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

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