Qualitative and Quantitative Evaluation of Morpho-Metabolic Changes in Bone Cartilage Complex of Knee Joint in Osteoarthritis Using Simultaneous 18F-NaF PET/MRI-A Pilot Study

Abstract

Background Articular cartilage (AC) loss and deterioration, as well as bone remodeling, are all symptoms of osteoarthritis (OA). As a result, an ideal imaging technique for researching OA is required, which must be sensitive to both soft tissue and bone health. Objective The aim of this study was to assess the potential of simultaneous 18F sodium fluoride (18F-NaF) positron emission tomography/magnetic resonance imaging (PET/MRI) to identify as well as classify osseous metabolic abnormalities in knee OA and to see if degenerative changes in the cartilage and bone on MRI might be correlated with subchondral 18F-NaF uptake on PET. Methods Sixteen (32 knees) volunteers with no past history of knee injury, with or without pain, were enrolled for the research from January to July 2021. The images of both knees were taken utilizing an molecular magnetic resonance (mMR) body matrix coil on a simultaneous PET/MRI biograph mMR. The acquisition was conducted after 45 minutes of intravenous infusion of 18F-NaF 185-370 MBq (5-10 mCi) over one PET bed for 40 minutes, while MRI sequences were performed simultaneously. Results All pathologies showed significantly higher maximum standardized uptake value (SUV _max ) than the background. Thirty-four subchondral magic spots were identified on 18F-NaF PET without any structural alteration on MRI. Bone marrow lesions (BMLs) and osteophytes with higher MRI osteoarthritis knee score (MOAKS) score showed higher 18F-NaF uptake (grade1˂grade2˂grade3). BMLs had corresponding AC degeneration. There was discordance between grade 1 osteophytes (86.6%), sclerosis (53.7%) and grade 1 BML in cruciate ligament insertion site (91.66%); they did not have high uptake of 18F-NaF. In case of cartilage, there was significant difference between AC grades and average subchondral SUV _max and T2* relaxometry (grade0˂grade1˂grade2˂grade3˂grade4). BMLs are much more metabolically active than other pathologies, while sclerosis is the least. We also found that the subchondral uptake was statistically increased in the areas of pathology: Conclusion 18F-NaF PET/MRI was able to detect knee abnormalities unseen on MRI alone and simultaneously assessed metabolic and structural markers of knee OA across multiple tissues in the joint. Thus, it is a promising tool for detection of early metabolic changes in OA.

Keywords: bone; cartilage; knee joints; osteoarthritis; simultaneous PET/MRI.

Fig. 1

18F sodium fluoride (18F-NaF) served as a one-stop modality to assess whole joint, that is, bone pathology, cartilage, and ligaments (sagittal T1 turbo spin echo) showed osteophyte (orange arrow) and sclerosis (yellow arrow) with corresponding 18F-NaF uptake ( B ; fused positron emission tomography/magnetic resonance imaging [PET/MRI]); ( C ) sagittal three-dimensional MEDIC) did not have any structural abnormality but when fused with PET ( D ) there was high uptake volume of interest (green arrow) termed as “subchondral magic spot”; ( E ) sagittal T2 spectral attenuated inversion recovery [SPAIR]) depicted grade 1 BML (blue arrow) with corresponding uptake in fused PET/MRI ( F ) G (axial T2*) showed degenerated medial and lateral trochlear cartilage (pink arrow) with corresponding fused PET/MRI ( H ) and T2* relaxometry ( I ) that raised values. Additional findings that can be obtained with PET/MRI scan. J (sagittal T2 SPAIR) and K (coronal T2 SPAIR) showed medial meniscal tear and image L (sagittal T2 SPAIR) showed mucoid degeneration of anterior cruciate ligament.

Fig. 2

Maximum standard uptake value (SUV _max ) and number of areas with respect to articular cartilage (AC) grade.

Fig. 3

T2* relaxometry value and number of areas with respect to articular cartilage (AC) grade.