Quantitative Assessment of Bone Marrow Edema-Like Lesion and Overlying Cartilage in Knees With Osteoarthritis and Anterior Cruciate Ligament Tear Using MR Imaging and Spectroscopic Imaging at 3 Tesla

BONE MARROW EDEMA PATTERN is defined as an area of high signal intensity in T2-weighted, fat-saturated magnetic resonance (MR) images or in short inversion time inversion-recovery (STIR) images. These lesions are present in knee osteoarthritis (OA) (1-3) and acute knee injuries, frequently associated with anterior cruciate ligament (ACL) tear (4-6). Although termed as “edema,” these lesions have shown surprisingly little edema based on histopathologic examination (7). Instead, this increase in signal has been attributed to a number of other factors, including abnormal trabeculae, bone marrow necrosis, swelling of fat cells, and marrow bleeding. Therefore, in this study, we term these lesions as bone marrow edema-like lesion (BMEL). While MR findings of BMEL are common, our knowledge concerning their natural history and significance is limited. In ACL tears, BMEL indicates a so-called bone bruise or impression fracture due to translational injury, where the anterolateral femur impacts the posterolateral tibia (kissing contusions) when the ACL is ruptured. A number of studies have proposed that the overlying cartilage has sustained irreversible injury during impact, and thus, cartilage degeneration can continue to occur despite the fact that functional stability of the knee is restored following ACL reconstruction (8,9). In OA, BMEL has been associated with the severity and progression of OA (2,10), and pain in OA (10,11). However, the causal relationship between these lesions and the disease progression is unclear. Better understanding of the interrelation of cartilage and the bone immediately under it is also necessary. Recent advances in MRI allow quantitative measures of volumetric and biochemical changes of cartilage prior to macroadaptive changes seen on traditional radiographs. In particular, T1ρ has been proposed as an attractive candidate to probe the early changes in cartilage extracellular matrix (ECM) during early stages of cartilage degeneration (12-15). T1ρ describes the spin-lattice relaxation in the rotating frame. It provides low-frequency information and probes changes in macromolecules. T1ρ relaxation rate (1/T1ρ) has been shown to decrease linearly with decreasing proteoglycan (PG) content in ex vivo bovine patellae (16). In vivo studies have also shown increased cartilage T1ρ values for patients with OA (17-19). One difficulty in studying BMEL lies in quantification of these lesions due to its ill-defined shape in MR images. Most previous imaging studies used qualitative (presence of high signal or not) (4,6,11), or semiquantitative evaluations (scoring the size of bone bruise volume) (1,10). Recent studies on measuring volume of BMEL manually or semiautomatically using MR images provide potential for quantitative evaluation of such lesions (5,20-22). Such quantification of BMEL may allow better classification of these lesions and potentially a better prediction to their prognosis. Furthermore, proton MR spectroscopy (MRS) provides a noninvasive method for quantifying biochemical or metabolic changes in tissues. 3D MR spectroscopic imaging (MRSI) will further allow examining the spatial distribution of the metabolic levels. In the case of bone marrow, the water and lipids contents are of interest (23-26). Different compartments of lipids can be also investigated using MRS. Of particular interest are indices that provide measures of the unsaturation levels among the triglycerides, which may have potential medical applications (27,28). MRS has been used widely in cerebral imaging and other tissues/organs such as prostate, breast, and muscle. However, few studies have investigated MRS in knee bone marrow (29). The objective of this study was to apply quantitative MRI and 3D MRSI methods to assess BMEL in patients with OA and ACL tears. Specifically, the goals were: 1) to establish an automatic method to calculate the volume of BMEL; 2) to quantify water and lipids changes in BMEL using 3D MRSI; and 3) to assess composition changes in cartilage overlying areas of BMEL using measurements of T1ρ relaxation time.