American Association of Physicists in Medicine (AAPM) Report No.204 recommends the size-specific dose estimates (SSDE), wherein SSDE=computed tomography dose index-volume (CTDIvol )×size correction factor (SCF), as an index of the CT dose to consider patient thickness. However, the study on SSDE has not been made yet for area detector CT (ADCT) device such as a 320-row CT scanner. The purpose of this study was to evaluate the SCF values for ADCT by means of a simulation technique to look into the differences in SCF values due to beam width. In the simulation, to construct the geometry of the Aquilion ONE X-ray CT system (120 kV), the dose ratio and the effective energies were measured in the cone angle and fan angle directions, and these were incorporated into the simulation code, Electron Gamma Shower Ver.5 (EGS5). By changing the thickness of a PMMA phantom from 8 cm to 40 cm, CTDIvol and SCF were determined. The SCF values for the beam widths in conventional and volume scans were calculated. The differences among the SCF values of conventional, volume scans, and AAPM were up to 23.0%. However, when SCF values were normalized in a phantom of 16 cm diameter, the error tended to decrease for the cases of thin body thickness, such as those of children. It was concluded that even if beam width and device are different, the SCF values recommended by AAPM are useful in clinical situations.
It is generally known that the dose distribution around the high-density materials is not accurate with commercially available radiation treatment planning systems (RTPS). Recently, Acuros XB (AXB) has been clinically available for dose calculation algorithm. The AXB is based on the linear Boltzmann transport equation – the governing equation – that describes the distribution of radiation particles resulting from their interactions with matter. The purpose of this study was to evaluate the dose calculation accuracy around high-density materials for AXB under three X-rays energy on the basis of measured values with EBT3 and compare AXB with various dose calculation algorithms (AAA, XVMC) in RTPS and Monte Carlo. First, two different metals, including titanium and stainless steel, were inserted at the center of a water-equivalent phantom, and the depth dose was measured with EBT3. Next, after a phantom which reproduced the geometry of measurement was virtually created in RTPS, dose distributions were calculated with three commercially available algorithms (AXB, AAA, and XVMC) and MC. The calculated doses were then compared with the measured ones. As a result, compared to other algorithms, it was found that the dose calculation accuracy of AXB at the exit side of high-density materials was comparable to that of MC and measured value with EBT3. However, note that AXB underestimated the dose up to approximately 30% at the plane of incidence because it cannot exactly estimate the impact of the backscatter.
Static magnetic field non-uniformity and gradient magnetic field non-linearity can be considered as the causes of geometric distortion in MRI images. The impact of a distortion in imaging such as whole body imaging or whole spine imaging can be serious. A standard 2D-distortion correction method does not correct the distortion in the slice encoding direction. This study examined the effect of 3D-distortion correction with a correction effect in both the imaging plane and the slice-encoding plane using three MRI devices with differing static magnetic field intensities and boa diameters. Imaging of a nickel sulfate bottle phantom attached to the MRI device was conducted using a CT scan to measure the distortion rate based on the CT image. The result of the distortion rate at −39.1% in the Z-axis direction was reduced to −1.3%, and the distortion rate at about −9.8% in the magnetic X-axis was reduced to −1.7%. In addition, the reduction effect was greater on the 70 cm boa device compared to the 60 cm boa device, and it was also greater at 1.5 T compared to 3 T. 3D-distortion correction is believed to be useful for wide scope imaging using large FOV.