A method to estimate transmission profiles of bow-tie filters using rotating tube measurements.

PURPOSE: The purpose of this paper was to present a method of determining the dose profile of the beam bow-tie filter (BTF) without the need for fixing the x ray tube in a position or using special instruments or dosimeters other than the ordinary types of ion chambers used for CT dosimetry (e.g., Farmer chamber).

METHODS: The idea behind this method is to try to invert the integral of exposures from axial measurements by decomposing it into fractions per degree of tube rotation. Measurements of the CT tube output were taken with a full tube rotation while the chamber was fixed in air. Starting with isocenter the output measurements were performed at 1-cm interval above the isocenter. Measurements were repeated for three sizes of BTFs; small, medium, and large. Maximum fan angle per chamber position was computed and an effective fan angle was defined to account for the new angular range encountered per chamber position. Variation due to inverse-square law was isolated from each measurement and contribution from the effective fan angle was computed. Resulted profiles from this method were then compared to profiles obtained with direct measurements, when the tube was in a fixed position. Effects of over and under 360° rotation per scan on results accuracy were also investigated.

RESULTS: Using the direct measurements as the gold standard, results from this method were accurate to 4% for most of the BTFs angular ranges. The average relative error in the small BTF was 1.5%. In the medium BTF, the average relative error was <3% for up to 16° fan angle. With the large BTF, the mean error was about 4% for up to 22°. The relative error appeared to increase at larger fan angles especially with the large BTF; reaching an average of about 32% for fan angles between 22° and 27.5°.

CONCLUSION: The presented method is relatively easy to perform and provides BTFs profiles with reasonably good accuracy. Associated errors of >10% only appear in high angles of large and medium BTFs.