Monochromatic X-rays have been proposed for medical imaging, especially in the mammographic energy range. Our previous investigations have shown that the contrast of objects such as lesions or contrast media can be enhanced considerably by using monochromatic X-rays instead of the common polychromatic spectra. Admittedly, only one specific polychromatic spectrum and one monochromatic energy have been compared so far. In this work, we investigated the contrast yielded by a series of different X-ray spectra obtained by varying tube voltage and beam filtering. This resulted in spectra of different mean energies and spectral widths. The objects under examination were aqueous solutions containing different chemical elements such as I, Gd, Dy, Yb, and Bi. A monoenergetic spectrum at 17.5 keV was obtained using a mammographic X-ray tube with a Mo anode and a monochromator equipped with a HOPG crystal. Moreover, we simulated quasi-monoenergetic spectra at different energies and with different widths. As a result, we demonstrated that in many cases spectra with an energetic width of some keV yield an equivalent contrast to monoenergetic radiation at the same energy. Therefore, the advantage in image contrast of monochromatic X-rays at 17.5 keV over narrow-band polychromatic X-ray spectra obtained by appropriate filtering is only slight. Thus, the additional expenditure on a mammography system with HOPG monochromator that can deliver only a small X-ray dose and the unfavorable slot-scan geometry can be avoided. Moreover, we carried out simulations of monochromatic versus polychromatic spectra throughout the whole radiographic energy range. We found advantages in using monochromatic X-rays at higher energies and thicker objects that will justify their application for diagnostic imaging in a number of specific cases.
We are investigating the advantage of scatter removal by the slot-scanning method compared to antiscatter grids. We carry out model calculations for the signal-to-noise ratio simulating different geometrical settings for slot-scan systems. The results are compared with those for standard nonscanning mammography systems with and without anti-scatter grid. Monte Carlo simulations are performed in order to get a realistic amount of scatter radiation as input for the model estimates. We present the results as function of the compressed breast thickness equivalent to the scatter fraction. It is demonstrated that a perfect slot-scan system with 100% transmission of primary radiation and 100% suppression of scattered radiation improves SNR2, and correspondingly reduces dose, by a factor of less than 1.8, compared with conventional anti-scatter grids and otherwise the same detector DQE. For realistic geometry the advantage is considerably smaller. The advantage of scatter removal by employing a slot-scanning method is moderate because the scatter fraction is relatively low in mammography. For breast thickness up to 5 cm it turns out that it is advantageous to work without a grid due to the low scatter fraction, which questions a scatter reduction method in that region at all. The model can be used as a simple design tool.
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.
The requirements for medical X-ray detectors tend towards higher spatial resolution, especially for mammography. Therefore, we have investigated common absorber materials with respect to the possible intrinsic limitations of their spatial resolution.
Primary interaction of an incident X-ray quantum is followed by a series of processes: Rayleigh scattering, Compton effect, or the generation of fluorescence photons and subsequent electrons. Lateral diffusion of carriers relative to their drift towards the electrodes also broadens the point-spread function. One consequence is that the spatial resolution of the detector, expressed in terms of the modulation transfer function (MTF), is reduced.
Monte Carlo simulations have been carried out for spectra with tube voltages of 28-120 kV using the program ROSI (Roentgen Simulation) based on the well-established EGS4 algorithm. The lateral distribution of deposited energy has been calculated in typical materials such as Se, CdTe, HgI2, and PbI2 and used to determine the line spread function.
The complex absorption process is found to determine the spatial resolution of the detector considerably. The spectrum at energies closely above the K-edge of the absorber material tends to result in a reduced MTF. At energies above 50 keV, electron energy loss increasingly reduces spatial resolution in the high frequency range. The influence of fluorescence is strongest in the 5-20 lp/mm range. If a very high spatial resolution is required, a well-adapted semiconductor should be applied.