Attenuation‐based size metric for estimating organ dose to patients undergoing tube current modulated CT exams. Issue 2 (29th January 2015)

• Record Type: Journal Article
• Authors: Bostani, Maryam; McMillan, Kyle; Lu, Peiyun; Kim, Hyun J.; Cagnon, Chris H.; DeMarco, John J.; McNitt‐Gray, Michael F.
• Is Part Of: Medical physics. Volume 42:Issue 2(2015)
• Page Start: 958
• Page End: 968
• Publication Date: 2015-01-29
• Subjects: approximation theory -- biological tissues -- biomedical equipment -- bio‐optics -- computerised tomography -- correlation methods -- data acquisition -- dosimetry -- feature extraction -- feature selection -- image segmentation -- kidney -- liver -- lung -- medical image processing -- Monte Carlo methods -- physiological models -- regression analysis -- water -- X‐ray tubes; Computed tomography -- Dosimetry/exposure assessment -- Dose‐volume analysis -- Algorithms -- Segmentation; Computerised tomographs -- Biological material, e.g. blood, urine; Haemocytometers -- Methods or arrangements for processing data by operating upon the order or content of the data handled -- Digital computing or data processing equipment or methods, specially adapted for specific applications -- Image data processing or generation, in general -- X‐ray tubes -- Devices sensitive to very short wavelength, e.g. x‐rays, gamma‐rays or corpuscular radiation -- Transforming x‐rays -- X‐ray technique -- Scintigraphy; CT -- Monte Carlo -- dosimetry -- tube current modulation -- water equivalent diameter; Lungs -- Computed tomography -- Dosimetry -- Monte Carlo methods -- Image scanners -- Kidneys -- Tissues -- Medical image segmentation; Medical physics -- Periodicals; Medical physics; Geneeskunde; Natuurkunde; Toepassingen; Biophysics; Periodicals; Periodicals; Electronic journals; 610.153
• DOI: 10.1118/1.4906132
• Languages: English
• ISSNs: 0094-2405

Abstract

Abstract : Purpose: Task Group 204 introduced effective diameter (ED) as the patient size metric used to correlate size‐specific‐dose‐estimates. However, this size metric fails to account for patient attenuation properties and has been suggested to be replaced by an attenuation‐based size metric, water equivalent diameter ( DW ). The purpose of this study is to investigate different size metrics, effective diameter, and water equivalent diameter, in combination with regional descriptions of scanner output to establish the most appropriate size metric to be used as a predictor for organ dose in tube current modulated CT exams. Methods: 101 thoracic and 82 abdomen/pelvis scans from clinically indicated CT exams were collected retrospectively from a multidetector row CT (Sensation 64, Siemens Healthcare) with Institutional Review Board approval to generate voxelized patient models. Fully irradiated organs (lung and breasts in thoracic scans and liver, kidneys, and spleen in abdominal scans) were segmented and used as tally regions in Monte Carlo simulations for reporting organ dose. Along with image data, raw projection data were collected to obtain tube current information for simulating tube current modulation scans using Monte Carlo methods. Additionally, previously described patient size metrics [ED, DW, and approximated water equivalent diameter ( D Wa )] were calculated for each patient and reported in three different ways: a single value averaged over the entire scan, a single value averaged over the region of interest, and a single value from a location in the middle of the scan volume. Organ doses were normalized by an appropriate mAs weighted CTDIvol to reflect regional variation of tube current. Linear regression analysis was used to evaluate the correlations between normalized organ doses and each size metric. Results: For the abdominal organs, the correlations between normalized organ dose and size metric were overall slightly higher for all three differently (global, regional, and middle slice) reported DW and D Wa than they were for ED, but the differences were not statistically significant. However, for lung dose, computed correlations using water equivalent diameter calculated in the middle of the image data ( D W, middle ) and averaged over the low attenuating region of lung ( D W, regional ) were statistically significantly higher than correlations of normalized lung dose with ED. Conclusions: To conclude, effective diameter and water equivalent diameter are very similar in abdominal regions; however, their difference becomes noticeable in lungs. Water equivalent diameter, specifically reported as a regional average and middle of scan volume, was shown to be better predictors of lung dose. Therefore, an attenuation‐based size metric (water equivalent diameter) is recommended because it is more robust across different anatomic regions. Additionally, it was observed that the regional size metric reported as a single value averaged over a region of interest and the size metric calculated from a single slice/image chosen from the middle of the scan volume are highly correlated for these specific patient models and scan types.