Introduction

Osteoarthritis (OA) is a heterogeneous disease characterised by diarthrodial joint multi-tissue failure¹; it is the most prevalent chronic rheumatic illness and the leading cause of disability worldwide². With the implementation of MRI, increased early detection rates could allow faster intervention and alleviation of burden upon the NHS. In addition, the analysis and comparison between histology of a cartilage tissue sample and MRI are critical for validating novel imaging methods and contrasts, particularly in molecular imaging research³. This study investigated the correlation between the OA cartilage tissue MRI contrast (under compression/relaxation) with histological staining. Implications are noteworthy as we can sensitise OA protocols during MRI analysis to target specific histological features and perform non-invasive monitoring of the disease in response to therapeutic interventions to improve the response of the impaired cartilage tissue to a natural load in OA.

Methods

Human Cartilage OA (n=7) Specimens were taken following knee replacement surgery (conducted at the University of Nottingham) from the tibia plateau cartilage. Control cartilage specimen (n=6) were obtained from cadavers. The cartilage and subchondral bone were sectioned into 1cm³ cubes, staged for OA, vacuum-sealed to prevent evaporation, and utilised in MRI studies. The samples were subsequently histochemically examined to determine the sites of PG and collagen in the cartilage. NMR and MRI of all spin species reported in this study were performed on a 9.4 T Bruker Avance III Microimaging system (Bruker, Germany) using 25mm dual tuned ¹H/²³Na microimaging coil (Bruker, Germany). ¹H multi-slice T1 weighted /T2* gradient echo MRI protocol (Paravision 6.01) was used to visualise anatomy of the cartilage with 20mm in-plane resolution and monitor the compression. ²³Na MRI was performed using non-slice selective gradient echo home-written protocol for TopSpin 3.2 environment and spectroscopically determined triple quantum filter (TQF) was integrated into the code to visualise stored sodium in the same cartilage specimen. Sodium levels of cartilage samples in the images were expressed in SNRs. Tripe quantum filtered spectroscopy was used to characterise both fast and slow sodium transverse relaxation and residual quadrupolar coupling constant in all cartilage specimens.

Results

The concept for co-registration of MRI contrast induced by sodium and water protons in the cartilage tissue is outlined in Figure 1. Changes in collagen fibres probed by T2* of water protons in the cartilage during compression/relaxation cycle for OA and Control specimen are shown in Figure 2. The co-localisation of free and bound sodium in OA and cartilage specimens visualised by ²³Na MRI during compression/relaxation cycle are shown in Figure 3 where sodium levels were compared using sodium SNRs. 23Na triple quantum filtered spectroscopy was used to characterise sodium transverse fast and slow relaxation, as well residual quadrupolar coupling constant. Only slow component of sodium transverse relaxation demonstrated statistically significant outcome that is displayed in Figure 4 for all studied control and OA specimens.

Discussion

We demonstrate that microimaging can be used to correlate proton induced T2* contrast with collagen response during compression/relaxation cycle. We demonstrate that sodium contrast reflects GAGs behaviour during compression/relaxation cycle for both control and OA specimens. We show that both free and bound sodium are located within the cartilage tissue that consists mostly of the extracellular matrix enriched with glycosaminoglycans (GAGs) at similar levels during compression/relaxation cycle. Hence the characterisation of the cartilage tissue was performed using triple quantum filtered sodium spectroscopy. We found that the slow component of sodium transverse relaxation was sensitive to distinguish between OA and control cases only when compression/relaxation cycle applied. Without compression we found no difference between control and OA specimen in all studied sodium and proton markers.

Conclusion

We provide the evidence that the slow component of sodium triple quantum filtered relaxation is a marker for OA during compression/relaxation cycle. Our data demonstrate that the compression/relaxation cycle subjects the cartilage tissue to undergo changes that result in the same relaxed state as the tissue was initially for the controls while in the case of OA the tissue never restores to its original pre-compression state. This is indicative of different mechanical response to the normal stress, probably associating with the loss of GAGs in the cartilage scaffold linked to OA progression. This hypothesis should be further tested in vivo.

Acknowledgements

GEP and TM thank the Medical Research Council for funding (Grant No. MC_PC_15074).

References

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