Lothar Bätz

Requirements on amorphous semiconductors for medical X-ray detectors

Abstract Solid state X-ray detectors based on large-area amorphous semiconductors, such as amorphous silicon or selenium, have been developed. The requirements for various applications in medical diagnosis determine the boundary conditions for different detectors. Key parameters are image-receptor size, spatial resolution, image frequency, signal-to-noise ratio, and long-term stability. Conventional thorax radiographs are 43×35 cm 2 in size. Therefore, to replace film, large detectors are required. Mammograms need to display microcalcifications of some 100 μm in detail. Fluoroscopic images are taken at a rate of 30 images/s, or even faster. Moreover, all detectors need to deliver optimum image quality at the lowest tolerable dose. The lifetime should be at least 10 years. In this paper we discuss different types of detectors that could fulfill those needs. Two fundamental concepts are compared: a scintillator coupled to a photodiode and a direct converting semiconductor, both being arranged on an amorphous silicon-switching matrix.

Spatial Frequency-Dependent Signal-to-Noise Ratio as a Generalized Measure of Image Quality

A generalized, objective image quality measure can be defined for X-ray based medical projection imaging: the spatial frequency-dependent signal-to-noise ratio SNR = SNR(u,v). This function includes the three main image quality parameters, i.e. spatial resolution, object contrast, and noise. The quantity is intimately related to the DQE concept, however its focus is not to characterize the detector, but rather the detectability of a certain object embedded into a defined background. So also effects from focus size and radiation scatter can be quantified by this method. The SNR(u,v) is independent of basic linear post-processing steps such as appropriate windowing or spatial filtering. The consideration of the human visual system is beyond the scope of this concept. By means of this quantity, different X-ray systems and setups can be compared with each other and with theoretical calculations. Moreover, X-ray systems (i.e. detector, beam quality, geometry, anti-scatter grid, basic linear post-processing steps etc.) can be optimized to deliver the best object detectability for a given patient dose. In this paper SNR(u,v) is defined using analytical formulas. Furthermore, we demonstrate how it can be applied with a test phantom to a typical flat panel detector system by a combination of analytical calculations and Monte Carlo simulations. Finally the way this function can be used to optimize an X-ray imaging device is demonstrated.

Design of a contrast-enhanced dual-energy tomosynthesis system for breast cancer imaging

Digital breast tomosynthesis (DBT) is a three-dimensional X-ray imaging modality that has the potential to decrease the superimposition effect of breast structural noise, thereby increasing lesion conspicuity. To further improve breast cancer detection, our work has been devoted to develop a prototype for contrast-enhanced dual-energy tomosynthesis (CEDET). CEDET involves the injection of an iodinated contrast agent and measures the relative increase in uptake of contrast in the suspected breast cancer lesion. Either temporal or dual-energy subtraction techniques may be used to implement CEDET. Both 2D contrast-enhanced dual-energy mammography and 3D tomosynthesis can be applied. Here we present the design of a prototype CEDET system based on the Siemens MAMMOMAT Inspiration and employing two additional high-energy filters in addition to the standard Rh filter, the latter being used for the low-energy acquisitions. A quality factor of squared signal-difference-to-noise-ratio of iodine per pixel area and average glandular dose as a function of breast thickness is used to optimize the filter material, the filter thickness, and the tube voltage. The average glandular dose can be calculated from the entrance surface air kerma using computed conversion coefficients DgN for the used X-ray spectra. We also present the results of DQE measurements of the amorphous selenium detector involved. Finally, results of phantom tests for tomosynthesis acquisition and first clinical data in the 2D mode will be shown.