Optimization of abdomen Multiplanar Reformation (MPR) with isotropic data sets aquired for 16-Detector CT scanner

Abstract

The technology of 16-detector CT scanner enables the submillimeter section acquisition which in turn yields the isotropic data set that provides the generation of MPR image with the similar image quality to that in the axial images. The purpose of this study is to determine the optimal parameter setting for abdomen MPR with isotropic data set in 16-detector CT scanner. With the scanning of a 0.38 mm tungsten carbide bead, then applied the MTF calculation to evaluate the spatial resolution as a function of CT parameters. The spatial frequency at 10% of MTF was considered as spatial resolution of the image. At the collimations of 16x0.75 and 16x1.5 mm, the spatial frequencies at 10% of MTF were 0. 72 and 0.36 cycles/mm, for all setting of rotation time the approximate spatial frequency were 0.7 cycles/mm, for helical pitch values of 0.5 to 1.5 the spatial frequencies ranged from 0.74 to 0.68 cycles/mm, for the body kernel of B10f to B80f the spatial frequencies ranged from 0.84 to 0.48 cycles/mm, for the slice thickness of 0.75, 1.0 and 2.0 mm of axial data set, the spatial frequencies were 0.72, 0.64 and 0.40 cycles/mm for the MPR images respectively. The characteristics of image noise were studied by scanning a 32 cm diameter of PMMA phantom. The standard deviation of CT number was measured for all planes of MPR and then averaged to represent the image noise. The measured image noise in MPR images were 18.32 and 10.87 HU for 16x0.75 and 16x1.5 mm collimations, for 0.5 to 1.5 sec rotation time the noise ranged from 18.31-18.66 HU, for helical pitch values of 0.5 to 1.5 the noise ranged from 17.11 to 21.30 HU, for the body kernel of B10f to B80f the noise ranged from 15.20 to 48.86 HU. The displayed CTDIvol at the constant effective mAs of 140, were changed with the collimation settings of 16x0.75 and 16x1.5 mm of 10.92 and 9.8 mGy. The low contrast detectability was quantitative assessed by using the Catphan low-contrast module images with the CNRs calculation. As the effective mAs increasing from 100 to 200, the calculated CNRs increased from 2.03 to 2.53.As the MPR slice thickness increasing from 1.0 to 5.0 mm, CNRs ranged from 1.26 to 2.35. For qualitative assessment, the raw data of 7 patients who underwent abdomen CT were retrospective reconstructed to create the coronal MPR images with the slice thickness of 1.0 to 5.0 mm. These images were scored by two radiologists following the diagnostic preference and the 5 mm slice thickness was consistently preferred. Therefore, in order to achieve a good image quality of abdomen MPR images, the acquisition parameters were: 16x0.75 mm collimation, 0.75 mm axial slice thickness with 0.7mm image interval for 350 mm DFOV, 0.5 sec rotation time, helical pitch of 1.0 at 120 kVp, 140 effective mAs (for standard patient size of 70 kg) and reformatted to 5.0 mm slice thickness of MPR without the exceed of radiation dose than 35 mGy of CTDIw given by the European Guideline. Our results support that MPR images show the better spatial resolution, improved in image noise and also CNR than that in the axial images. Therefore, the use of MPR application could be beneficial in adding up the confidence for interpretation of abdomen CT examination.