1. 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

2. Diagnosis: C. Slice sensitivity profile

3. 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.

4. 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)

5. References/Bibliography:
Polacin et al, Evaluation of Section Sensitivity Profiles and Image Noise in Spiral CT, Radiol 185(1):29-35, 1992