Maria Giuseppina Bisogni

Characterization of a PET detector head based on continuous LYSO crystals and monolithic, 64-pixel silicon photomultiplier matrices

The characterization of a PET detector head based on continuous LYSO crystals and silicon photomultiplier (SiPM) arrays as photodetectors has been carried out for its use in the development of a small animal PET prototype. The detector heads are composed of a continuous crystal and a SiPM matrix with 64 pixels in a common substrate, fabricated specifically for this project. Three crystals of 12 mm × 12 mm × 5 mm size with different types of painting have been tested: white, black and black on the sides but white on the back of the crystal. The best energy resolution, obtained with the white crystal, is 16% FWHM. The detector response is linear up to 1275 keV. Tests with different position determination algorithms have been carried out with the three crystals. The spatial resolution obtained with the center of gravity algorithm is around 0.9 mm FWHM for the three crystals. As expected, the use of this algorithm results in the displacement of the reconstructed position toward the center of the crystal, more pronounced in the case of the white crystal. A maximum likelihood algorithm has been tested that can reconstruct correctly the interaction position of the photons also in the case of the white crystal.

Performance of a 4096-pixel photon counting chip

A 4096 pixel Photon Counting Chip (PCC) has been developed and tested. It is aimed primarily at medical imaging although it can be used for other applications involving particle counting. The readout chip consists of a matrix of 64 by 64 identical square pixels, whose side measures 170 micrometers and is bump-bonded to a similar matrix of GaAs or Si pixel diodes covering a sensitive area of 1.18 cm2. The electronics in each cell comprises a preamplifier, a discriminator with variable threshold and a 3-bit threshold tune as well as a 15-bit counter. Each pixel can be individually addressed for electrical test or masked during acquisition. A shutter allows for switching between the counting and readout modes and the use of static logic in the counter enables long data taking periods. Electrical test of the chip have shown a maximum counting and readout modes and the use of static logic in the counter enables long data taking periods. Electrical test of the chip have shown a maximum counting rate of up to 2 MHz in each pixel. The minimum reachable threshold is 1400 e with a variation of 350 e rms that can be reduced to 80 e rms after tuning with the 3-bit adjustment. Electrical noise at the input is 170 e rms. Several read-out chips have been bump bonded to 200 micrometers thick GaAs pixel detectors. Test with (gamma) -ray and (beta) sources have been carried out. A number of objects have been imaged and a 260 micrometers thick aluminum foil which represents a contrast to the surrounding air of only 1.9 percent has been correctly imaged.

Energy, Timing and Position Resolution Studies With 16-Pixel Silicon Photomultiplier Matrices for Small Animal PET

A high resolution small animal PET scanner that employs Silicon Photomultiplier (SiPM) matrices as photodetectors is under development at the University of Pisa and INFN Pisa. The first SiPM matrices fabricated by the Center for Scientific and Technological Research, FBK-irst (Trento, Italy), are being evaluated for this purpose. The devices are composed of 16 (4 times4) pixel elements of 1 mmtimes1 mm in a common substrate. The first tests have been carried out employing the ASIC MAROC2 for the readout. Energy and timing resolution, and position determination tests have been performed coupling both pixellated and continuous LYSO scintillator crystals to the matrix, and the results have been compared with the ones obtained for single SiPMs. The first tests on position determination with continuous crystals and SiPM matrices have been performed. An intrinsic spatial resolution of 0.61 mm FWHM has been obtained.

Novel Silicon Photomultipliers for PET Applications

Silicon photomultipliers (SiPMs) with quantum efficiency maximized for a wavelength between 420 and 470 nm have been developed at ITC-irst Trento (Italy), and are being tested for their application in the construction of a ultra high resolution small animal PET tomograph. The devices have an area of 1 mm times 1 mm and 625 microcells. The breakdown voltage is around 30 V, and the gain of the order of 106. The intrinsic timing resolution is 70 ps rms at the single photoelectron level. The first tests as readout for scintillators show an energy resolution of 21% FWHM with Na-22 employing LSO crystals. The first matrices of SiPMs have been produced and are being tested.

Silicon photomultipliers and SiPM matrices as photodetectors in nuclear medicine

A high performance detector head with matrices of silicon photomultipliers (SiPMs) is under development at the University of Pisa. The detector head will be employed in the construction of a high spatial resolution, MR compatible small animal PET scanner. Silicon photomultipliers from FBK- irst (Trento, Italy) are being evaluated for this purpose. SiPM elements of 1 mm x 1 mm size and SiPM matrices composed of four (2 x 2) pixel elements have been tested. An intrinsic timing resolution of 60 ps sigma has been measured. The results with LSO crystals show an energy resolution of 20% FWHM at 511 keV, and a coincidence timing resolution of 600 ps rms. New devices with improved characteristics and active area, as well as SiPM matrices with 16 (4 x 4) SiPM elements have been produced, and will be evaluated.

Low contrast imaging with a GaAs pixel digital detector

A digital mammography system based on a GaAs pixel detector has been developed by the INFN (Istituto Nazionale di Fisica Nucleare) collaboration MED46. The high atomic number makes the GaAs a very efficient material for low energy X-ray detection (10-30 keV is the typical energy range used in mammography). Low contrast details can be detected with a significant dose reduction to the patient. The system presented in this paper consists of a 4096 pixel matrix built on a 200 /spl mu/m thick semi-insulating GaAs substrate. The pixel size is 170/spl times/170 /spl mu/m/sup 2/ for a total active area of 1.18 cm/sup 2/. The detector is bump-bonded to a VLSI front-end chip which implements a single-photon counting architecture. This feature allows to enhance the radiographic contrast detection with respect to charge integrating devices. The system has been tested by using a standard mammographic tube. Images of mammographic phantoms will be presented and compared with radiographs obtained with traditional film/screen systems. Monte Carlo simulations have been also performed to evaluate the imaging capability of the system. Comparison with simulations and experimental results will be shown.

GaAs pixel radiation detector as an autoradiography tool for genetic studies

Abstract We present an autoradiography tool to be used mainly for genetic studies. It performs a quantitative analysis of radioactivity and can follow a dynamic process. We designed several applications, in particular one aimed at detecting hybridization of radio-labeled DNA fragments with known DNA-probes deposited on a micro-array. The technique is based on GaAs pixel array detector and low threshold, large dynamic range and good sensitivity integrated electronics developed for medical applications, suitable to detect markers (gamma or beta) such as 14C, 35S, 33P, 32P, 125I, even at very low activities. A Monte Carlo simulation of β− detection in GaAs is presented here in order to study the spatial resolution characteristics of such a system. For several biological applications, the electronics is required to perform at high temperatures (from 37° to 68°): we present here studies of noise and minimum threshold as a function of the temperature.

AUTORADIOGRAPHY WITH SILICON STRIP DETECTORS

A digital autoradiography system based on double sided silicon strip detectors (1.6 × 1.6 mm2 active surface with 100 μm pitch) has been developed and successfully tested with beta-emitting tracers. It is shown here that the system is able to perform imaging of organic material with specific sensitivity as small as 0.002 nCi/mm2, and to record activity measurements with good linearity in the range 0.002–20 nCi/mm2. Autoradiographic images of clusters of mammary cells marked with ortho-(32P)phosphate, obtained with an exposure time of about 10 min are presented.

A Medipix2-based imaging system for digital mammography with silicon pixel detectors

In this paper we present the first tests of a digital imaging system based on a silicon pixel detector bump-bonded to an integrated circuit operating in single photon counting mode. The X-rays sensor is a 300 /spl mu/m thick silicon, 14 by 14 mm/sup 2/, upon which a matrix of 256 /spl times/ 256 pixels has been built. The read-out chip, named MEDIPIX2, has been developed at CERN within the MEDIPIX2 Collaboration and it is composed by a matrix of 256 /spl times/ 256 cells, 55 /spl times/ 55 /spl mu/m/sup 2/. The spatial resolution properties of the system have been assessed by measuring the square wave resolution function (SWRF) and first images of a standard mammographic phantom were acquired using a radiographic tube in the clinical irradiation condition.

An innovative detection module concept for PET

The design of a Positron Emission Tomography detection module capable of working inside a Magnetic Resonant Imaging system is the main objective of the 4D-MPET project. Combining the two imaging technologies offers better soft tissue contrast and lower radiation doses by providing both functional and morphological information at the same time. The proposed detector will feature a three-dimensional architecture based on two tiles of Silicon Photomultipliers coupled to a single LYSO scintillator on both its faces. Silicon Photomultipliers are magnetic-field compatible photo-detectors with a very small size enabling novel detector geometries that allow the measurement of the Depth of Interaction as well as a high detector packing fraction to maximize system sensitivity. Furthermore they can be fabricated using standard silicon technology, have a large gain in the order of 106 and are very fast thus allowing evaluating the Time of Flight. Among the other features of the proposed detection system, the architecture of the innovative readout electronics will be also described which plays a relevant role for the achievement of the desired performance and is based on custom integrated circuits. Simulation results of the whole system show good performance in terms of time and spatial resolution: a timestamp of 100 ps is the ultimate performance achievable with the use of a double threshold technique along with fast electronics. Time over threshold is exploited to provide the energy information with a bin size of 400 ps. Moreover, a z resolution of 1.4 mm Full Width at Half Maximum can be achieved. The proposed detector can also be exploited in other tracking applications, such as High Energy Physics and Astrophysics.