Cartilage Properties Quantified With 7T MRI

Abstract

Both the younger population with cartilage defects and the elderly population with OA can benefit from proper assessment of their articular cartilage, for which we use 7T MR imaging in this thesis. In order to get new imaging techniques embedded in the clinic, a number of stages must be completed before a new technique reaches the patient. Ideally, one could start with a perfectly functioning MR, with a dedicated coil and validated sequences. Therefore, we developed a double tuned proton-sodium knee coil for this purpose, which has been described in chapter 2. Once we had produced and validated this coil, the next step was to validate sequences for clinical use, but not before choosing the imaging targets. Within this thesis, the choice has been made to pursue two components of articular cartilage, being GAG content and collagen structure/content. Upon target selection, the next phase towards clinical implementation of new sequences is ex vivo validation. We chose to validate our sodium scans in ex vivo tibial plateaus of patients who underwent a total knee replacement. The design and results of this work, including the (lack of) correlation with DMMB analyses, are shown in chapter 3. Within chapter 4 we scaled these ex vivo experiments up from small cartilage samples to whole joint samples, in this case carpal joints of Shetland ponies. These Shetland ponies were part of a larger experiment which tested the effect of a blunt groove model and a sharp groove model. We used T2* mapping to gain insight in the collagen fiber network of the cartilage in the healthy contralateral control sites and the grooved sites. With this technique we were able to show differences in average relaxation time between grooved and healthy cartilage – paving the way for implementation in patients to discriminate between healthy and damaged cartilage. The following step after ex vivo validation is in vivo feasibility. We designed a feasibility study for gagCEST MRI, to assess the feasibility and potential of gagCEST MRI in a group of patients, being cartilage repair patients. We showed (chapter 5) that we could apply gagCEST MRI in a clinically feasible scanning time of seven minutes and demonstrated high stability, reproducibility and clinical applicability. Now that we showed the feasibility, we tried to prove that gagCEST is able to measure what we expect it does: (early) cartilage damage. In chapter 6, we implemented a pilot study where we acquired gagCEST images from patients before TKA, whereafter we retrieved the cartilage resurfacing cuts to assess those with electromechanical indentation and biochemical assays. We showed that there is indeed a good correlation of gagCEST with electromechanical indentation, on both the lateral and medial condyle. Summarizing, this thesis shows the applicability of 7T MRI within imaging and quantification of articular cartilage in various stages of (technical) development. This thesis describes several steps which are needed to implement imaging protocols in clinical practice. These steps are ex vivo feasibility and validation, in vivo feasibility and finally in vivo validation and implementation.