Abstract
PurposeTo determine the effect of patient’s vertical off-centering and scout direction on the function of automatic tube voltage selection (ATVS) and tube current modulation (TCM) in chest computed tomography (CT). MethodsChest phantom was scanned with Siemens and GE CT systems using three clinical chest CT protocols exploiting ATVS and a fixed 120 kVp chest protocol. The scans were performed at five vertical positions of the phantom (−6 to +6 cm from the scanner isocenter). The effects of scout direction (posterior-to-anterior, anterior-to-posterior, and lateral) and vertical off-centering on the function of ATVS and TCM were studied by examining changes in selected voltage, radiation dose (volume CT dose index, CTDIvol), and image noise and contrast. ResultsBoth scout direction and vertical off-centering affected ATVS. The effect differed between the vendors for the studied geometry, demonstrating differences in technical approaches. The greatest observed increase in CTDIvol due to off-centering was 91%. Anterior-to-posterior scout produced highest doses at the uppermost table position, whereas posterior-to-anterior scout produced highest doses at the lowermost table position. Dose varied least using lateral scouts. Vertical off-centering impacted image noise and contrast due to the combined effect of ATVS, TCM, structural noise, and bowtie filters. ConclusionsPatient vertical off-centering and scout direction affected substantially the CTDIvol and image quality in chest CT examinations. Vertical off-centering caused variation also in the selected tube voltage. The function of ATVS and TCM methods differ significantly between the CT vendors, resulting in differences in CTDIvol and image noise characteristics.
Highlights
Medical radiation exposure to patients in diagnostics has increased mostly because of the growing use of computed tomography (CT) [1,2,3]
Several studies have previously shown remarkable effects of patient off-centering on patient radiation dose and image quality due to function of beam shaping filters and geometric magnification/minification resulted in the scout images [20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36]
The previous patient positioning studies have mainly investigated the function of tube current modulation (TCM) and only in a single limited study has the effect of automatic tube voltage selection (ATVS) on radiation dose been investigated [30]
Summary
Medical radiation exposure to patients in diagnostics has increased mostly because of the growing use of computed tomography (CT) [1,2,3]. The increased use of CT in medical imaging has driven optimization efforts, including both technical and clinical approaches, to decrease radiation dose for the patients while maintaining the sufficient image quality for diagnosis. Despite the effective technical CT optimization tools, the role of user remains important to achieve optimal results both in terms of image quality and radiation dose. Several studies have previously shown remarkable effects of patient off-centering on patient radiation dose and image quality due to function of beam shaping filters and geometric magnification/minification resulted in the scout images (planning radiographs) [20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36]. The optimal function of the bowtie filter and TCM techniques assume that the patient is centered on the scan isocenter
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