|
Medical x-ray imaging systems must be carefully designed to ensure that images can be produced with the highest possible signal-to-noise ratio (SNR) for a given x-ray dose to the patient. In most practical systems, images result from the conversion of primary x-rays into secondary quanta such as optical photons or electrical charge, in multiple cascaded stages. The average number of quanta at each stage (per incident x ray) is often evaluated as the product of all preceding system gain factors and displayed graphically as a "quantum-accounting diagram" (QAD). The stage with the fewest quanta forms the quantum sink, and is generally the noise-determining stage. This "zero spatial-frequency" type analysis is simplistic, however, as it ignores the spatial spreading of secondary quanta that will further degrade image noise. We have recently extended the above approach by introducing a spatial-frequency dependent QAD, in which the number of quanta at each stage is expressed by the product of all preceding system gains and squared modulation transfer functions (MTFs). The results are displayed graphically and used to determine the quantum-sink stage as a function of spatial frequency. The visual impact of the non-zero spatial frequency quantum sink is illustrated in a Monte Carlo simulation of the cascading process. A hypothetical system consisting of a scintillating phosphor optically coupled to a CCD camera is used for illustrative purposes. It is shown that an inefficient optical system results in the addition of a quantum mottle to the images due to a secondary quantum sink in the number of optical quanta. The mottle appears to be uniform in frequencies, but in combination with the effect of the screen MTF masks high-frequency detail more than low-frequency detail. This secondary quantum sink can be minimized both by: i) increasing the efficiency of the optical system; and, ii) improving the highfrequency response of the screen. Increasing the optical efficiency reduces the secondary quantum mottle, thus improving visualization particularly at high frequencies. Improving the high-frequency response makes a slight improvement on the quantum mottle, and also increases the contrast of the high-frequency patterns. The combination improves visualization at high spatial frequencies. Interpretation of the QAD is assisted by a direct comparison with the corresponding Monte Carlo images. It is concluded that the secondary quantum sink results in a visible deterioration of image quality at any specified frequency when the QAD at that frequency is less than approximately five times the primary quantum sink QAD value.
|
