A fan-shaped x-ray beam is utilized to successively measure the x-ray attenuation across the patient for the required number of projection angles. 3.1, left) a multicellular linear detector array and the x-ray tube are mounted on the same carrier, which rotates around the patient. 1įigure 3.1 shows the two principal measurement geometries used in today's CT scanners. It turns out that the resulting ''beam hardening artifacts" can be corrected to a sufficient degree by a simple nonlinear correction to the measured line integrals. The resulting attenuation coefficients are usually expressed with reference to water:Įquations 3.1 and 3.2 ignore the fact that the spectrum of the x-rays entering the patient is generally non-monochromatic. In CT scanners the x-ray attenuation according to equation 3.2 is measured along a variety of lines within a plane perpendicular to the long axis of the patient with the goal of reconstructing a map of the attenuation coefficients a for this plane. This model is explored further in section 14.1.1. With the recent introduction of spiral CT (section 3.3), x-ray CT, like other imaging modalities, has started to move from a slice-by-slice to a volume imaging method, with more isotropic spatial resolution in all three dimensions allowing a better three-dimensional representation of anatomical structures. With today's typical scan parameters there is a discrepancy between the planar resolution, which is less than 1 mm, and the slice thickness, which customarily is several millimeters. An improvement in the transaxial or planar spatial resolution is sensible only if the resolution as measured along the scanner axis (i.e., the slice thickness) is improved simultaneously. Further increasing the contrast resolution will require an increase in the patient dose, since the efficiency of the detector systems is already close to its theoretical limits. The greatest progress has been made in reducing scan times and in improving spatial resolution. Technological Improvements in the First 20 Years of Computed Tomography.
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