Slice Sensitivity Profiles in CT Scanning
A detailed examination of the differences in slice thickness between axial and helical CT scanner modes. Learn about slice sensitivity profiles and their impact on image reconstruction.
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Presentation Transcript
Wednesday Case of the Day Category Physics Authors: Eric L. Gingold, PhD & Jaydev K. Dave, PhD Department of Radiology, Thomas Jefferson University, Philadelphia, PA 19107 Question: The ACR phantom images on the right are both nominal 5 mm reconstructions from the same scanner. However, the 0.5 mm spaced wire ramps (arrows) show a pronounced difference in slice thickness. What is the reason for this difference? Noise index mAs Slice sensitivity profile Bowtie compensation filter A. B. C. D.
Diagnosis: C. Slice sensitivity profile
Correct Answer: C. Different slice sensitivity profile The upper image was scanned in axial mode, and the lower image in helical mode. An important difference between axial and helical scanning is the shape of the slice sensitivity profile (SSP). The axial SSP is quite sharp, while a helical SSP is broad, and depends on the helical pitch, the interpolation and the reconstruction. From: Polacin et al, Evaluation of Section Sensitivity Profiles and Image Noise in Spiral CT, Radiol 185(1):29-35, 1992.
Additional Information (Vendor-Specific) GE CT scanners offer two types of helical reconstruction, called Full and Plus modes Full mode produces a thinner slice profile Plus mode has a 20% wider slice profile than Full but requires 15-20% less mA to achieve the same image noise The lower image in the COD question was reconstructed using the wider Plus mode. This explains the apparently larger slice width seen in the COD question (based on wire ramps observed in the ACR phantom)
References/Bibliography: Polacin et al, Evaluation of Section Sensitivity Profiles and Image Noise in Spiral CT, Radiol 185(1):29-35, 1992
