Study using high-resolution CT and MRI of rabbit knee joint for correlation of T2 and BMD

Introduction: MRI relaxometry (T2) is a great indicator of degradation of articular cartilage by osteoarthritis (OA) ¹, it alters molecular composition and structural integrity together with an increase of water². The addition of µCT measurements for bone-density properties, we show that by accurately correlating the two quantitative maps we can accurately study the changes in cartilage and underlying bone which is inadequately measured in MRI.

Method:Four intact normal rabbit knee joints placed in a polyurethane tube with some saline (to avoid drying) were imaged using a Bruker AVANCE IIIHD system with a 7T/9 cm vertical-bore magnet with a MiniWB57 probe and a 40 mm coil. All MRI experiments used a MSME sequence with a FOV of 30 mm and a matrix size of 256x128, which yielded MRI T2 Maps of the intact joint at 58.6 µm/pixel nominal size. ImageJ (NIH, MA) was used to calculate T2 and collect the bulk cartilage T2. The intact knee joints were then imaged in µCT by Skyscan1174 (Bruker) with experimental parameters set at 50 kV, 800 mA, 12 averages, 0.5° rotation step, 180° rotation, 0.2 mm Al filter, and 652 × 512 data matrix. All CT images were reconstructed using NRecon (Bruker) with the same parameters and a global intensity threshold was kept constant for all scans to include the full range of attenuation values. CTan ((Bruker, Kontich, Belgium) was used to collect all the BMD values from the corresponding SBP regions under the cartilage ROIs.

Results:Figure 1 shows the coronal µCT image at 60.8 µm/pixel (isotropic) resolution and the almost identical MRI proton-density T2 map at 58.6 µm/pixel resolution. Figure 2 shows a linear relationship of T2 with BMD where T2 is inversely proportional to the BMD. Here we show the underlying bone properties that are undetectable with MRI.

Discussion&Conclusion: We show with the comparing µCT coronal image and the quantitative T2 map of intact rabbit joint, that by studying the bone density and the corresponding cartilage T2 values, we can quantitatively measure the T2 to BMD linear relationship. Most clinical MRI studies of topographical knee joint spacing lack a true plateau due to the large slice thickness to improve signal to noise ratio or adding a slice gap that causes the loss of information due to time constraints in experimental setup.