The detection limitations inherent in statistically limited computed tomographic (CT) images are described through the application of signal detection theory. The delectability of large-area, loc-contrast objects is shown to be chiefly dependent upon the low-frequency content of the noise power spectral density. For projection data containing uncorrelated noise, the resulting ramplike, low-frequency behavior of the noise power spectrum of CT reconstructions may be conveniently characterized by the number of noise-equivalent x-ray quanta (NEQ) detected in the projection measurements. The NEQ for a given image may be determined either from a measurement of the noise power spectrum or from the noise granularity computed with an appropriate weighting function. A measure of the efficiency of the scanner does utilization is proposed, which compares the average dose required by an ideal scanner to obtain the same NEQ.
Keywords: detectability in CT images, computed tomography (CT), noise equivalent quanta (NEQ), noise power spectrum (NPS), noise granularity, dose efficiency, signal detection theory
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