BACKGROUND: Image quality and radiation dose to the patient are important factors in computed tomography (CT). To provide constant image quality, tube current modulation (TCM) performed by automatic exposure control (AEC) adjusts the tube current to the patient's size and shape. PURPOSE: To evaluate the effects of patient centering on tube current-time product (mAs) and image noise. MATERIAL AND METHODS: An oval-shaped acrylic phantom was scanned in various off-center positions, at 30-mm intervals within a 500-mm field of view, using three different CT scanners. Acquisition parameters were similar to routine abdomen examinations at each site. The mAs was recorded and noise measured in the images. The correlation of mAs and noise with position was calculated using Pearson correlation. RESULTS: In all three scanners, the mAs delivered by the AEC changed with y-position of the phantom (P<0.001), with correlation values of 0.98 for scanners A and B and -0.98 for scanner C. With x-position, mAs changes were 4.9% or less. As the phantom moved into the y-positions, compared with the iso-center, the mAs varied by up to +70%, -34%, and +56% in scanners A, B, and C, respectively. For scanners A and B, noise in two regions of interest in the lower part of the phantom decreased with elevation, with correlation factors from -0.95 to -0.86 (P<0.02). In the x-direction, significant noise relationships (P<0.005) were only seen in scanner A. CONCLUSION: This study demonstrates that patient centering markedly affects the efficacy of AEC function and that tube current changes vary between scanners. Tube position when acquiring the scout projection radiograph is decisive for the direction of the mAs change. Off-center patient positions cause errors in tube current modulation that can outweigh the dose reduction gained by AEC use, and image quality is affected.
BACKGROUND: Image quality and radiation dose to the patient are important factors in computed tomography (CT). To provide constant image quality, tube current modulation (TCM) performed by automatic exposure control (AEC) adjusts the tube current to the patient's size and shape. PURPOSE: To evaluate the effects of patient centering on tube current-time product (mAs) and image noise. MATERIAL AND METHODS: An oval-shaped acrylic phantom was scanned in various off-center positions, at 30-mm intervals within a 500-mm field of view, using three different CT scanners. Acquisition parameters were similar to routine abdomen examinations at each site. The mAs was recorded and noise measured in the images. The correlation of mAs and noise with position was calculated using Pearson correlation. RESULTS: In all three scanners, the mAs delivered by the AEC changed with y-position of the phantom (P<0.001), with correlation values of 0.98 for scanners A and B and -0.98 for scanner C. With x-position, mAs changes were 4.9% or less. As the phantom moved into the y-positions, compared with the iso-center, the mAs varied by up to +70%, -34%, and +56% in scanners A, B, and C, respectively. For scanners A and B, noise in two regions of interest in the lower part of the phantom decreased with elevation, with correlation factors from -0.95 to -0.86 (P<0.02). In the x-direction, significant noise relationships (P<0.005) were only seen in scanner A. CONCLUSION: This study demonstrates that patient centering markedly affects the efficacy of AEC function and that tube current changes vary between scanners. Tube position when acquiring the scout projection radiograph is decisive for the direction of the mAs change. Off-center patient positions cause errors in tube current modulation that can outweigh the dose reduction gained by AEC use, and image quality is affected.
Authors: Ranish Deedar Ali Khawaja; Sarabjeet Singh; Atul Padole; Alexi Otrakji; Diego Lira; Da Zhang; Bob Liu; Andrew Primak; George Xu; Mannudeep K Kalra Journal: Radiat Prot Dosimetry Date: 2017-08-01 Impact factor: 0.972
Authors: Sergio Salerno; Giuseppe Lo Re; Davide Bellini; Marco Rengo; Maurizio Marrale; Maria Chiara Terranova; Laura Scopelliti; Andrea Laghi Journal: Radiol Med Date: 2019-03-08 Impact factor: 3.469
Authors: Lienard A Chang; Donald L Miller; Choonsik Lee; Dunstana R Melo; Daphnée Villoing; Vladimir Drozdovitch; Isabelle Thierry-Chef; Sarah J Winters; Michael Labrake; Charles F Myers; Hyeyeun Lim; Cari M Kitahara; Martha S Linet; Steven L Simon Journal: Health Phys Date: 2017-12 Impact factor: 1.316
Authors: Timothy P Szczykutowicz; Robert K Bour; Nicholas Rubert; Gary Wendt; Myron Pozniak; Frank N Ranallo Journal: J Appl Clin Med Phys Date: 2015-07-08 Impact factor: 2.102