Fat quantification in the sacroiliac joint syndesmosis – a new semi- automatic volumetric approach

Fat is physiologically embedded within the interosseous ligaments in the posterior sacroiliac (PSIJ). This composite of fat and ligaments is hypothesised to serve a shock-absorbing, stabilising function for the sacroiliac joint and the lumbopelvic transition region. Using a novel Python-based software (VolSEQ), total PSIJ volume and fat volume was computed semi-automatically. Differences within the cohort and the viability of the program for the quantification of fat in routine CT scans were assessed. In 39 CT scans of heathy patients the PSIJ were first manually segmented as a region of interest (ROI) in OSIRIX. Within VolSEQ, ‘fat’ Hounsfield units (− 150 to − 50 HU) are selected and patient DICOM files of the patient and associated region of interest file from OSIRIX were imported and the pixel sub volumes were then automatically computed. Volume comparisons were made between sexes, sides and ages (≤ 30, 31–64 and > 65 years). PSIJ volumes in both software (VolSeq vs OSIRIX) were non-different (both 9.7 ± 2.8cm 3 ; p  = 0.9). Total PSIJ volume ( p  = 0.3) and fat volume ( p  = 0.7) between sexes was non-different. A significant difference in total PSIJ volume between sexes ( p  < 0.01) but not in fat volume ( p  = 0.3) was found only in the ≥ 65 years cohort. Fat volume within the PSIJ remains unchanged throughout life. PSIJ volume is sex-dependant after 65 years. VolSEQ is a viable and user-friendly method for sub-volume quantification of tissues in CT.