Quantification of bone microarchitecture and strength with photon-counting detector CT at different radiation doses: an ex-vivo and in-vivo comparison with HR-pQCT

Purpose To compare trabecular microarchitecture measurements at the distal radius and tibia from photon-counting detector CT (PCD-CT) at varying radiation doses with high-resolution peripheral quantitative CT (HR-pQCT). Methods Two intact wrist and two intact ankle specimens from an 88-year-old man were scanned with PCD-CT at radiation doses of 2.5, 5, 10, and 20 mGy and with HR-pQCT. Additionally, clinical in-vivo HR-pQCT and PCD-CT scans at the radius and tibia were acquired of a 40-year-old woman with osteoporosis. After bone segmentation, the segmented PCD-CT and HR-pQCT scans were three-dimensionally registered. Cubic volumes (edge length: ex-vivo 6 mm, in-vivo 5 mm) were defined at corresponding locations in the PCD-CT and HR-pQCT scans based on the three-dimensional registration. For each cube, trabecular volume fraction (Tb.BV/TV), thickness (Tb.Th), number (Tb.N), separation (Tb.Sp), and heterogeneity (Tb.1/N.SD) were quantified and compared between corresponding PCD-CT and HR-pQCT cubes. Results Ex-vivo , linear correlation coefficients ( R 2 ) between PCD-CT and HR-pQCT were 0.85–0.97 at 2.5mGy and remained stable with increasing radiation dose for all parameters except Tb.1/N.SD. For Tb.1/N.SD, R2 increased between 2.5 and 5 mGy and remained stable at higher doses. At each radiation dose, Tb.BV/TV, Tb.N, and Tb.Th values were higher and Tb.Sp and Tb.1/N.SD lower on PCD-CT than on HR-pQCT. In-vivo , R 2 was 0.89–0.95 (radius) and 0.82–0.97 (tibia). Conclusions PCD-CT strongly correlated with HR-pQCT in trabecular microarchitecture measurements at the distal radius and tibia at low, clinically acceptable, radiation dose. Between-modality differences in microarchitecture values are likely related to chosen image analysis settings.