M.G. Bisogni

Energy and Timing Resolution Studies With Silicon Photomultipliers (SiPMs) and 4-Pixel SiPM Matrices for PET

A high performance detector head with matrices of silicon photomultipliers (SiPMs) as photodetectors is under development at the University of Pisa and INFN Pisa. The silicon photomultipliers fabricated at FBK-irst (Trento, Italy) are being evaluated for this purpose. Single SiPM detectors of 1 mm x 1 mm size and the first SiPM test matrices composed of four (2 x 2) pixel elements in a common substrate have been tested with LYSO crystals. The energy and timing resolution of single SiPMs have been evaluated, obtaining an energy resolution of 20% FWHM at 511 keV, and a coincidence timing resolution of 1.4 ns FWHM. In addition, the first tests performed with the SiPM matrices prove the feasibility of such devices.

Use of silicon and GaAs pixel detectors for digital autoradiography

Solid state detectors made of Si (4.8/spl times/8 mm/sup 2/) and GaAs (6.4/spl times/8 mm/sup 2/) pixel matrices bump-bonded to the Omega2 and Omega3 electronic read-out systems, developed at CERN for H.E.P. experiments, have been used to obtain autoradiographic images of clusters of human epithelial cells and DNA fragments separated via electrophoresis, both labeled with /sup 32/P. The system has shown a good minimum detectable activity per unit area of 2.10/sup -4/ cps mm/sup -2/, and has proved linear for a count rate in the range 0.2-20 cpa, typical of autoradiography. The pixel dimensions are 75/spl times/500 /spl mu/m/sup 2/ (Si-Omega2) and 50/spl times/500 /spl mu/m/sup 2/ (GaAs-Omega3), respectively. We are able to clearly localize clusters of cells which have incorporated the radioactive tracer and DNA fragments on an electrophoretic gel on paper (blots).

Experimental study of LEC GaAs detectors for X-ray digital radiography

Abstract In previous studies, various semi-insulating LEC GaAs crystals were irradiated with photons in the diagnostic energy range (20–100 keV), in view of a possible application in digital radiography. Solid-state and irradiation measurements, together with Monte Carlo simulations, have indicated good candidates for this application among the crystals we have investigated. In this paper we present results concerning the detection characteristics (detection efficiency, charge-collection efficiency and energy resolution as functions of the bias voltage) of one of these materials and the images obtained by a pixel detector made on the same material and a bump-bonded electronic system.

GaAs detector optimization for different medical imaging applications

Abstract We have investigated the detection performance of GaAs detectors made with different thickness and contact geometries. A comparison is made between these detection capabilities and the imaging requirements for the following medical applications: digital mammography, digital chest radiography and nuclear medicine. Experimental results and preliminary images are presented and discussed.

Characterization of a mammographic system based on single photon counting pixel arrays coupled to GaAs x-ray detectors

The authors report on the imaging capabilities of a mammographic system demonstrator based on GaAs pixel detectors operating in single photon counting (SPC) mode. The system imaging performances have been assessed by means of the transfer functions: The modulation transfer function (MTF), the normalized noise power spectrum, and the detective quantum efficiency (DQE) have been measured following the guidelines of the IEC 62220-1-2 protocol. The transfer function analysis has shown the high spatial resolution capabilities of the GaAs detectors. The MTF calculated at the Nyquist frequency (2.94 cycles/mm) is indeed 60%. The DQE, measured with a standard mammographic beam setup (Mo/Mo, 28 kVp, with 4 mm Al added filter) and calculated at zero frequency, is 46%. Aiming to further improve the systems image quality, the authors investigate the DQE limiting factors and show that they are mainly related to system engineering. For example, the authors show that optimization of the image equalization procedure increases the DQE(0) up to 74%, which is better than the DQE(0) of most clinical mammographic systems. The authors show how the high detection efficiency of GaAs detectors and the noise discrimination associated with the SPC technology allow optimizing the image quality in mammography. In conclusion, the authors propose technological solutions to exploit to the utmost the potentiality of GaAs detectors coupled to SPC electronics.

Interconnection techniques of GaAs pixel detector on silicon ASIC electronics

To realize innovative instrumentation for mammography, a digital mammographic head based on several gallium arsenide detectors and on the Medipix chip (developed by the Medipix collaboration at CERN) has been designed. This device is able to perform single photon counting readout and allows a full detection efficiency at the mammographic energies. To make such detection units, industrial processes for the production of GaAs pixel detectors and for their bump-bonding to the read-out VLSI electronics have been developed by Alenia Marconi Systems (AMS S.p.A.). The detection unit is a 200 /spl mu/m thick GaAs matrix of 64/spl times/64 square pixels (170 /spl mu/m pitch), previously optimized and fully characterized, connected via bump bonding to the electronic chip. Specific studies were performed to develop a process that meets the requirements of low cost, high yield and high reliability for the bump bonding process of the GaAs detector to the silicon electronics. To ensure a high-quality electrical contact between the matrix detector and the silicon read out electronics a bump-bonding process has been set-up. It is based on a In evaporation process. In this work we present the technical characteristics of the bump bonding process and the results obtained in terms of bump-bonding yield and detection performance of the GaAs based detection units which have been realized and fully tested and are now ready to be implemented in the mammographic head.

Digital system based on a bichromatic x-ray source and a single-photon counting device: a single-exposure dual-energy mammography approach

In this work we exploit the advantages of using a bi-chromatic X-rays source coupled with a single photon counting pixel detector to perform a feasibility study for dual energy mammography. This technique allows enhancing the contrast between different breast tissues by composing two images acquired at two different energies. The high and low energy images have been acquired by a single X-ray shot. The bi-chromatic beam has been produced per diffraction of polychromatic photons by a monochromator crystal. The imaging system is based on a single photon counting silicon pixel detector. The data read-out is performed by a VLSI Integrated Circuit bump-bonded to the sensor. The energy threshold of each electronics channel can be individually trimmed. We set the threshold of one pixel below 16 keV while the threshold of the neighboring pixel between 16 and 32 keV. With a single exposure the information from both energies is recorded. After separation between low and high threshold pixels, we obtained two independent images. We acquired radiographs of phantoms made of three different materials. Appling a dual energy algorithm, we obtained synthesized images where any of the three materials is removed from the radiograph, enhancing the contrast between the two remaining.

Imaging spectroscopic performances for a Si based detection system

We present the imaging and spectroscopic capabilities of a system based on a single photon counting chip (PCC) bump-bonded on a Si pixel detector. The system measures the energy spectrum and the flux, produced by a standard mammographic tube. We have also made some images of low contrast details, achieving good results.

Detection performance of SI GaAs detectors for nuclear medicine

AbstractThiscollaborationhasalreadyobtainedencouragingresultsabouttheperformanceof200and600mmthicksemi-insulatingGaAsdetectors,intermsofchargecollectionefficiency(c.c.e.),detectionefficiencyandimagingcapabilities,tobeusedassinglephotoncountingdetectorswithphotonsintheradiographicenergyrange.Thepossibleuseofsuchdetectorsinnuclearmedicineimpliesdetectionofmoreenergeticphotons,uptothe140keVofthe 99m Tcsource;thusefficientdetectorsmustbethickerthanthosepreviouslystudied.Inthispaper,wepresenttheresultsoftheinvestigationofelectricalanddetectionproperties(spectra,c.c.e.,energyresolution)ofaseriesofdetectors,whosethicknessrangesfrom 200to1000mm, when irradiated with radioactive sources over the energy range used in nuclear medicine.# 2001 ElsevierScienceB.V.Allrightsreserved. 1. IntroductionIn nuclear medicine, in applications as scinti-graphy and single photon emission computedtomography(SPECT)thephotonstobedetectedare emitted by a radioisotope present into thepatient thanks to a radiopharmaceutical (radio-isotope-taggedcompound),whichcanbeinjected,ingested or breathed [1], in order to ‘‘trace’’variousfunctionsofthebody.Oneoftheprimaryadvantages associated with the use of radionu-clides is the large signal (the emitted radiation)obtainedfromtherelativelysmallmassofradio-nuclide employed for a given study. The minutemass of the radiolabeled material allows for theobservation without disturbance of the systemunder study through pharmacological or toxico-logicaleffect.Thephysicalcharacteristicsofradio-nuclides that are desirable for nuclear medicineimaginginclude:asuitablephysicalhalf-life;decayvia photon emission; associated photon energyhigh enough to penetrate the body tissue withminimal tissue attenuation and low enough toallowingforminimalthicknessofcollimatorsepta;absenceofparticulateemission.

Two VLSI CMOS ICs for digital X-ray 2-D imaging system: low noise front-end amplifier and 80 MHz digital encoder interface

We describe a VLSI-based data acquisition to perform X-Y coincidences with a two side microstrip digital detector, optimized for applications in digital mammography. A two components chip set is described in detail: a low noise Front-End analog amplifier and an 80 MHz synchronous digital encoder. These two components can be used to solve the problems arising with the use of a large number of channels and low signal amplitude as in our application.