Early detection of breast cancer based on mammography requires outstanding imaging capabilities which are hardly met by the conventional radiographic systems based on film-screen techniques. On one hand, the visualization of micro-calcifications needs high resolution images at a relatively highimage-contrast, while on the other, the detection of a small tumor on a background of healthy tissue requires the capability to image details at low image-contrast.
Although a general consensus from a clinical point of view has not yet been reached, one can assume that the detection of a microcalcification would require the capability to resolve details of the order of 0.2 mm diameter with a contrast not lower than 10 % , while the identification of carcinoma would profit from the ability to detect contrasts as low as 1 % for details possibly as small as 2 to 3 mm.
The next step is to improve our capability to visualize image details at low contrast where the resolution of the imaging system is limited by the exposure which has to be kept low to reduce the total dose delivered to the body ( a key issue for mammographic screening). Further improvement in transmission radiography can be obtained by suppression of the background in the image due to the diffused radiation from the body.
Rejection of the physical background due to scattered Compton radiation from the body is difficult to obtain in single-exposure radiography while it is possible in scanning slot systems or with dedicated detector configurations in multiple-exposure digital radiography.
Multiple-exposure radiography allows the introduction of a novel technique of scattering suppression. The experimental evaluation of this technique is the main scope of our current work.
In our approach, a pair of X-ray masks positioned upstream (collimator) and downstream (anti-scatter grid) the patient, are coupled mechanically and moved in four steps in a square pattern in order to irradiate the full area in four consecutive short exposures.
In this way the total dose delivered to the body is the same as in the case of a single full-field exposure.The two masks are shaped in a projective geometry as an array of squared apertures whose dimensions scale with the side of the active X-ray detector element (at a fixed distance between phantom and detector)
The above scattering rejection scheme can be implemented in many ways. Two main configurations are considered in the following :
An array of digital X-ray detectors (shown as black squares in the picture) arranged in a projective geometry with the two masks. We have studied, by means of MonteCarlo simulation, the geometrical parameters of this configuration in order to optimize the rejection of the scattered radiation from the body and to minimize the presence of image artifacts.
Preliminary results have been presented ( proceedings of Vienna International Conference on Instrumentation - VCI 2001 - February 2001 to be published on Nucl. Instr. & Methods).
