Optimal Dose Utilization With Variable X-Ray Intensity In Digital Radiography

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Abstract
For the case of radiographic contrast media flowing through an otherwise stationary object, a theoretical analysis shows that image enhancement with optimal dose utilization can be achieved by varying the x-ray intensity during the sequence of exposures. The need for variable intensity derives from the conflicting requirements of continuous visualization of the time course of the contrast media and the fact that dose utilization in a difference image is maximized when all the x-ray photons are divided between minimum and maximum opacification. A set of optimal weights for combining multiple images are derived as a function of the x-ray intensity in each image and the effects of photon noise, camera (video system) noise, and digital truncation noise. It is shown theoretically that controlled variation of x-ray intensity may improve the signal to noise to dose ratio by approximately a factor of 2 when compared to conventional matched filtering. If the x-ray intensity remains constant for all images, the optimal filter weights reduce to those of the expected matched filter. A simple implementation of optimal filtering using only two intensities is described mathematically and demonstrated experimentally.

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