Masteruppsatser / Medicinsk strålningsvetenskap

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Browsing Masteruppsatser / Medicinsk strålningsvetenskap by Subject "Anti-scatter grid"

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    Automatic exposure control for digital radiography without a physical anti-scatter grid
    (2025-09-04) Ring, Louise; University of Gothenburg/Institute of Clinical Sciences; Göteborgs universitet/Institutionen för kliniska vetenskaper
    Purpose: This study aimed to find a factor R for different parameter settings that can be used to adjust the AEC preset value, in order to maintain a constant amount of primary radiation to the detector when the anti-scatter grid is removed. Method and materials: A factor R was calculated as the fraction between dose-area product (DAP) with and without an anti-scatter grid, from each exposure on polymethylmethacrylate (PMMA) phantoms, multiplied with a primary transmission factor Tp. These measurements lead to a deeper understanding of how varying PMMA thickness, kV and field size would influence R. Second, anthropomorphic phantoms were examined, with a built-in AEC assistance detector (BiAA), while increasing their thicknesses and using different kV and field sizes. An adult chest phantom were examined in anteroposterior (AP) and lateral (LAT) projections (AP/LAT thickness: 21/29, 27/35, 32/39 cm). An adult pelvis and hip phantom (thickness: 23, 29, 35 cm) was examined in AP projection. A pediatric chest phantom (thickness: 15 cm) was examined in AP projection. Results: The Rs from examination of the PMMA resulted in an increase as the thickness increased. This trend was also observed when the field size increased. However, as the kV increased, R decreased. The adult chest (thickness: 21, 27 and 32 cm) AP examination resulted in Rs of 1.13, 1.36 and 1.59. The adult chest (thickness: 29, 35, 39 cm) LAT examination resulted in Rs of 1.20, 1.36 and 1.51, with 141 kV. The adult pelvis (thickness: 23, 29, 35 cm) AP examination resulted in Rs of 2.89, 3.49 and 4.13 with 70 kV. The Rs for the hip examination were 2.34, 2.95 and 2.62. The pediatric chest (thickness: 15 cm) AP examination resulted in an R of 1.08. Discussion and conclusion: R had a similar relative increase for the examination of the adult chest and pelvis with increasing thickness, but R was higher for the pelvis. The change in R between the different thicknesses and phantoms was substantial, which implies the importance of optimizing the examination protocol according to patient size during a gridless examination with AEC. The difference in R also means that the protocol should include a combination of R for a specific anatomic region and patient size, for a finer adjustment of the AEC preset dose. The results of the pediatric phantom measurement, where two different mA were used, the lower mA increased the exposure time. However, the exposure time remained shorter than the minimum recommended by the generator manufacturer. This implies that for smaller thicknesses, the AEC may not be reliable. This study can be used as a base for further studies, and the performance of virtual anti-scatter grids and its impact on image quality needs to be investigated.