Giovanna Dipasquale

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.

Spectroscopic and imaging capabilities of a pixellated photon counting system

We are studying the performance of various thickness GaAs pixel detectors bump-bonded to a dedicated photon counting chip (PCC) for medical imaging applications in different energy ranges. In this work we present the experimental results obtained with a 600 μm thick pixel matrix (64×64 square pixels, 170 μm side) in the 60–140 keV energy range to evaluate the possible use of such a system in the nuclear medicine field. In particular, we have measured the spectroscopic properties of the detector (charge collection efficiency, energy resolution and detection efficiency) and evaluated the discrimination capability of the electronics. Then we have measured the imaging properties of the whole system in terms of Point Spread Function and using a home made thyroid phantom. We present also a comparison with a traditional gamma camera and an evaluation, made by both experimental measurements and software simulations, of the imaging characteristics related to the use of a collimation system.

The Calma Project

The CALMA (Computer_Assisted Library for MAmmography) project was born as a collaboration between radiologists and physicists. Its goal is to collect a set of digital mammographic images and to work out a suitable Computed Assisted Diagnosis tool to be used in screening mammography. The project is an Italian collaboration among the Istituto Nazionale di Fisica Nucleare sections of Bologna, Pisa, Torino, Udine and Trento and the Hospitals of Bari, Bologna, Livorno, Udine and Trento. Some preliminary results obtained in the classification of breast disease are described here.

Comparison of imaging properties of several digital radiographic systems

Digital radiography is a field of great interest; a wide range of different imaging systems is now becoming available and it is very important to have powerful instruments to characterize and compare their imaging properties. We have evaluated imaging properties for several digital radiographic systems in terms of resolution and noise properties. For the resolution properties, we have used the Modulation Transfer Function (MTF) and the presampling MTF, for undersampled systems. We have characterized noise properties by Detective Quantum Efficiency, that is an absolute parameter that allows comparison between different systems.

Evaluation of the imaging properties of a direct detection single photon counting based system

We have tested the imaging properties of a system based on a GaAs matrix that performs the direct detection of photons in the range of radiographic interest. The detector is bump-bonded to a VLSI single photon counting electronics. The imaging properties of the system have been evaluated in terms of the presampling MTF and the DQE(ν), that allow an absolute characterization of imaging properties of digital systems. We have tested other radiographic imaging systems in standard clinical conditions, using the same functions and we have compared them with the GaAs matrix system.

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.