A. Studen

Development of silicon pad detectors and readout electronics for a Compton camera

Abstract Applications in nuclear medicine and bio-medical engineering may profit using a Compton camera for imaging distributions of radio-isotope labelled tracers in organs and tissues. These applications require detection of photons using thick position-sensitive silicon sensors with the highest possible energy and good spatial resolution. In this paper, research and development on silicon pad sensors and associated readout electronics for a Compton camera are presented. First results with low-noise, self-triggering VATAGP ASICs are reported. The measured energy resolution was 1.1 keV FWHM at room temperature for the 241 Am photo-peak at 59.5 keV .

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.

Timing in thick silicon detectors for a Compton camera

In the scope of construction of a PET apparatus based on detection of Compton scattering in silicon (Compton camera), the timing properties of 1-mm-thick silicon pad and double-sided microstrip, detectors are studied. Timing in pad detectors is also investigated for 140.5 keV /sup 99m/Tc and 364.5 keV /sup 131/I gamma rays in view of a SPECT application. Compton scattering and energy loss of the Compton electron in the silicon detector are simulated using the GEANT package. The electric field in the detector is calculated numerically for a fully depleted detector in the abrupt junction approximation, taking into account the geometry and varying the reverse voltage. Signal formation is studied using Ramos theorem and pulse shaping properties of the trigger circuit. A time-walk cut is seen to be directly corresponding to a deposited-energy cut. At 10 keV threshold in 1-mm-thick detectors, 10 ns timing windows are shown to reject a significant portion of events, degrading efficiency or limiting the angular range in a prohibitive way. More involved techniques are therefore suggested, either in the electronics circuit or in later stages of the trigger.

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.

Verification of High Dose Rate

A system for in-vivo tracking of 192Ir source during high dose rate or pulsed dose rate brachytherapy treatment was built using 1 mm thick silicon pad detectors as image sensors and knife-edge lead pinholes as collimators. With source self-images obtained from a dual-pinhole system, location of the source could be reconstructed in three dimensions in real time. The system was tested with 192Ir clinical source (kerma rate in air at 1 m 2.38 Gy/h) in air and plexi-glass phantom. The locations of the source were tracked from a distance of 40 cm in a field of view of 20 × 20 × 20 cm3. Reconstruction precision, defined as the average distance between true and reconstructed source positions, with data collected in less than 1 s with 22 GBq 192Ir source was about 5 mm. The reconstruction precision was in our case mainly limited by imperfect alignment of detectors and pinholes. With perfect alignment the statistical error would allow precision of about 1 mm which could further be improved with larger detector placed at larger distance from the pinhole. However already the modest precision of few millimeters is sufficient for in-vivo detection of larger deviations from planned treatment caused by various misadministrations or malfunctioning of the brachytherapy treatment apparatus. Usage of silicon detectors offers a possibility for building a compact device which could be used as an independent online quality assurance system. In this paper details about sensors, readout system and reconstruction algorithm are described. Results from measurements with clinical source are presented.

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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.

Ir Source Position During Brachytherapy Treatment Using Silicon Pixel Detectors

PET probes are showing a lot of promise in extending performance of the conventional PET ring. The underlying idea is to supplement basic PET data with information collected in the finely segmented probe placed close to the region of interest. The benefit is two fold: a) data collected near the object are less prone to errors related to scattering and acolinearity and b) the object itself is magnified in the proximity focus. The principle would be beneficial to clinical applications where spatial resolution below the current limit is required in a narrow field of view. The probe should therefore have excellent spatial resolution, should be compact and robust and should be able to handle large count rates of the clinical environments. Based on those we decided to explore devices with high-resistivity silicon as the sensitive material. They provide high spatial resolution, are compact and robust, and can handle the foreseen rates. We constructed a prototype, based on 1 mm thick silicon wafers, cut into 40 by 26 mm2 detectors further segmented into 1 × 1 mm2 square pads, effectively providing 1 mm3 sensitive voxels. For a module, two such detectors were placed in a back-to-back arrangement, providing filling factor in excess of 70 %. Stacking multiple modules is foreseen to compensate for low stopping power of silicon. The sensors are read out by 128 channel VATAGP7, GM-Ideas sourced application sensitive integrated circuit. Each module requires 16 chips, placed on 4 custom made PCB boards (hybrids) which are read independently. The modules were characterized and will be placed in a test PET ring. A simple point sources and phantoms will be imaged to confirm the predicted benefits.