Monte-Carlo based optimization of tube voltage and tube current as a function of object size and shape for clinical CT

Abstract

Purpose: CT image quality can be quantified by spatial resolution, contrast and image noise. Spatial resolution basically depends on the scanner geometry, on the scan mode and on reconstruction parameters and can be quantified to a high degree of accuracy using linear system theory. Noise and contrast, however, depend in a complicated way on the object itself and on the x-ray quality. Current practice in medical CT is to use scan protocols with a fixed spectrum, e.g. 120 kV. We investigated the relationship between object size, object shape, image quality and dose as a function of the x-ray spectrum. Method and materials: Based on the image signal and noise by mathematical simulation and on the Monte Carlo-based computation of the 3D dose distribution we predict the optimal spectrum for a given object. Water-equivalent phantoms of oval and circular shape with diameters ranging from 10 to 500 mm with water and iodine inserts were simulated and reconstructed using polychromatic CT spectra from 30 to 200 kV. Signal S and noise N within the central insert and CTDI-type weighted dose D applied to the phantom was used to compute the signal-to-noise ratio at unit dose SNRD^2=S*S/N*N*D. For a given object size, shape and insert type the optimum tube voltage is defined to be the one with the maximum SNRD. The corresponding optimal tube current is to be chosen proportional to SNRD^-2. Results: SNRD decreases with increasing object size. For a given object size and shape SNRD shows a well pronounced maximum at the optimal tube voltage. Tube voltage should be adapted to object size and shape. E.g. 2:1 ovals that mimic the human thorax with 120, 240, 360 and 480 mm width are best imaged at 80, 110, 180 and 200 kV to obtain the optimal SNRD. For pediatric CT scans low tube voltages and low currents should be used. Given that the optimal tube voltage is used tube current must be doubled for an increase of object size of about 5 cm (effective half value layer). Conclusions: The optimal spectrum strongly depends on the object size. For small objects such as in pediatric CT, CT mammography or micro CT, low tube voltages and low mAs values must be used; for large objects higher energies than in practice today are of interest.