A recently proposed application of quantitative computed tomography is in the study of cerebral blood flow and partition coefficient using stable xenon as a freely diffusible, radio-opaque tracer. Central to the method is the calibration factor describing the relationship between CT number and xenon concentration in the brain. In this paper we examine the influence of temporal fluctuations, kVp, radial position and beam hardening on this calibration factor through the analysis of a series of phantom measurements. We conclude that under clinically realistic conditions and with correlations for temporal fluctuations, the error associated with the calibration factor is less than 2%. Furthermore, errors of this magnitude translate into errors of less than 3% in derived blood flow and partition coefficient values obtained using xenon-enhanced computed tomography.
Monte Carlo simulations have been used to study the accuracy which can be expected in the quantification of blood flow and the partition coefficient using xenon-enhanced computed tomography in the presence of noise. We have demonstrated that the markedly asymmetric frequency distribution of estimates increases in size rapidly with an increase in the standard error of the input CT data. On the basis of our results, we recommend that controllable sources of noise (eg. CT number drift) be corrected and that estimates be obtained by averaging CT data and then fitting, rather than averaging blood flow and partition coefficients derived from individual pixels, as the latter procedure results in the introduction of considerable bias.
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