Introduction
Juvenile idiopathic arthritis is the most common rheumatic disease in childhood [
1]. Cartilage can be damaged by the autoimmune-mediated inflammation that originates in the synovial membrane. Ongoing inflammation can subsequently extend to the cartilage and result in degradation of cartilage and bone matrix. Osteochondral damage is presumably irreversible and associated with disability and decreased quality of life [
2,
3]. Previous research also indicates that damaged cartilage matrix facilitates binding of synovial inflammatory cells [
4]. Thus, even when inflammation subsides, a damaged matrix leaves the child at increased risk for more cartilage degradation whenever a flare occurs. Damage to the cartilage matrix is characterized by loss of proteoglycan and collagen, as demonstrated in studies focusing on rheumatoid arthritis [
5‐
9]. This microstructural damage is important to recognize because intensification of anti-rheumatic treatment might prevent irreversible cartilage damage. Current imaging techniques, such as radiography, US imaging and conventional MRI, can detect structural bone damage and synovial inflammation but cannot detect microstructural damage to cartilage [
10].
T
1ρ is an MRI parameter that is hypothesized to quantify proteoglycan loss of the cartilage matrix in vitro [
11] and in vivo [
12]. T
1ρ uses a weak radiofrequency pulse to lock protons in phase, which slows transverse relaxation and reduces the effect of dipolar interactions. As a result, the measured relaxation of protons can be attributed to time-constant T
1ρ. Earlier studies in osteoarthritis and rheumatoid arthritis [
12‐
16] showed that T
1ρ values increase as a consequence of decreased proteoglycan content. Cartilage integrity was studied once in pediatric patients using T
1ρ: knee cartilage of 10 healthy children was assessed and a mean T
1ρ value of 76.6 ms was reported [
17]. Cartilage in children is distinct from articular cartilage in adults. Postnatally, chondrocytes are small and there is a scant matrix without a distinct zonal orientation [
18]. Animal studies demonstrated that proteoglycan content is highest postnatally, with a gradual decrease in proteoglycans during aging, while collagen content increases and collagen fibers show increasing isotropy, resulting in development of a zonal organization of the cartilage [
19‐
21].
The primary aim of this pilot study was to evaluate feasibility (motion artefacts and patient comfort) of T
1ρ knee cartilage imaging in children with juvenile idiopathic arthritis; secondarily we studied repeatability of T
1ρ values derived by manual cartilage segmentation. Moreover, we explored T
1ρ values in juvenile idiopathic arthritis by comparing T
1ρ values with conventional MRI scores using the juvenile arthritis MRI scoring system [
22].
Discussion
In this pilot study, we show the feasibility of T1ρ for assessing knee cartilage integrity in children with juvenile idiopathic arthritis. All 13 children in the study underwent the T1ρ acquisition protocol without discomfort and all images were of sufficient quality. None of the 13 children showed structural cartilage damage on conventional MRI. We found excellent repeatability for derivation of T1ρ values using manual cartilage segmentations on the T1ρ images.
Concerning the assessment of cartilage integrity, we found high correlation between T
1ρ values and the juvenile arthritis MRI score, an MRI-based disease activity score, in the seven children who had inflammation in the knee, but we also observed that T
1ρ values in children with actively inflamed knees were not different compared to T
1ρ values in those with non-inflamed knees. Nevertheless, cartilage of the lateral patellar region of interest demonstrated significantly higher T
1ρ values as compared to cartilage in children without knee inflammation on MRI. The patellar cartilage borders the patellofemoral synovium, which is often affected if knee arthritis is present [
33]. Correspondingly, in our study, 4 of the 7 children with active arthritis indeed showed inflamed patellofemoral synovium. Thus, we hypothesize that increased T
1ρ values might represent pre-erosive microstructural damage to proteoglycans and collagen in the cartilage matrix that is not visualized using conventional MR sequences.
Several studies confirmed that T
1ρ values can be used to detect macrostructural and microstructural damage to cartilage in osteoarthritis [
34‐
36]. In rheumatoid arthritis, the use of T
1ρ was first described in a study involving five people with rheumatoid arthritis [
12]. In this study of cartilage specimens after total knee arthroplasty, T
1ρ values correlated with histological Safranin-O staining and macroscopic grade of severity of cartilage degeneration. In another study, radiocarpal cartilage was evaluated in a 3-month follow-up study of nine people with rheumatoid arthritis who used anti-rheumatic medication [
37]. T
1ρ values correlated with treatment response, showing the potential of T
1ρ to measure changes in cartilage structure following treatment. Our results seem comparable with the findings in both osteoarthritis and rheumatoid arthritis [
10,
12,
37] in which people with more severe disease activity were found to have higher T
1ρ values. When comparing the absolute T
1ρ values, we found (lowest-to-highest) 31 ms to 55 ms. Values in people with rheumatoid arthritis have been found to be 38–62 ms [
12], and values in healthy pediatric patients 66–77 ms [
17]. Note our values are somewhat lower. For the first comparison, this is probably attributable to the more severely affected cartilage in these children with rheumatoid arthritis who were scheduled for total knee arthroplasty. Another factor to take into account when comparing results from different studies is the spin lock frequency because T
1ρ values are higher at increased spin lock frequency. Our scans were acquired at lower frequency (400 Hz) than the scans of people with rheumatoid arthritis and healthy pediatric subjects (both acquired at 500 Hz) [
12,
17].
We found a correlation between the juvenile arthritis MRI score and T1ρ values as well as erythrocyte sedimentation rate and T1ρ values. This supports the hypothesis that inflammation in the knee negatively affects the cartilage. We could not confirm our hypothesis that increasing age, and thus lower proteoglycan content, leads to lower T1ρ values because we observed no correlation between age and T1ρ values in this small cohort. However, this could be influenced by the age dispersion in our cohort because all but one child was older than 10 years.
Concerning body mass index, literature shows contradictory results. A recent study found correlation between body mass index and T
1ρ in the knee [
38] while others decline a relation between body mass index and T
1ρ values in hip cartilage and intervertebral disc cartilage, respectively [
39,
40]. In our study, body mass index was not correlated to T
1ρ values. It should be noted that our cohort consisted of mainly non-obese adolescents, thus we cannot exclude that age and body mass index could influence T
1ρ values in, for example, a 4-year-old or heavily obese child.
Limitations of this study are that none of the children had structural cartilage damage on MRI, hence it was not possible to examine the T
1ρ value in actual erosive cartilage damage. Moreover, histochemical proof of the hypothesized pre-erosive microstructural proteoglycan loss in the cartilage is lacking. However, obtaining cartilage specimens using biopsy is not feasible because this would harm the joints of these children. Furthermore, our T
1ρ experiments were performed at a spin lock frequency of 400 Hz because of specific absorption rate limitations and the need to keep the acquisition time short enough. Therefore, the used spin lock preparation did not completely remove all the contributions of dipolar interactions to the relaxation process. As a consequence, our readout is not completely specific to proteoglycan content and likely also reflects changes in the collagen matrix, such as degradation or swelling. Additional studies are needed in order to decouple the two contributions and gain more insight into the biochemical modifications induced by the disease. Another limitation is our segmentation. Although the cartilage segmentations were performed meticulously by an experienced reader, we cannot rule out that the cartilage–bone and cartilage–soft-tissue boundaries were imperfect. This could have influenced our results, especially if possible fluid pixels from joint effusion were wrongly included in the segmentation. To prevent this, we used three imaging planes when drawing the segmentations. Second, we performed our segmentation on full-thickness articular cartilage. Because cartilage has a zonal orientation, it would be interesting to subdivide the cartilage into a superficial and deep layer and study spatial variation in more detail. This could, for example, be performed using a normalization procedure to correct for different cartilage thicknesses in children, as has been done by authors studying cartilage with T2 relaxation time mapping in healthy children and children with juvenile idiopathic arthritis [
41‐
44]. However, because our primary goal was to evaluate the feasibility of T
1ρ, we did not perform such in-depth analyses of the cartilage; nevertheless, we would recommend a zonal analysis of the cartilage in studies that include a bigger sample of patients. Last, the small patient sample itself is considered a limitation and further work should focus on inclusion of more patients to validate the results of our pilot study. When more patients are included, the likelihood of scanning patients with structural cartilage damage would increase, which is important to affirm the assumption that higher T
1ρ values are seen in structurally abnormal cartilage as seen on conventional, qualitative MRI.
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