Measurements of computed tomography x-ray fields utilizing the partial volume effect

Due to the x-ray field geometry, exposure measurements on computed tomographic (CT) scanners require modification of widely used measurement techniques. Ionization chambers have geometries which do not allow measurement of narrow rotating x-ray fields. However, by utilizing the partial volume effect, an ion chamber can be calibrated to measure narrow x-ray fields with a spatial resolution of down to 1 mm. The number of scans required to reconstruct a complete profile depends on the resolution required, and is typically on the order of 10 to 20 scans. This is accomplished using a series of attenuating sleeves over the ionization chamber. The chamber and sleeves are calibrated against a standard chamber using large field geometries. Comparisons with TLD measurements are shown to verify the accuracy of the technique.