We aim to perform a meta-analysis of the corpus of experimental results accumulated for compositional MRI biomarkers of articular cartilage used in clinical studies. We summarized the data according to the statistical evidence that is reported. We report the correlation of MRI parameters with composition, the ability of MRI parameters to detect group differences between healthy and degraded cartilage, and the ability of MRI to detect cartilage damage.
Changes in articular cartilage, a hallmark of early osteoarthritis, include increased hydration, proteoglycan (PG) loss, and remodeling of the collagen network.[1-3] MRI has shown high promise to assess the biochemical composition of articular cartilage: water, PG and collagen. Almost all cartilage compositional imaging biomarkers used today in clinical studies were proposed within few years in the early 90’s: T1,[4] sodium MR,[5] magnetization transfer (MT),[6] T2,[7] diffusion MRI,[8] gadolinium enhanced MRI of articular cartilage (dGEMRIC),[9] and T1ρ.[10] Since then many studies have validated these imaging biomarkers in well-controlled ex vivo experiments.
We aim to perform a meta-analysis of the corpus of experimental results accumulated for those MRI biomarkers used in clinical studies. We summarized the data according to the statistical evidence that is reported. We report the correlation of MRI parameters with composition, the ability of MRI parameters to detect group differences between healthy and degraded cartilage, and the ability of MRI to detect cartilage damage.
Data compilation
We considered only studies that used a non-MRI based standard of reference to assess either composition or damage. We performed the search in Pubmed that resulted in 57 relevant publications. We complemented the results of the research with references known to the authors to identify a total of 67 manuscripts.
To synthesize the information presented in all 67 studies we applied the following criteria. We report only values averaged over the full cartilage thickness, which was the most commonly reported value. We only report the data from the group with the most severe damage to show the range of parameter variation.
Statistical methods
Correlations between MRI methods and composition were reported as the range of correlations with the mean calculated using the Fisher’s z-scale. To summarize differences between groups we used the Cohen's d as a measure of effect size.[11] Since only few studies have analyzed the performance of MRI biomarkers for the diagnosis, we only use descriptive statistics to summarize the results.
Group differences
Differences in MRI parameters between healthy and degraded samples are summarized in Figure 1. The most studies validated T2[4,7,12-41] and T1,[4,7,9,10,12,14,16-18,20,25,28-31,38,42-47] followed by diffusion,[8,14,18,27-30,38,48-53], dGEMRIC,[9,17,19,20,24,26,28-30,43,45,47,51,54,55] T1ρ,[10,16,19,23,34,35,37,45,47,56-60] magnetization transfer,[10,12,17,29,30,38,44,46,61,62] and Na-imaging and spectroscopy.[5,7,8,42,61,63,64] The largest Cohen’s d was for Na concentration, [Na], (d=-2.45). Diffusivity measurements have the next largest Cohen’s d=1.14 (n=12 studies), followed by km (-1.10). T1ρ, T1Gd, and FA showed moderate significance to detect changes (d=-0.86, 0.77, and 0.75 respectively). The large standard deviation of T2 and T1 indicate the rather modest ability of T2 and T1 to detect changes (d=0.39), likely explaining the low performance of clinical MRI sequences for the detection of early cartilage degeneration.
Correlation
MRI parameters have been correlated against the histological score,[4,21,26,34,35,37,40,53,57] the GAG content,[8,9,16,18,20,24,26,31,34,35,37,38,42,43,45,46,48,49,52,54-57,62,63] the collagen content,[20,24,26,35,37,46] and the water content (WC)[20,26,31,37,38]. The MRI parameters (T2, T1, diffusion, T1ρ, and T1Gd) show a moderate correlation with the histological grade, with correlation coefficients between 0.5 and 0.7 (Fig. 2). However, many studies reported non-significant correlations with histological grading for T2 (4 out of 8 studies), and T1ρ (3 out of 5 studies). Similarly, correlations with water content are weak for all parameters but T1Gd and diffusivity. Poor correlation was found between MRI and collagen content, indicating the suboptimal assessment of the collagen component of cartilage. However, we identified only a few studies that measure collagen composition. Several parameters showed a strong correlation with the GAG content. The highest correlations were for [Na] (r=0.98) followed by diffusion (r=-0.88), T1ρ (r=-0.79) and T1Gd (r=0.70).
Diagnostic accuracy
Only 10 of publications analyzed the diagnostic accuracy of MRI parameters for detecting cartilage damage, from which 6 comes from the same group.[26,29,30,38,47,53,65-67] We identified 4 in vivo studies on patients that undergo arthroscopy as a standard of reference.[68-70] The diagnostic accuracy is summarized in Table 1. Best univariate MRI classifiers were T1 (sensitivity(S)=78%, specificity(Sp)=87%), diffusion (S=78%,Sp=79%), T1Gd (S=79%,Sp=62%) and magnetization transfer (S=62%,Sp=66%). Multivariate classifications improved classification.
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