POS0974 LOW DOSE COMPUTED TOMOGRAPHY HOUNSFIELD UNITS: A RELIABLE METHODOLOGY FOR ASSESSING CHANGES IN VERTEBRAL BONE DENSITY IN RADIOGRAPHIC AXIAL SPONDYLOARTHRITIS

Abstract

BackgroundAssessing local vertebral bone loss in radiographic axial spondyloarthritis (r-axSpA) poses challenges, namely because Dual-energy X-ray absorptiometry (DXA) only allows assessment of lumbar vertebrae. The measurement of Computed Tomography (CT) vertebral Hounsfield Units (HU) has been shown to correlate with vertebral bone density as measured by DXA in trauma patients. We have recently reported the excellent reliability of low dose CT (ldCT) HU measurements from C3 to L5 in patients with r-axSpA1; however, HU change scores have never been studied.ObjectivesTo describe ldCT HU change scores and their inter-reader reliability per vertebra in patients with r-axSpA over 2 years.MethodsWe used spine ldCT scans of r-axSpA patients included in the multicenter 2-year Sensitive Imaging in Ankylosing Spondylitis (SIAS) study. A standardized protocol and automatic exposure control calibration in ldCT imaging acquisition were used. Baseline and 2-year ldCT scans were independently assessed by two trained readers. The HU measurements were performed as described in Figure 1. Mean (standard deviation, SD) for the change-from-baseline scores were provided by reader, together with the respective mean differences (SD). Inter-reader reliability for the change scores was assessed using intraclass correlation coefficients (ICC) absolute agreement, applying two-way random effect models. Agreement was assessed using the smallest detectable change (SDC = 1.96 x SDdifference /(√2*√k); SDdifference is the SD of the differences in change scores between the two readers, and k=2 readers) and Bland-Altman plots. The percentage of vertebrae in which readers agreed on the direction of the change (positive vs negative) and on change scores > |SDC|were also computed.ResultsWhole spine ldCT scans from 49 r-axSpA patients (mean age of 49 (SD 10) years; 88% male and 84% HLA-B27 positive) were included. In total, 1,053 vertebrae were assessed by both readers. The HU mean change values varied from -23 to 28 and -23 to 29 for reader 1 and 2, respectively – Table 1. More vertebrae showed a positive change in the cervical spine compared with the thoracic and lumbar spine. Inter-reader reliability was excellent (ICC: 0.91 to 0.99). SDC varied from 4 to 7, mean and median of 5. Bland-Altman plots showed homoscedasticity throughout the whole spine, with negligible systematic error between the readers. The two readers agreed on the direction of the change score in 88-96% of vertebrae, and agreement on change scores > |SDC| was obtained in 64-96% of the vertebrae.Table 1.Change scores for both readers, mean differences and intraclass correlation coefficients (ICC) from C3 to L5Vertebra§Mean change (SD)Mean Difference (SD)Change > 5 #Change < -5 #Change >|5| #ICCReader 1Reader 2C318 (56)17 (56)0.2 (5.0)5532870.97C418 (53)17 (52)0.3 (5.5)5932910.98C528 (70)29 (70)-0.7 (5.0)6132930.99C623 (62)23 (62)0.4 (5.8)5039890.99C7-3 (60)-2 (59)-0.7 (6.9)4049890.98T1-6 (87)-6 (88)0.6 (5.0)4749960.98T23 (45)3 (45)0.7 (4.0)4145860.97T31 (43)2 (43)0.6 (4.4)2935640.95T4-2 (48)-1 (47)0.2 (5.3)3945840.94T50.02 (48)-0.3 (48)0.3 (5.1)3945840.91T6-3 (44)-3 (45)-0.1 (4.6)3545800.95T7-4 (43)-4 (43)-0.1 (4.5)2951800.99T8-1 (38)-1 (40)0.1 (4.6)3545800.98T9-9 (50)-9 (50)0.02 (5.4)2259810.99T100.2 (59)1 (59)-0.4 (5.2)3339720.96T11-8 (53)-7 (53)-0.6 (4.3)3543780.94T12-23 (59)-23 (60)-0.3 (4.4)3553880.99L1-9 (33)-7 (33)-1.9 (6.3)2741680.97L21 (46)2 (45)-1.1 (4.6)2941700.98L3-4 (35)-2 (34)-1.2 (6.3)2439630.92L4-2 (24)1 (22)-1.5 (5.4)3137680.91L58 (43)9 (43)-1.0 (6.0)4733800.97§ C3-C7: n=44; T1-L5: n=49# % of vertebrae in which both readers agreed on a change score > smallest detectable change.ConclusionLdCT measurement of HU is a reliable method to assess changes in bone density at each vertebra from C3 to L5. This methodology can aid the study of bone loss in r-axSpA, a disease affecting the whole spine.