Author: Yanle Hu, Shuai Leng, Cynthia H. McCollough, Maryam Sadeghian, Joe Swicklik 👨🔬
Affiliation: Mayo Clinic Arizona, Mayo Clinic 🌍
Purpose: To assess the dependency of CT numbers on position in the field-of-view (FOV) using single-energy (SE) and dual-energy (DE) images and energy-integrating detector (EID) and photon-counting detector (PCD).
Methods: A cortical bone insert (𝜌e ≈ 1.78) was placed inside a body size (40 x 30 cm2) multi-energy CT phantom (Gammex) and scanned on an EID-CT (SOMATOM Force) and PCD-CT (NAEOTOM Alpha) using routine abdominal protocols. SE and DE scans were acquired for EID-CT at 120 kV and 90/Sn150 kV, respectively. PCD-CT’s standard mode (144x0.4 mm collimation) was used at 120 kV. For each configuration, scans were repeated with the bone insert at different locations: iso-center, off-center, and six peripheral locations. Images were reconstructed with a 3-mm thickness and a quantitative Qr40 kernel. 70 keV VMIs were reconstructed for DE-EID-CT and PCD-CT scans. CT numbers were measured within circular regions of interest (ROIs) drawn on the bone insert. Mean, standard deviation, coefficient of variation (CV), maximum-to-minimum, and central-to-peripheral differences (of mean CT numbers) were calculated and compared among modalities.
Results: CT numbers of the bone insert for PCD-CT, and for VMIs from DE-EID- and PCD-CT, were similar, all of which were higher than for SE EID-CT. For SE images, the CV was similar between EID-CT (0.84%) and PCD-CT (0.79%). The DE-EID-VMI had a higher CV (1.67%) than its SE counterpart, while PCD-VMI had the lowest CV (0.35%) among all modalities. The Max-to-Min CT number difference was 18.02, 25.51, 61.87, and 10.06 HU for SE-EID, SE-PCD, DE-EID-VMI, and PCD-VMI, respectively.
Conclusion: CT numbers vary depending on their position in the FOV and the variation is different among scanner technologies and image types, with VMIs from PCD-CT showing the least location dependency. This could potentially eliminate repeated calibration scans and improve the accuracy and precision of dose calculations in radiotherapy planning.