Radiation imaging physics
Evaluation of slice sensitivity profiles for helical and axial 4D-CT acquisitions on the Philips Brilliance CT Big Bore
4D-CT can be performed using two acquisition modes. One employs a cine axial scan and the other acquires the data using a very low pitch helical acquisition. This study evaluates the longitudinal resolutions of each of these methods by creating slice sensitivity profiles (SSP).
An SSP phantom was scanned using both axial and helical scanning modes. The phantom was scanned at the center of the field of view (FOV) and at several points offset from the FOV center. For helical scans, the pitch was varied from 0.04 to 0.2. With the axial scans, the phantom position relative to the center of the detector ring was varied. All of these were performed using a 16 × 1.5 mm collimation.
The SSP graphs were generated from these scanned datasets. The full width at half maximum (FWHM) of the graphs was used as the surrogate for longitudinal resolution. The results of these experiments demonstrated that axial scans have a slightly better resolution at CT isocenter (1.5 vs 1.8 mm). Helical mode scanning maintained this measurement for varying pitch factors and placement with the scan field. However, the resolution for axial scans degrades as the phantom is moved further from the center of the FOV and is positioned nearer to the edge of the detector rings.
When it is imperative to maintain uniform longitudinal resolution throughout the FOV, it is suggested that helical acquisition be utilized or the axial mode data be reconstructed to account for beam divergence. For axial scans, this will require either limiting the longitudinal extent of each cine axial acquisition and/or using a smaller reconstructed FOV.