Digital Mammography at Jožef Stefan Institute

The Breast Imaging Sytem - BIS Project

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Our group is part of the Experimental Particle Physics Department at Jožef Stefan Institute in Ljubljana, Slovenia. We are involved in development of a novel digital imaging system for mammography.

The conventional mammographic systems use film-screen combination for X-ray photon detection. The film screen has an excellent spatial resolution (>15 lp/mm), but its detection efficiency is not high. In order to reduce the dose received during medical examination, efficient detectors of X-rays should be used.

Silicon microstrip detectors were developed for high precission tracking of charged particles in high energy physics experiments. It was proposed recently, that they could be used in a so called "edge-on geometry" as efficient X-ray detectors. In this geometry the photons hit the detector from the side, so that a few milimeters to several centimeters of silicon are available for photon detection, while absorption length of 20 keV photons in silicon is 1 mm. The efficiency of such device is limited only by the dead layer between the cutting edge and the implants. The detectors that we are currently using, have a dead layer of approx. 250 microns, leading to efficiency of approx. 80% at 20 keV.

Moreover, the image acquisition is digital and the detector is operated in a single photon counting mode. We use a custom designed integrated circuit CASTOR, which was developed by LEPSI, Strassbourg, France. Due to its low noise the counting of photons with energy above 12 keV is practicaly noise free (without false counts). The only fluctuations in the image are then due to a Poissonian character of photon emmision and detection. Therefore the fluctuations in the image are only limited by statistical quantum fluctuations.

A silicon microstrip detector acts a  linear pixel detector. Pixel dimensions are given by strip pitch and detector thickness. images of objects are obtained by scanning in one direction. This is called a slot-scanning technique. This technique also contributes to scatter rejection which is one of the contrast degrading factors.

The advantages of the novel system compared to the conventional screen-film technique are:

The only drawback of the silicon detectors is their spatial resolution, which is lower that in the case of screen film. Our preliminary results show, that a spatial resolution up to 10 lp/mm can be achieved.

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