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OBJECTIVE
The purpose was to evaluate the intra- and inter-observer reliability of combined quantitative 3D-volumetric single-photon emission computed tomography (SPECT)/CT analysis including size, intensity and localisation of tracer uptake regions and total knee arthroplasty (TKA) position.

MATERIALS AND METHODS
Tc-99m-HDP-SPECT/CT of 100 knees after TKA were prospectively analysed. The anatomical areas represented by a previously validated localisation scheme were 3D-volumetrically analysed. The maximum intensity was recorded for each anatomical area. Ratios between the respective value and the mid-shaft of the femur as the reference were calculated. Femoral and tibial TKA position (varus-valgus, flexion-extension, internal rotation- external rotation) were determined on 3D-CT. Two consultant radiologists/nuclear medicine physicians interpreted the SPECT/CTs twice with a 2-week interval. The inter- and intra-observer reliability was determined (ICCs). Kappa values were calculated for the area with the highest tracer uptake between the observers.

RESULTS
The measurements of tracer uptake intensity showed excellent inter- and intra-observer reliabilities for all regions (tibia, femur and patella). Only the tibial shaft area showed ICCs <0.89. The kappa values were almost perfect (0.856, p < 0.001; 95 % CI 0.778, 0.922). For measurements of the TKA position, there was strong agreement within and between the readings of the two observers; the ICCs for the orientation of TKA components for inter- and intra-observer reliability were nearly perfect (ICCs >0.84).

CONCLUSION
This combined 3D-volumetric standardised method of analysing the location, size and the intensity of SPECT/CT tracer uptake regions ("hotspots") and the determination of the TKA position was highly reliable and represents a novel promising approach to biomechanics.