1. A method comprising: determining voxels in a slice of interest in a three dimensional computed tomography scan of the interior of a body based on a first set of measurements of radiation transmitted through the body; determining, based on the first set of measurements, a first background image for radiation transmitted through the body in a first direction without the effects of the voxels in the slice of interest; determining a current image based on a different current measurement of radiation transmitted through the body in the first direction; and determining a first difference between the current image and the first background image to show contrast in the slice of interest.