique has been developed, with a scatter detector composed of a stack of five thick silicon pad detectors, and a scintillator as absorption detector. The silicon sensors are 4 cm times 1 cm, 1 mm thick, with 1.4 mm times 1.4 mm pad dimensions. The scatter detector performance has been optimized, and an energy resolution about 1.4 keV FWHM is obtained. The absorption detector consists of three Nal(Tl) scintillators that are placed around the scatter detector. A spatial resolution of 5 mm FWHM has been measured reconstructing an image of a point-like 133 Ba source placed at 11 cm distance from the scatter detector. Additional studies show the improvement of the detector resolution with increasing incident photon energy and relative distance from the scatter to the absorption detector. Simulations have also been performed for a complete understanding of the data, and to predict the expected resolution at near field operation of the device. The prototype construction is the first step in the development of a Compton probe for prostate imaging.

Timing in silicon pad detectors for Compton cameras and high resolution PET

Silicon pad detectors and self-triggering readout electronics have been developed for use in Compton cameras and very high resolution PET applications. Events of interest in both techniques are registered when one or more detector interactions occur within a narrow time coincidence window. And in both methods poor time resolution leads directly to poor discrimination between true and random coincidences. Initial coincidence timing measurements between two silicon detectors each having 512 1.4 mmtimes1.4 mm pads in a 32times16 array have demonstrated a cusp-shaped spectrum of 80 ns FWHM with long tails. Ideally, for PET and Compton camera applications, time resolution would be less than 10 ns FWHM. This investigation demonstrates that the shape of the spectrum and to a large extent its width result from time-walk due to (1) the wide range of energies deposited in the silicon detectors from Compton interactions, (2) the leading-edge threshold trigger employed in the readout ASIC, and (3) the ~200 ns peaking time in the fast-channel of the readout ASIC. Although walk is a major component of the timing spectrum, it is not the only reason for broadening as determined by both simulations and measurements correlating pulse-height with threshold-crossing time

Performance of the MADEIRA PET probe prototype

This paper reports the characterization of a detector module, the building block to be used for the MADEIRA PET probe prototype. The prototype will be used in synchronization with a conventional PET ring, amplifying the basic image with a subset of events with high spatial resolution. For image improvement, the crucial parameters are the spatial and timing resolution of the probe, while the energy resolution can be used in event classification. The final prototype is made of high-resistivity silicon detectors, 1 mm thick with 1040 square pads with a size of 1 mm. The performance was characterized on an evaluation module featuring the same electronics and sensor material, save for sensor geometry where a sensor with 256 square pads with a side of 1.4 mm was used. The pads were read out with VATAGP7, a Gamma Medica-Ideas designed application specific integrated circuit (ASIC). The ASIC provides a logic trigger signal and an analog output for every out of 128 input channels. The measurement of the timing resolution was performed, using a positron source and a timing reference detector. A similar measurement was performed with a test pulse, ie. a charge injection into the ASIC, to study inherent resolution of the firststage electronics. We calibrated the analog output of the ASIC using standard gamma sources (241Am), and we determined the overall energy resolution. The comparators were calibrated by a test pulse, looking at the rate of triggers versus threshold. The gain variation was compensated with internal 3-bit DACs. The evaluation module was successfully characterized, exhibiting energy resolution of 1.5 keV FWHM after gain alignment. The spread of the comparator levels can be decreased below 1 keV. The timing performance allows timing windows of 20 ns to be used. Based on the performance of the evaluation module we conclude that the final prototype can be used as a PET insert probe.

A study of the effects of strong magnetic fields on the image resolution of PET scanners

Very high resolution images can be achieved in small animal PET systems utilizing solid state silicon pad detectors. In such systems with sub-millimeter intrinsic resolutions, the range of the positron is becoming the dominant contribution to image blur. The size of the positron range effect depends on the initial positron energy and hence the radioactive tracer used. For higher energy positron emitters, such as 68Ga and 94mTc which are gaining importance in small animal studies, the width of the annihilation point distribution dominates the spatial resolution. This positron range effect can be reduced by embedding the field of view of the PET scanner in a strong magnetic field. In order to confirm this effect experimentally we have developed a high resolution PET instrument based on silicon pad detectors that can operate in a 7 T magnetic field. In this paper we present the preliminary results of a study of the effects of magnetic fields up to 7 T on PET image resolution for 22Na and 68Ga point sources.

An FPGA based DAQ system for the readout of Madeira PET probe

Madeira project aims to significantly improve three-dimensional (3D) nuclear medicine imaging technologies via a compact photon-sensitive probe interfaced to an external conventional PET ring and placed close to the region of interest. The probe consists of several modules densely packed. Each module is made of two high-resistivity silicon detectors of 1 mm thickness and 1040 square pixels of 1 mm2. The detectors are placed back-to-back at a distance of 0.8 mm. The pads are read out with the VATAGP7 chip, a Gamma Medica — Ideas designed application specific integrated circuit (ASIC). A FPGA based DAQ system has been designed and developed at IFIC-Valencia to read out the probe. The system consists of several DAQ boards working in parallel which control the acquisition process. The DAQ board has trigger and coincidence capabilities to be used in coincidence with a conventional PET scanner. This work describes the characteristics of the system and its architecture, and also the future activities.

Very High Resolution Small Animal PET in Strong Magnetic Fields

Very high resolution images can be achieved in small animal PET systems utilizing solid state silicon detectors (Compton PET). In such systems with sub-millimeter intrinsic resolutions, the range of the positron is the largest contribution to image blur. The size of the positron range effect depends on the initial positron energy and hence the radioactive tracer used. For higher energy positron emitters, such as 124I and 94mTc which are gaining importance in small animal studies, the variation of the annihilation point dominates the spatial resolution. It has been suggested that this positron range effect can be reduced by embedding the PET field of view in a strong magnetic field. Conventional PET systems using scintillators and photomultiplier tubes require extensive modifications to operate in magnet fields; however, our silicon detector based system can operate in magnetic fields with minimal modifications. In this paper we present a progress report of embedding our small animal PET test-bench in magnetic fields up to 7 Tesla.

Spectroscopy study of imaging devices based on silicon Pixel Array Detector coupled to VATAGP7 read-out chips

Spectroscopic and timing response studies have been conducted on a detector module consisting of a silicon Pixel Array Detector bonded on two VATAGP7 read-out chips manufactured by Gamma-Medica Ideas using laboratory gamma sources and the internal calibration facilities (the calibration system of the read-out chips). The performed tests have proven that the chips have (i) non-linear calibration curves which can be approximated by power functions, (ii) capability to measure the energy of photons with energy resolution better than 2 keV (exact range and resolution depend on experimental setup), (iii) the internal calibration facility which provides 6 out of 16 available internal calibration charges within our region of interest (spanning the Compton edge of 511 keV photons). The peaks induced by the internal calibration facility are suitable for a fit of the calibration curves. However, they are not suitable for measurements of equivalent noise charge because their full width at half maximum varies with their amplitude. These facts indicate that

Development and test of TAB bonded micro-cables for silicon detectors in a Compton prostate probe

This contribution describes the work made towards packaging optimization in the frame of the development of an endorectal probe for imaging the prostate. This application is based on the concept of electronic collimation for single gamma detection taking advantage of the Compton scattering of the photons in a stack of 1 mm thick silicon pad detectors and their later absorption in an external detector. This concept allows to remove the mechanical collimators used in the usual gamma cameras and, therefore, provides the possibility of improving both sensitivity and resolution. Packaging of the silicon sensors and their associated electronics is of paramount importance in this application. To address this important aspect of the device we have explored the tape automated bonding (TAB) technique for the connection of the silicon sensors to the readout electronics and for routing the signals to the outside. TAB offers an elegant solution not only for a flexible and compact interconnection between the readout ASICs and the sensors but also for ASIC selection and testing prior to the assembly of the detector and their associated readout electronics