Hartmut Sklebitz

OPDIMA: large-area CCD-based x-ray image sensor for spot imaging and biopsy control in mammography

The detection characteristics of digital x-ray and film-screen mammography systems are different and thus current film-screen techniques are not ideal for digital mammography. Therefore optimum technical parameters required for digital mammography are likely to be different compared with film-screen mammography. The goal of this study is to evaluate the optimum technical parameters for full-field digital mammography by experimental and computer simulation methods. A General Electric Full Field Digital Mammography (FFDM) prototype unit using Cesium Iodide (CsI) on an amorphous Silicon photodiode array was used for the experimental measurements. Using breast equivalent phantoms, images were acquired for a set of x-ray target-filters for a range of peak kilovoltage, varying breast composition and thickness, with and without an anti-scatter grid. The signal-to-noise ratio (SNR) and figure-of-merit (FOM) were determined for simulated calcification and mass targets, independently by the two methods. The results for noise, contrast, SNR and FOM were compared and agree within 5% and 6% respectively. Combined results are presented for the case of 50% glandular - 50% adipose tissue breast composition using the grid and for the calcification target. Based on the FOM approach, preliminary results suggest that a Rhodium target-filter combination will be beneficial for higher breast thickness and for denser breasts.

Lens optical coupling versus fiber optical coupling: the impact on x-ray TV image quality with CCDs and pick-up tubes

The limited light sensitivity of the TV-pick-up devices and the desire for low dose imaging in X-ray fluoroscopy causes the use of extremly high aperture lens optics between X-ray image intensifier and TV-sensor (1). Therefore a contrast reducing optical feedback between the TV-sensor and the output screen of the X-ray image intensifier may occur, if the light of the output screen of the X-ray image intensifier is reflected from the sensors photoconductor. The contrast relations in the case of lens coupling of X-ray IL of different output structures with some CCD-sensors and different kinds of pick-up tubes have been measured and will be discussed. One possible way to reduce contrast loss due to stray light is direct fiber optical coupling. We investigated such systems with fiber optic pick-up tubes and a CCD equipped with a fiber optic plate in front of the sensor area coupled via a fiber optical taper to an X-ray image intensifier with fiber optic output window. The drawback of every direct fiber optical coupling is, however, that the light flux to the TV-sensor cannot be controlled. Therefore an adaptation of such a system to the practical needs of varying indMdual applications cannot be performed. Some imaging properties oflens coupling, fiber optical coupling and mixed coupling (fiber optics and lens optics in one system) have been evaluated and will be compared. Some precautions against loss in contrast and requirements regarding signal to noise ratio will be outlined.