Abstract
The purpose of this study was to investigate the frequency and impact of vertical mis‐centering on organ doses in computed tomography (CT) exams and evaluate the effect of a commercially available positioning compensation system (PCS). Mis‐centering frequency and magnitude was retrospectively measured in 300 patients examined with chest‐abdomen‐pelvis CT. Organ doses were measured in three postmortem subjects scanned on a CT scanner at nine different vertical table positions (maximum shift ± 4 cm). Organ doses were measured with optically stimulated luminescent dosimeters inserted within organs. Regression analysis was performed to determine the correlation between organ doses and mis‐centering. Methods were repeated using a PCS that automatically detects the table offset to adjust tube current output accordingly. Clinical mis‐centering was >1 cm in 53% and 21% of patients in the vertical and lateral directions, respectively. The 1‐cm table shifts resulted in organ dose differences up to 8%, while 4‐cm shifts resulted in organ dose differences up to 35%. Organ doses increased linearly with superior table shifts for the lung, colon, uterus, ovaries, and skin (R 2 = 0.73–0.99, P < 0.005). When the PCS was utilized, organ doses decreased with superior table shifts and dose differences were lower (average 5%, maximum 18%) than scans performed without PCS (average 9%, maximum 35%) at all table shifts. Mis‐centering occurs frequently in the clinic and has a significant effect on patient dose. While accurate patient positioning remains important for maintaining optimal imaging conditions, a PCS has been shown to reduce the effects of patient mis‐centering.
Highlights
IntroductionModern computed tomography (CT) scanners are equipped with several technological innovations that serve to optimize radiation dose.[1,2]
The frequency of clinical mis‐centering is likely dependent on the technologist training methods and years of experience, so it is not expected that one hospital would have the same frequency of clinical patient mis‐centering as another
Similar findings were reported in other studies, where one group found that 17% of their patients were mis‐centered by >3 cm,[10] and another group reported that 22% of their patients were mis‐centered by >3 cm with maximum errors ranging from 6.6 cm posterior to 3.4 cm anteriorly.[6]
Summary
Modern computed tomography (CT) scanners are equipped with several technological innovations that serve to optimize radiation dose.[1,2]. Among these technologies, automatic tube current modulation (TCM) adjusts the tube current to the specific size and shape of the patient in order to produce diagnostic image quality with minimal radiation exposure to the patient.[3–5]. The optimal function of a bowtie filter and TCM techniques require that the patient is centered appropriately in the CT gantry.[8– 14]. When a patient is placed on the CT table, the technologist should attempt to position the patient in the center of the gantry using the gantry‐mounted laser system. The technologist acquires a localizer radiograph, which serves to measure patient attenuation for proper TCM, as well as help verify correct patient positioning. The technologist should correct the patient's position and acquire a new localizer radiograph
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