AGREEMENT BETWEEN IN VIVO AND EX VIVO PHOTON-COUNTING CT MEASURES OF SUBCHONDRAL BONE FEATURES IN PATIENTS WITH KNEE OSTEOARTHRITIS

INTRODUCTION

Bone changes are integral to the onset and progression of OA. Many aspects remain poorly understood due to the inability to assess bone architecture in vivo. Research has relied on ex vivo imaging, hindering evaluation of early-stage disease and longitudinal analysis. Conventional CT lacks the resolution to visualise subchondral bone microstructure. While ex vivo Photon Counting CT (PCCT) has demonstrated imaging comparable to μCT, its ability to capture bone microstructure in vivo in knee OA patients under clinical conditions remains unproven.

OBJECTIVE

The aim of this study was to compare in vivo and ex vivo PCCT of subchondral bone features in patients with knee OA.

METHODS

Pre-surgery in vivo and post-surgery ex vivo PCCT (Siemens Naeotom Alpha, Siemens Healthineers, Germany) of the tibial plateau from participants with severe knee OA referred to arthroplasty surgery from January 2022 through September 2023 were compared. Acquisition/reconstruction details: a tube current of 120 kV, a matrix size of 1024 × 1024, a slice thickness of 0.2 mm, and a FOV of 150 × 150 mm. 18 in vivo/ex vivo PCCT pairs were included. The ex vivo scans was registered to the in vivo scans. Linear regression and Bland-Altman plots were used to assess correlation and agreement between in vivo and ex vivo measures of bone volume fraction (BV/TV), trabecular thickness (Tb.Th.), and attenuation in healthy and sclerotic trabecular bone. Delineated areas of bone sclerosis were compared using the Dice coefficient and Hausdorff distance, Fig. 1.

RESULTS

Comparing in vivo and ex vivo scans strong correlations were found for BV/TV, R²=0.82 and attenuation in both healthy, R²=0.89, and sclerotic, R²=0.79, bone, while a moderate correlation was found for Tb.Th., R²=0.55. Bias for BV/TV and Tb.Th. was -4.1% and -0.598mm, respectively, and -41.4 HU and -81.1 HU for healthy and sclerotic bone, respectively. A proportional bias was observed for BV/TV and Tb.Th., Fig. 2. There was excellent agreement between the segmentations of sclerotic areas, Dice coefficient = 0.91 and Hausdorff distance = 0.11mm.

CONCLUSION

In patients with severe knee OA, BV/TV and attenuation can be obtained with high correlation and small bias between in vivo and ex vivo scans. Tb.Th. showed moderate correlation and larger bias. Subchondral bone sclerosis, a key OA feature, is well translated from ex vivo to in vivo PCCT. Longitudinal studies using in vivo PCCT are feasible, but caution may be advised when measuring Tb.Th.