CT Number Accuracy and Association With Object Size: A Phantom Study Comparing Energy-Integrating Detector CT and Deep Silicon Photon-Counting Detector CT.

Abstract

Background: Variable beam hardening based on patient size causes variation in CT numbers for energy-integrating detector (EID) CT. Photon-counting detector (PCD) CT more accurately determines effective beam energy, potentially improving CT number reliability. Objective: To compare EID CT and deep silicon PCD CT in terms of the effect of changes in object size on CT number, and in CT number accuracy. Methods: A phantom with polyethylene rings of varying sizes (mimicking patient sizes), as well as inserts of different materials, was scanned on an EID CT scanner in single-energy (SE) mode (120-kV images) and in rapid kV-switching dual-energy (DE) mode (70-keV images), and on a prototype deep silicon PCD CT scanner (70-keV images). ROIs were placed to measure materials' CT number. Slopes of CT number as function of object size were computed. Materials' ideal CT number at 70 keV was computed using the U.S. National Institute of Standards and Technology XCOM toolkit. Root mean square error (RMSE) between measured and ideal numbers was calculated across object sizes. Results: Slope (in HU/cm) was significantly closer to zero (i.e., less variation in CT number as function of size) for PCD than for SE EID for air (1.2 vs 2.4), water (-0.3 vs -1.0), iodine (-1.1 vs -4.5), and bone (-2.5 vs -10.1), and for PCD than for DE EID for air (1.2 vs 2.8), water (-0.3 vs -1.0), polystyrene (-0.2 vs -0.9), iodine (-1.1 vs -1.9), and bone (-2.5 vs -6.2) (p<.05). For all tested materials, PCD had smallest RMSE, indicating CT numbers closest to ideal numbers; specifically, RMSE (in HU) for SE EID, DE EID, and PCD was 32, 44, and 17 for air; 7, 8, and 3 for water; 9, 10, and 4 for polystyrene; 31, 37, and 13 for iodine; and 69, 81, and 20 for bone. Conclusion: For numerous materials, deep silicon PCD, in comparison with SE EID and DE EID, demonstrated lower CT number variability as a function of size and CT numbers closer to ideal numbers. Clinical Impact: Greater reliability of CT numbers for PCD CT is important given dependence of diagnostic pathways on CT numbers.