Quantitative MRI techniques of cartilage composition

Abstract

Due to aging populations and increasing rates of obesity in the developed world, the prevalence of osteoarthritis (OA) is continually increasing. Decreasing the societal and patient burden of this disease motivates research in prevention, early detection of OA, and novel treatment strategies against OA. One key facet of this effort is the need to track the degradation of tissues within joints, especially cartilage. Currently, conventional imaging techniques provide accurate means to detect morphological deterioration of cartilage in the later stages of OA, but these methods are not sensitive to the subtle biochemical changes during early disease stages. Novel quantitative techniques with magnetic resonance imaging (MRI) provide direct and indirect assessments of cartilage composition, and thus allow for earlier detection and tracking of OA. This review describes the most prominent quantitative MRI techniques to date-dGEMRIC, T2 mapping, T1rho mapping, and sodium imaging. Other, less-validated methods for quantifying cartilage composition are also described-Ultrashort echo time (UTE), gagCEST, and diffusion-weighted imaging (DWI). For each technique, this article discusses the proposed biochemical correlates, as well its advantages and limitations for clinical and research use. The article concludes with a detailed discussion of how the field of quantitative MRI has progressed to provide information regarding two specific patient populations through clinical research-patients with anterior cruciate ligament rupture and patients with impingement in the hip. While quantitative imaging techniques continue to rapidly evolve, specific challenges for each technique as well as challenges to clinical applications remain.

Keywords: Magnetic resonance imaging; biomarker; cartilage; osteoarthritis (OA); quantitative.

Figure 1

Representation of articular cartilage composition at three stages of health. Healthy cartilage (A) demonstrates several distinct layers between the articular surface and bone interface, marked by orientation of collagen fibrils. The collagen matrix is highly structured and proteoglycans are abundant. In early stages of OA (B), proteoglycans are initially depleted, and the collagen matrix begins to break down. In later stages of OA (C), proteoglycans become severely depleted and the morphological structure of the cartilage becomes compromised. Current research in quantitative MR applications to cartilage aims to detect early arthritic changes in cartilage before more severe morphological changes occur

Figure 2

Quantitative MR imaging of osteoarthritis. A number of quantitative MR techniques are increasingly applied to obtain biochemical information regarding cartilage health, especially in the knee. While a conventional MR image may show no major morphological defects in the cartilage (A), quantitative techniques provide evidence of early osteoarthritic changes (B-D). Spatial variation in dGEMRIC T1 and T1rho relaxation times in the anterior weight-bearing femoral cartilage and trochlear cartilage suggests an initial depletion of PGs. T2 mapping reveals minor variations in the posterior weight-bearing femoral cartilage that are also indicative of early OA

Figure 3

Quantitative MR imaging of ACL tear knees. T1rho mapping (A,B) is applied to demonstrate the traumatic effects of ACL tear on cartilage biochemistry, compared to healthy controls. Increased heterogeneity of T1rho relaxation times within weeks of injury (B) suggests these changes occur along with the traumatic event. Sodium imaging (C,D) is also applied to reveal the impact of ACL tear on GAG content (Courtesy of Caroline Jordan, Ph.D., Stanford Dept. of Bioengineering)

Figure 4

Quantitative MRI of hip impingement. Sagittal T1-weighted image (A) of a patient with mixed impingement demonstrates swelling and increased signal in the anterior-superior labrum, suggesting a tear. T2 (B) and T1rho (C) maps of the same patient demonstrate applications of quantitative imaging to the hip