Evaluation of the anode voltage of a CT scanner during clinical use: the impact of bowtie filters

Monitoring the performance parameters of X‑ray diagnostic equipment, including CT scanners, is mandatory during the acceptance and periodic testing within medical institutions. This article addresses the importance of monitoring the anode voltage of X‑ray emitter. This is one of the key technical parameters during the diagnostic studies of the patients, which can affect the quality of visualization and safety of equipment operation. The impact of bowtie filters on the accuracy of anode voltage measurement was studied by using the following CT scanner: Somatom Definition AS, SOMATOM go.All (Siemens, Germany). The anode voltage was evaluated by noninvasive methods, involving universal dosimeters, with the equipment operating in the user mode. The tests were conducted in a clinical setting without a partial removal of the gantry (a structural element of the CT scanner, containing the “X-ray emitter—detector” system) or connection to a high-voltage circuit of the device. The distance between the sensors of the measuring device and the displacement of the center of the active zone relative to the position of the central axis of the X‑ray beam of the CT scanner were taken into account. The normalized kerma and anode voltage curves, as well as their approximating analytical functions were obtained using a Piranha R&F/M 657 universal dosimeter (RTI Electronics AB, Sweden) by displacing it relative to the laser centralizer of the CT scanner. Based on the calculation scheme of noninvasive estimation of the anode voltage, a formula was derived for determining the relative deviation of the measured value of this voltage. It was shown that such deviation is associated with the effect of the bowtie filter at the output of the X‑ray emitter of the CT scanner, and with the displacement of the center point of the active zone of the measuring device relative to the central axis of the X‑ray beam of the CT scanner. The validity of the data calculated based on the derived formula was confirmed experimentally. The obtained results will be useful for the personnel performing maintenance or monitoring of the performance parameters of X‑ray CT scanners.