Functional imaging in OA: role of imaging in the evaluation of tissue biomechanics

Abstract

Functional imaging refers broadly to the visualization of organ or tissue physiology using medical image modalities. In load-bearing tissues of the body, including articular cartilage lining the bony ends of joints, changes in strain, stress, and material properties occur in osteoarthritis (OA), providing an opportunity to probe tissue function through the progression of the disease. Here, biomechanical measures in cartilage and related joint tissues are discussed as key imaging biomarkers in the evaluation of OA. Emphasis will be placed on the (1) potential of radiography, ultrasound, and magnetic resonance imaging to assess early tissue pathomechanics in OA, (2) relative utility of kinematic, structural, morphological, and biomechanical measures as functional imaging biomarkers, and (3) improved diagnostic specificity through the combination of multiple imaging biomarkers with unique contrasts, including elastography and quantitative assessments of tissue biochemistry. In comparison to other modalities, magnetic resonance imaging provides an extensive range of functional measures at the tissue level, with conventional and emerging techniques available to potentially to assess the spectrum of preclinical to advance OA.

Keywords: Cartilage degeneration; Elastography; Magnetic resonance imaging (MRI); Radiography; Ultrasound; WOMAC.

Figure 1. Noninvasive imaging can probe the pathomechanics of osteoarthritis (OA)

Structural changes in the progression of OA are characterized by cartilage damage and loss, following [156]. Radiography indirectly identifies advanced OA through joint space narrowing. Functional imaging and quantitative MRI show promise to probe early OA, prior to the expression of gross changes in cartilage structure and morphology [143].

Figure 2. Imaging commonly evaluates joint and tissue function in OA by radiography, MRI and ultrasound over a wide range of spatial resolutions

Radiography has historically identified advanced OA through the assessment of joint space narrowing as the cartilage is worn allowing bony surfaces to contact. MRI is a versatile method that can acquire data to assess joint space narrowing, directly visualize cartilage structure and morphology, and reveal internal patterns of strain in tissues active deep within the joint. Multiple contrasts (e.g. relaxivity, internal strains) are a promising new advance in MRI assessment of OA [151, 152]. Ultrasound shows promise for real time assessment of the joint tissues. Various functional measures are depicted over common spatial resolution ranges for each imaging modality.

Figure 3. Multicontrast imaging of human joint cartilage is possible through complementary MRI techniques

Standard MRI (e.g. fast spin echo pulse sequence) depicts morphology of the left knee joint in the coronal plane (M=medial, L=lateral, S=superior, I=inferior). Spatial maps of relaxivity (e.g. T₂) can be visualized in cartilage, ligament, and meniscus tissues. Compressive loading to the joint in the inferior to superior direction, determined by dualMRI, results in complex internal strain patterns that can be related to disease severity, material properties of the tissues, and contact conditions in the loaded joint. The data is adapted from [2].