Affiliation:
1. Division of Diagnostic Imaging Physics Department of Radiology Massachusetts General Hospital Boston Massachusetts USA
2. Department of Radiology Mayo Clinic Rochester Minnesota USA
3. Department of Radiology Boston University Medical Center Boston Massachusetts USA
Abstract
AbstractBackgroundIt is challenging to assess the accuracy of volume CT Dose Index (CTDIvol) when the axial scan modes corresponding to a helical scan protocol are not available. An alternative approach was proposed to directly measure using helical acquisitions and relatively small differences (< 20%) from CTDIvol were observed.PurposeTo visually demonstrate the 3D dose distribution for both axial and helical CT acquisitions and quantitively compare and CTDIvol.Methods3D dose distribution within the standard CTDI phantoms (16 and 32 cm diameter) from a single CT projection, Dp(x,y,z) was first generated using Monte Carlo simulation (GEANT4) with 9×108 photons per combination of tube voltage (80–140 kV), collimation width (1–8 cm), and z‐axis location of the central ray of the x‐ray beam, with a spatial resolution of 1 mm3. These dose distributions from one single projection were analytically ensembled to simulate 3D dose volumes DA(x,y,z) and DH(x,y,z) for axial and helical scans, respectively, with different helical pitches (0.3–2) and scan lengths (100–150 mm). 2D planar dose distributions were obtained by integrating the inside 100 mm of the dose volumes. CTDIvol and were calculated using the planar dose data at corresponding pencil chamber locations and the percentage differences (PD) were reported.ResultsHigh spatial resolution 3D CT dose volumes were generated and visualized. PDs between and CTDIvol had strong dependency on scan length and peripheral chamber locations, with subtle dependency on collimation width and pitch. PDs were mostly within the range of ± 3% for a scan length of 150 mm with four peripheral chamber locations.ConclusionsWith a scan length covering the entire phantom length, directly measured from helical scans can serve as an alternative to CTDIvol only if all four peripheral locations were measured.
Reference28 articles.
1. The Measurement, Reporting, and Management of Radiation Dose in CT
2. Comprehensive Methodology for the Evaluation of Radiation Dose in X-Ray Computed Tomography
3. The CTDI paradigm: a practical explanation for medical physicists;Dixon RL;J Am Coll Radiol,2010
4. Radiation Dose in X-ray Computed Tomography
5. Report No. 233 ‐ Performance Evaluation of Computed Tomography Systems ‐ The Report of AAPM Task Group 233;Samei E;Am Assoc Phys Med,2019