The Breast Imaging Sytem - BIS Project
Introduction
Fields of Research
Publications
Interesting
Links
Introduction
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:
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very efficient detection of X-ray quanta, leading to lower dose per investigation
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scater rejection
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fluctuation in the image are due to quantum fluctuations only (which is
the lowest possible to achieve)
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immediate digital image, suitable for further image processing
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the silicon technology is well know, available and relatively chip (not
much further R&D are required)
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.
Fields of research
Our fields of research include:
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detector and readout electronics study
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the readout circuit CASTOR
was tested; gain and noise were determined
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a simple test system was assembled and simple objects' images were made
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a batch of CSEM manufactered detectors was used to study the performace
of devices at different cutting conditions (html)
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a silicon microstrip detector with 0.100 mm strip pitch was designed and
fabricated at the Faculty of Electrical Engineering of University of Ljubljana;
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imaging system development and tests
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the assembled system was used to make images of a CIRS
tissue equivalent
mammographic phantom (html)
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modelled microcalcifications and tumors were visible and recognized at
skin entrance doses lower than 1 mGy
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the modulation transfer function of the system in the direction of scanning
was measured
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a maximum entropy method was used to deconvolve the obtained image with
a line spread function of the system resulting in effectively reducing
the pixel size for a factor of 3 (PS)
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Monte Carlo simulations
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the possibility of using the semiconducting detectros as a photon detection
medium
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optimisation of spectra, scatter reduction