Abstract
The purpose of this study was to compare the resulting full width at half maximum of slice sensitivity profiles (SSP) generated by several commercially available point response phantoms, and determine an appropriate imaging technique and analysis method. Four CT phantoms containing point response objects designed to produce a delta impulse signal used in this study: a Fluke CT‐SSP phantom, a Gammex 464, a CatPhan 600, and a Kagaku Micro Disc phantom. Each phantom was imaged using 120 kVp, 325 mAs, head scan field of view, 32×0.625 mm helical scan with a 20 mm beam width and a pitch of 0.969. The acquired images were then reconstructed into all available slice thicknesses (0.625 mm−5.0 mm). A computer program was developed to analyze the images of each dataset for generating a SSP from which the full width at half maximum (FWHM) was determined. Two methods for generating SSPs were evaluated and compared by choosing the mean vs. maximum value in the ROI, along with two methods for evaluating the FWHM of the SSP, linear interpolation and Gaussian curve fitting. FWHMs were compared with the manufacturer's specifications using percent error and z‐test with a significance value of p<0.05. The FWHMs from each phantom were not significantly different (p≥0.089) with an average error of 3.5%. The FWHMs from SSPs generated from the mean value were statistically different (p≤3.99×1013). The FWHMs from the different FWHM methods were not statistically different (p≤0.499). Evaluation of the SSP is dependent on the ROI value used. The maximum value from the ROI should be used to generate the SSP whenever possible. SSP measurement is independent of the phantoms used in this study.PACS number: 87.
Highlights
Helical CT allows for the rapid and continuous volumetric scanning of patients by moving the patient through the gantry while tomographic data are acquired
Are commonly available CT phantoms, such as the CatPhan or Gammex 464, acceptable for SSP measurement, or is a specialized SSP phantom required? Dose the image reconstruction interval impact the results? How should the reconstructed images be analyzed? The purpose of this study was to determine if any of these factors affect the measurement of the slice sensitivity profile, to develop an optimized procedure for acquiring phantom data, and to create an algorithm to efficiently analyze the images while reducing human error
Percent errors were computed from comparisons of measured full width at half maximum (FWHM) values to those provided in the technical reference manual by the manufacturer.[14]
Summary
Helical CT allows for the rapid and continuous volumetric scanning of patients by moving the patient through the gantry while tomographic data are acquired. In order for an axial slice to be reconstructed, the helical dataset needs to be interpolated to create a full set of projection data within the plane of reconstruction. The use of projection data from outside the reconstructed volume leads to a reduction in axial resolution.[1,2,3,4,5] The slice sensitivity profile (SSP) allows the measurement of the axial resolution of helical CT systems. The increase in photons allows thicker slices to use a lower dose to achieve the same SNR. This relationship between slice thickness and image quality makes the SSP an important factor during CT protocol optimization. SSP measurement is a recommended part of CT acceptance testing, as indicated by the AAPM Report 39 and IEC 61223-3-5 reports.[10,11]
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