Detection of knee osteoarthritis, which represents more than 80% of osteoarthritis total burden [8], in an early stage is a must to treat it effectively [9]. We can only differentiate early alterations in cartilage morphology from the normal articular cartilage surface using a spatial resolution of 0.3 mm, which is considered beyond the sequences used for morphological cartilage imaging [10]. T2 mapping sequence is not dependent on a spatial resolution to detect early alterations in articular cartilage but rather depicts areas of increased water content and collagen matrix ultrastructure alteration in degenerative cartilage [11,12,13,14,15]. So, T2 mapping helps detect early and potentially reversible cartilage damage before the onset of symptoms and morphological alterations, allowing early action by disease-modifying agents [16].
In our study, adding a T2 mapping sequence to a standard MRI protocol significantly improves the sensitivity for cartilage lesion detection, especially the early ones, within the knee joint from 73.3 to 96.7% (p=0.001). This agrees with Liebl et al. [4] who suggested, in a case-control study using 130 subjects that knees with incident tibiofemoral osteoarthritis had significantly higher mean T2 values in each compartment compared with controls similar to Dunn et al. [17] case/control study of 55 subjects that showed significantly (p <0.05) increases in T2 relaxation times between diseased (34.4-41.0 ms) and healthy (32.1-35.0 ms) knees. Also, Kijowski et al. [18], prospectively studied 150 subjects, found that increased T2 values in cartilages corresponded to cartilage lesions arthroscopically, and stated that increased articular cartilage T2 value could be used as an indicator of cartilage degeneration in OA.
The relative lower sensitivity of the standard MRI protocol of knee cartilage for identifying early cartilage degeneration is close to the previous studies’ findings, which have reported sensitivities for detecting cartilage abnormalities and defects ranging from 9-62% at both 1.5 T and 3.0 Tesla [19,20,21,22,23,24].
On the other hand, our study reported that the T2 mapping sequence showed a decrease in specificity for detecting knee cartilage lesions from 100% by routine MRI to 90% by adding T2 mapping. This decrease may be from the magic angle effect leading to a false increase in T2 relaxation time or early asymptomatic cartilage degeneration.
While all these studies, including our study, showed an association between increased T2 values and cartilage degeneration, Hirose et al. study [25] reported no significant change in T2 values with cartilage degeneration relative to normal cartilage does exist in the literature.
Our study results show that, as reported by Crema et al. [5] and Dautry et al. [26], T2 mapping imaging may add a robust value to MR imaging ability to identify focal knee chondral lesions when T2 mapping abnormalities and pain location are correlated. We found a correspondence between pain location and cartilage with T2 prolongation time in our study. T2 mapping focal abnormalities in the symptomatic compartments of 29 out of 30 patients (5 out of 5 symptomatic anterior compartments, 11 out of 11 symptomatic medial compartments, and 3 out of 4 symptomatic lateral compartments) are found.
Concerning T2 values in different knee joint compartments, our study results—similar to the Dautry et al. [26] study—showed no significant difference (p = 0.55) between the mean T2 values of medial, lateral femorotibial cartilage, and anterior patellofemoral cartilage of OA patients. Contrariwise, another study by Mosher et al. [27], involving persons with and without radiographic knee OA, showed a positive correlation with higher T2 values in the medial compartment cartilage and a higher degree of knee pain.
In agreement with the cross-sectional CT arthrography retrospective study by Omoumi et al. [28] carried on 535 consecutive knees showing that cartilage of the posterior aspect of the medial condyle was statistically thicker in osteoarthritic knees compared to non-osteoarthritic ones (p<0.001), our study results showed that there was a significant difference between cartilage thickness in OA patients and controls (p = 0.03). On the contrary, Mittal et al. [29] reported a lack of difference in average articular cartilage thickness between OA patients and controls (p>0.05). Also, Li et al. [30] reported that there was no significant difference in the average cartilage thickness in OA patients and control subjects (p = 0.37). They suggested that there is no significant correlation found between T2 values and cartilage thickness (p > 0.05), different from our study results that observed a significant moderate negative correlation (r = −0.43) between T2 values and cartilage thickness (p = 0.001).
Few studies in literature inspect the effect of physical activity on T2 mapping values. For example; one study by Mosher et al. [31], tested the effect of running/training on T2 mapping values eliciting the so-called functional cartilage MRI T2 mapping. Our study, however, has eliminated the effect of physical activity by necessitating an average of 30-45 min of rest for all patients prior to the examination. The aim of that was to cancel the effect of knee loading in an attempt to study the degenerative effect solely without the interference of other factors.
Regarding correlation with the gold standard techniques, one study in literature by Bazaldua et al. [32] correlated T2 mapping values with arthroscopic results and found that the T2 mapping sequence could identify 88 (92.6%) of the total number of lesions found with arthroscopy, while the conventional sequence protocol (conventional T2 sequence), evaluated by certified radiologists, was able to find 83 lesions (87.3%).
However, in our study, patients were referred for initial MR assessment in the context of a conservative treatment plan rather than arthroscopy (the gold standard) which was not a near plan. Accordingly, we resorted to correlation with clinical severity scoring of pain, stiffness, and physical disability as a reference and a good alternative to the non-applicable gold standard in addition to using a case-control study pattern. Based on that, we were able to find a credible correlation between the T2 values and WOMAC scores.
According to the results of our study, there was a highly significant positive correlation between total WOMAC score and T2 values of osteoarthritic patients (p>0.001), supporting the reliability of T2 mapping (i.e., the higher the T2 value; the higher the WOMAC score). While cartilage thickness of osteoarthritic patients showed a moderately negative correlation with WOMAC score (p = 0.04) (i.e., the smaller the cartilage thickness, the higher the WOMAC score).
Regarding age, our study showed a significant medium positive correlation (r = 0.32) between age and T2 values (p = 0.02) that agrees with ÇAĞLAR et al. study [33], which demonstrated that T2 values elevation correlated with age in all compartments measured in the subgroups of both patient and control groups.
Regarding gender, our study showed that there is no significant difference between T2 values of males and females (p = 0.5) that agrees with Mosher et al. study 19 [34], which comparing differences in T2 values between healthy men and women found no differences between genders. Also, ÇAĞLAR et al. study [33] found no statistically significant difference between the male and female cartilage T2 relaxation times in the subgroups with no knee joint pathology and the total group in all three compartments.
Regarding the literature [5,6,7], the “T2 mapping” sequence is a sensitive technique for hyaline cartilage lesions. However, it has several limitations. T2 value of the cartilage is dependent on field strength [35], moderately dependent on temperature [36], and more remarkably on physical activity, loading of the knee [37], and age [38]. Therefore, in our study, MR imaging was done on the same MRI field strength, at the same temperature, and after 30 min resting time.
Our study had several limitations; the most important was the lack of confirmatory arthroscopic procedure, as it would be inadequate for arthroscopy to assess for early or subtle intrinsic cartilage abnormalities shown on T2 maps. The second limitation was that T2 mapping is susceptible to the magic angle effect; therefore, we avoid the measurement of cartilage oriented at 55° to the main magnetic field. The third limitation was the non-recording of the body mass index of the patients and controls.