Richard L. VanMetter

An Analysis Of The Fundamental Limitations Of Screen-Film Systems For X-Ray Detection I. General Theory

A model is developed for the overall DQE of an x-ray screen-film system, and this is related to the component DQEs of the screen and film. The model characterizes the screen in terms of its x-ray absorption efficiency, MTF, and the statistics associated with the conversion of absorbed x-rays to emitted light photons. This model establishes that the film need be characterized solely by its DQE with respect to the emitted light photons. We show that the only explicit exposure dependence of the screen-film DQE results from the film DQE, while the spatial-frequency dependence is dominated by the screen MTF. However, depending on the screen gain, a significant exposure/spatial-frequency domain may exist where the overall DQE is independent of both of these factors and depends mainly on x-ray absorption efficiency.

Principles Governing The Transfer Of Signal Modulation And Photon Noise By Amplifying And Scattering Mechanisms

A general analysis is made of the influence of stochastic amplifying and scattering mechanisms on the transfer of signal modulation and photon noise in imaging processes. In this way we quantify the spatial-frequency dependence of signal and noise as they propagate through a multistage imaging system. Whereas by definition the signal structure (or modulation) is transferred via the MTF, the input photon noise is effectively unmodulated signal and as such bypasses the MTF. However, stochastic amplification of photon noise by one stage of an imaging process may produce noise structure that constitutes an effective signal spectrum to the next. Thus, in general, it is necessary to cascade these two components of the noise spectrum separately at each stage.

An Analysis Of The Fundamental Limitations Of Screen-Film Systems For X-Ray Detection. II. Model Calculations

A range of model DQE calculations illustrates the detailed influence of the most important screen-film parameters. The screen is characterized by typical values for the primary x-ray absorption, spread function associated with secondary photons, and statistics associated with the gain of the conversion process. An existing film DQE model is used to explore the effect of film parameters on screen-film performance. Examples are given of the conditions under which screen-film DQE becomes independent of both film DQE and screen-film MTF. The influence of all these factors on system performance is discussed.

The Role Of Screen And Film In Determining The Noise-Equivalent Number Of Quanta Recorded By A Screen-Film System

We apply an existing model for the relationship governing the DQE characteristics of a screen-film system to the calculation of absolute values of the noise-equivalent number of quanta (NEQ). The roles of the screen and film are investigated separately, and the influence of interactive properties such as screen amplification and grain size is explored. Hence factors limiting the NEQ of existing screen-film systems are identified.

The Effect Of Bias Exposure On The Detective Quantum Efficiency Of Radiographic Screen-Film Systems

The detective quantum efficiency (DQE) of radiographic screen-film systems is limited at low exposures by the threshold response of silver halide grains. Model calculations predict improvements in DQE at low x-ray exposures with a uniform (light) bias exposure to the film. The DQE gains predicted are compared with those established in the literature for the case where the signal exposure is also a direct light exposure. An optimum bias exposure has been calculated that balances the increase in grain sensitivity against the increase in fog to maximize DQE for the limit of very small exposures.