Dynamic contrast-enhanced MRI of synovitis in knee osteoarthritis: repeatability, discrimination and sensitivity to change in a prospective experimental study

Imaging-based measures of synovitis in knee osteoarthritis: A scoping review and narrative synthesis

Background

Synovitis has emerged as a tissue target of importance in OA research and is commonly evaluated with medical imaging.

Objectives

The purpose of this scoping review was to identify and describe the medical imaging techniques and definitions used by knee OA researchers to assess synovitis, summarize their advantages and disadvantages, and identify opportunities for future work.

Eligibility criteria

We included full-text peer-reviewed English publications including adults diagnosed with knee OA. Studies were included if one or more medical imaging modalities were used to assess synovitis in the knee.Studies of inflammatory arthritis, joint replacement, and synovial joints other than the knee were excluded. Animal studies and literature syntheses were also excluded.

Sources

MEDLINE, SCOPUS, and Google scholar databases were systematically searched for publications (2000-2023) using the following medical subject headings (MeSH): "osteoarthritis, knee", "magnetic resonance imaging", "ultrasonography", "synovitis".

Results

1154 articles were identified from searching medical databases. After removal of duplicates, abstract screening, and full text reading, 251 articles were included in the final review. MRI is the most common modality employed to assess knee synovitis, followed by US imaging. Varied imaging techniques used in the assessment of joint synovitis may be targeting divergent constructs of synovial remodeling and inflammation, which complicates interpretation of results.

Conclusions

There is no consensus on the best method for imaging of knee synovitis in OA. Future work may benefit from the evaluation of synovitis separate from joint effusion, and their associations with histologic findings to discriminate between features of synovial inflammation and remodeling.

Genicular Arteries Embolization for Patients with Osteoarthritis, Their Selection, and Follow-Up Based on MRI Findings

Osteoarthritis (OA) is a leading cause of disability worldwide, with its prevalence rising due to aging populations. Management ranges from conservative treatments such as weight management and pharmacologic therapy to surgical interventions such as total joint replacement. However, treating moderate knee OA remains challenging for patients unresponsive to conservative care but not yet surgical candidates. Genicular artery embolization (GAE) has emerged as a minimally invasive procedure targeting abnormal angiogenesis and inflammation in OA. This article explores GAE's mechanism, patient-selection criteria, and effectiveness in pain reduction and functional improvement. Studies suggest that GAE has the potential to significantly improve pain and function in mild to moderate OA, with sustained benefits. Patient selection is crucial for optimal outcomes, with imaging playing a key role. While conventional MRI assesses structural damage, Dynamic Contrast-Enhanced MRI (DCE-MRI) offers superior insights by evaluating synovitis, quantifying cartilage degradation, and monitoring treatment response. Due to its strong correlation with pain scores and status as the best surrogate marker for inflammation in synovitis, DCE-MRI holds significant potential to enhance patient selection and treatment monitoring for GAE.

Infrapatellar fat pad as a source of biomarkers and therapeutic target for knee osteoarthritis

BACKGROUND AND OBJECTIVE

Osteoarthritis (OA) is a multifactorial and highly prevalent disease in elderly adults; however, its pathogenesis, diagnosis, and treatment are unmet needs nowadays. Research efforts have focused on elucidating the molecular mechanisms involved in the pathogenesis, onset, and progression of OA to facilitate early detection and effective therapeutic approaches. Infrapatellar fat pad (IPFP) represents a promising novel source of OA biomarkers given that it is an active player in OA. This review aims to investigate the current literature regarding the potential of the IPFP as a source of diagnostic and prognostic biomarkers for OA as well as potential target for novel therapies.

METHODS

A literature search was conducted in the PubMed database in June 2024. We included cross-sectional and longitudinal studies based on IPFP from human OA patients, oriented in the identification of imaging, biochemical, and molecular biomarkers in the IPFP.

RESULTS

After screening and evaluation, we included a total of 61 studies. Most of the imaging publications (n = 47) on IPFP are based on magnetic resonance imaging (MRI) that revealed potential semiquantitative and quantitative imaging biomarkers linked to inflammation, fibrosis, pain, and joint degeneration imaging parameters. Biochemical and molecular studies (n = 14) pointed out an increase in interleukin-6 (IL-6), fatty acid-binding protein 4 (FABP4), adiponectin, and lysophosphatidylcholine (LysoPC) in the IPFP during OA progression.

CONCLUSIONS

Imaging, biochemical, and molecular studies indicate OA potential biomarkers in the IPFP related to inflammation, lipid dysregulation, and fibrosis. The combination of imaging and biochemical biomarkers could provide a better prediction of OA onset and the identification of OA progressors at an early stage. The IPFP study could also reveal potential therapeutic targets with the vision of better precision medicine.

Recent evolution in imaging techniques for assessment of synovitis in osteoarthritis

Synovitis remains an important marker of osteoarthritis (OA) disease incidence and progression, and is best assessed using imaging. In general, MRI with intravenous contrast is considered the gold standard method for assessing synovitis because it can effectively differentiate inflamed synovium and adjacent joint effusion and other surrounding structures. However, administration of intravenous gadolinium is not always desirable. Several emerging methods are being explored for the visualization of synovitis using non-contrast-enhanced MRI (NCE-MRI) but currently underestimate the amount of inflammation. Ultrasound is another approach that is able to measure and quantify synovitis; however, as with other applications of ultrasound, it is observer-dependent, which may affect reproducibility. Radiography does not play a role in synovitis assessment due to its inability to differentiate intraarticular soft tissues. CT, when contrast enhanced, has been shown to effectively detect synovitis and may be a viable alternative to MRI when MRI is contraindicated or not available. Nuclear medicine techniques such as PET-CT, PET-MRI, and SPECT-CT are not routinely used due to high cost, radiation exposure, and image acquisition times. However, novel radiotracers/biomarkers are being investigated. AI approaches have been investigated for their ability to predict clinical and structural outcomes and for automated detection and quantification including features such as effusion-synovitis.

Fluctuation of Bone Marrow Lesions and Inflammatory MRI Markers over 2 Years and Concurrent Associations with Quantitative Cartilage Loss

OBJECTIVE

To assess whether change of semiquantitatively magnetic resonance imaging (MRI)-defined bone marrow lesions (BMLs) and inflammatory markers is associated with change in quantitatively-assessed cartilage loss in the femorotibial joint (FTJ) in knees with radiographic osteoarthritis (OA) over 24 months.

DESIGN

Participants were included from the IMI-APPROACH and the Osteoarthritis Initiative FNIH studies. Semiquantitative MRI assessment was performed for BMLs, Hoffa- and effusion-synovitis. Quantitative cartilage thickness measurements were performed manually. Definitions of change included number of subregions with BMLs, change in sum and change in maximum increase in size. Change in Hoffa-synovitis and effusion-synovitis was categorized in addition. Between-group comparisons regarding cartilage loss in the FTJ, medial and lateral compartments were performed using analysis of variance (ANOVA).

RESULTS

A total of 629 participants were included. Knees without any BMLs at baseline (BL) and follow-up (FU) had significantly less cartilage loss compared to the other subgroups. Change in both directions in the sum score of BMLs was associated with increased rates of cartilage loss. Maximum increase in size of BMLs was associated with increased rates of cartilage loss (FTJ increase by 2 grades -0.183 mm, 95% CI [-0.335, -0.031], by 3 grades -0.306 mm, [-0.511, -0.101]). Worsening of Hoffa-synovitis was associated with increased rates of cartilage loss.

CONCLUSION

Knees without BMLs at BL and FU showed lowest rates of cartilage loss. Knees with an increase in BML size showed increased rates of concurrent cartilage loss. Approaches with the aim to inhibit BML development, avoidance of increase in size and avoidance of Hoffa-synovitis worsening may have beneficial effects on cartilage loss.

Perfusion Imaging of the Musculoskeletal System

Perfusion imaging is the aspect of functional imaging, which is most applicable to the musculoskeletal system. In this review, the anatomy and physiology of bone perfusion is briefly outlined as are the methods of acquiring perfusion data on MR imaging. The current clinical indications of perfusion related to the assessment of soft tissue and bone tumors, synovitis, osteoarthritis, avascular necrosis, Keinbock's disease, diabetic foot, osteochondritis dissecans, and Paget's disease of bone are reviewed. Challenges and opportunities related to perfusion imaging of the musculoskeletal system are also briefly addressed.

Advanced Magnetic Resonance Imaging and Molecular Imaging of the Painful Knee

Chronic knee pain is a common condition. Causes of knee pain include trauma, inflammation, and degeneration, but in many patients the pathophysiology remains unknown. Recent developments in advanced magnetic resonance imaging (MRI) techniques and molecular imaging facilitate more in-depth research focused on the pathophysiology of chronic musculoskeletal pain and more specifically inflammation. The forthcoming new insights can help develop better targeted treatment, and some imaging techniques may even serve as imaging biomarkers for predicting and assessing treatment response in the future. This review highlights the latest developments in perfusion MRI, diffusion MRI, and molecular imaging with positron emission tomography/MRI and their application in the painful knee. The primary focus is synovial inflammation, also known as synovitis. Bone perfusion and bone metabolism are also addressed.

Imaging of early-stage osteoarthritis: the needs and challenges for diagnosis and classification

In an effort to boost the development of new management strategies for OA, there is currently a shift in focus towards the diagnosis and treatment of early-stage OA. It is important to distinguish diagnosis from classification of early-stage OA. Diagnosis takes place in clinical practice, whereas classification is a process to stratify participants with OA in clinical research. For both purposes, there is an important opportunity for imaging, especially with MRI. The needs and challenges differ for early-stage OA diagnosis versus classification. Although it fulfils the need of high sensitivity and specificity for making a correct diagnosis, implementation of MRI in clinical practice is challenged by long acquisition times and high costs. For classification in clinical research, more advanced MRI protocols can be applied, such as quantitative, contrast-enhanced, or hybrid techniques, as well as advanced image analysis methods including 3D morphometric assessments of joint tissues and artificial intelligence approaches. It is necessary to follow a step-wise and structured approach that comprises, technical validation, biological validation, clinical validation, qualification, and cost-effectiveness, before new imaging biomarkers can be implemented in clinical practice or clinical research.

Update on recent developments in imaging of inflammation in osteoarthritis: a narrative review

Synovitis is an important component of the osteoarthritis (OA) disease process, particularly regarding the "inflammatory phenotype" of OA. Imaging plays an important role in the assessment of synovitis in OA with MRI and ultrasound being the most deployed imaging modalities. Contrast-enhanced (CE) MRI, particularly dynamic CEMRI (DCEMRI) is the ideal method for synovitis assessment, but for several reasons CEMRI is not commonly performed for OA imaging in general. Effusion-synovitis and Hoffa-synovitis are commonly used as surrogate markers of synovitis on non-contrast-enhanced (NCE) MRI and have been used in many epidemiological observational studies of knee OA. Several semiquantitative MRI scoring systems are available for the evaluation of synovitis in knee OA. Synovitis can be a target tissue for disease-modifying OA drug (DMOAD) clinical trials. Both MRI and ultrasound may be used to determine the eligibility and assess the therapeutic efficacy of DMOAD approaches. Ultrasound is mostly used for evaluation of synovitis in hand OA, while MRI is typically used for larger joints, namely knees and hips. The role of other modalities such as CT (including dual-energy CT) and nuclear medicine imaging (such as positron-emission tomography (PET) and its hybrid imaging) is limited in the context of synovitis assessment in OA. Despite research efforts to develop NCEMRI-based synovitis evaluation methods, these typically underestimate the severity of synovitis compared to CEMRI, and thus more research is needed before we can rely only on NCEMRI.

Quantitative bone marrow lesion, meniscus, and synovitis measurement: current status

Imaging plays a pivotal role in osteoarthritis research, particularly in epidemiological and clinical trials of knee osteoarthritis (KOA), with the ultimate goal being the development of an effective drug treatment for future prevention or cessation of disease. Imaging assessment methods can be semi-quantitative, quantitative, or a combination, with quantitative methods usually relying on software to assist. The software generally attempts image segmentation (outlining of relevant structures). New techniques using artificial intelligence (AI) or deep learning (DL) are currently a frequent topic of research. This review article provides an overview of the literature to date, focusing primarily on the current status of quantitative software-based assessment techniques of KOA using magnetic resonance (MR) imaging. We will concentrate on the imaging evaluation of three specific structural imaging biomarkers: bone marrow lesions (BMLs), meniscus, and synovitis consisting of effusion synovitis (ES) and Hoffa's synovitis (HS). A brief clinical and imaging background review of osteoarthritis evaluation, particularly relating to these three structural markers, is provided as well as a general summary of the software methods. A summary of the literature with respect to each KOA assessment method will be presented overall as well as with respect to each specific biomarker individually. Novel techniques, as well as future goals and directions using quantitative imaging assessment, will be discussed.

Osteoarthritis year in review 2022: imaging

PURPOSE

This narrative review summarizes original research focusing on imaging in osteoarthritis (OA) published between April 1st 2021 and March 31st 2022. We only considered English publications that were in vivo human studies.

METHODS

The PubMed, Medline, Embase, Scopus, and ISI Web of Science databases were searched for "Osteoarthritis/OA" studies based on the search terms: "Radiography", "Ultrasound/US", "Computed Tomography/CT", "DXA", "Magnetic Resonance Imaging/MRI", "Artificial Intelligence/AI", and "Deep Learning". This review highlights the anatomical focus of research on the structures within the tibiofemoral, patellofemoral, hip, and hand joints. There is also a noted focus on artificial intelligence applications in OA imaging.

RESULTS

Over the last decade, the increasing trend of using open-access large databases has reached a plateau (from 17 to 37). Compositional MRI has had the most prominent use in OA imaging and its biomarkers have been used in the detection of preclinical OA and prediction of OA outcomes. Most noteworthy, there has been an accelerated rate of publications on the implications of artificial intelligence, used in developing prediction models and performing trabecular texture analysis, in OA imaging (from 17 to 154).

CONCLUSIONS

While imaging has maintained its key role in OA research, publication trends have shown an emphasis on the integration of AI. During the past year, MRI has maintained the highest prevalence in usage while US and CT remain as readily available modalities. Finally, there has been a notable uptake in the development and validation of AI techniques used to perform texture analysis and predict OA progression.

What is the Effect of Bevacizumab on Cartilage and Synovium in a Rabbit Model of Hemophilic Arthropathy?

BACKGROUND

Hemophilic arthropathy can cause recurrent hemarthroses and severe damage to the synovium and articular cartilage. Previous studies have shown that vascular endothelial growth factor (VEGF) plays an essential role in neoangiogenesis. Bevacizumab, a monoclonal VEGF inhibitor, is used clinically to prevent angiogenesis. However, its effects on hemophilic arthropathy are unknown.

QUESTIONS/PURPOSES

Using a hemophilic arthropathy rabbit model, we asked: Does an intra-articular injection of bevacizumab (1) inhibit VEGF, (2) decrease signal intensity in dynamic contrast-enhanced MRI (DCE-MRI) as an assessment of capillary permeability and neoangiogenesis, (3) reduce cartilage damage, (4) reduce synovial changes, and (5) affect macroscopic changes during the development of hemophilic arthropathy?

METHODS

Twenty-five male New Zealand rabbits were divided into four groups. Eight knees from four rabbits were used as the control group. We used an established animal model for hemophilic arthropathy in the remaining 21 rabbits. Animals were assigned randomly to three groups with seven rabbits in each group. One group was used to establish mild arthropathy, and the other two were used to establish severe arthropathy. Autologous blood from the rabbits' ears was injected into the right and left knees twice per week for 8 weeks to represent mild arthropathy and for 16 weeks to represent severe arthropathy. In the mild arthropathy group, bevacizumab was injected into the right knee once every 2 weeks. Bevacizumab was injected into the right knee of rabbits in one of the severe arthropathy groups once every 2 weeks for 16 weeks, and intra-articular bevacizumab injections were administered to the right knees of rabbits in the other severe arthropathy group once every 2 weeks after the eighth week. An equal volume of 0.9% saline was injected into the left knee of rabbits in all arthropathy groups. To explore the efficacy of bevacizumab, joint diameters were quantitatively measured, and cartilage and synovial changes were examined. Degeneration of articular cartilage was evaluated with the semiquantitative Osteoarthritis Research Society International grading system. Synovial damage was analyzed with a semiquantitative microscopic scoring system. In addition, we evaluated perfusion and angiogenesis using DCE-MRI (quantitative signal intensity changes). Immunohistochemical testing was used to measure VEGF levels (analyzed by Western blotting).

RESULTS

Intra-articular bevacizumab treatment inhibited VEGF in our rabbit model of hemophilic arthropathy. VEGF protein expression levels were lower in the mild arthropathy group that received intra-articular bevacizumab (0.89 ± 0.45) than the mild arthropathy control group (1.41 ± 0.61) (mean difference -0.52 [95% CI -0.898 to -0.143]; p = 0.02). VEGF levels were lower in the severe arthropathy group that received treatment for 16 weeks (0.94 ± 0.27) than in the control knees (1.49 ± 0.36) (mean difference -0.55 [95% CI -0.935 to -0.161]; p = 0.01). In the severe arthropathy group, the Osteoarthritis Research Society International score indicating cartilage damage was lower in the group that received intra-articular bevacizumab treatment from the beginning than in the control group (median 17 [range 13 to 18] versus 18 [range 17 to 20]; difference of medians 1; p = 0.02). Additionally, the scores indicated synovial damage was lower in the group that received intra-articular bevacizumab treatment from the beginning than the control group (median 5 [range 4 to 9] versus 9 [range 8 to 12]; difference of medians 4; p = 0.02). The mean of mean values for signal intensity changes was higher in the nontreated severe groups than in the group of healthy knees. The signal intensity changes were higher in the severe arthropathy control groups (Groups BC and CC) (median 311.6 [range 301.4 to 361.2] and 315.1 [range 269.7 to 460.4]) than in the mild arthropathy control group (Group AC) (median 234.1 [range 212.5 to 304.2]; difference of medians 77.5 and 81, respectively; p = 0.02 and p = 0.04, respectively). In the severe arthropathy group, discoloration caused by hemosiderin deposition in the cartilage and synovium was more pronounced than in the mild arthropathy group. In the severe arthropathy group treated with intra-articular bevacizumab, joint diameters were smaller than in the control group (Group BT median 12.7 mm [range 12.3 to 14.0] versus Group BC median 14.0 mm [range 13.1 to 14.5]; difference of medians 1.3 mm; p = 0.02).

CONCLUSION

Hemarthrosis damages the synovial tissues and cartilage in the knees of rabbits, regardless of whether they are treated with intra-articular bevacizumab. However, intra-articular injection of bevacizumab may reduce cartilage and synovial damage in rabbits when treatment is initiated early during the development of hemophilic arthropathy.

CLINICAL RELEVANCE

If the findings in this study are replicated in larger-animal models that consider the limitations of our work, then a trial in humans might be appropriate to ascertain whether intra-articular injection of bevacizumab could reduce cartilage damage and synovial changes in patients with hemophilia whose hemarthroses cannot otherwise be controlled.

Osteoarthritis year in review 2021: imaging

PURPOSE

To provide a narrative review of original articles on imaging of osteoarthritis (OA) published between January 1, 2020 and March 31, 2021, with a special focus on imaging of inflammation, imaging of bone, cartilage and bone-cartilage interactions, imaging of peri-articular tissues, imaging scoring methods for OA, and artificial intelligence (AI) applied to OA imaging.

METHODS

The Embase, Pubmed, Medline, Cochrane databases were searched for original research articles in the English language on human, in vivo, imaging of OA published between January 1, 2020 and March 31, 2021. Search terms related to osteoarthritis combined with all imaging modalities and artificial intelligence were applied. A selection of articles reporting on one of the focus topics was discussed further.

RESULTS

The search resulted in 651 articles, of which 214 were deemed relevant to human OA imaging. Among the articles included, the knee joint (69%) and magnetic resonance imaging (MRI) (52%) were the predominant anatomical area and imaging modality studied. There were also a substantial number of papers (n = 46) reporting on AI applications in the field of OA imaging.

CONCLUSION

Imaging continues to play an important role in the assessment of OA. Recent advances in OA imaging include quantitative, non-contrast, and hybrid imaging techniques for improved characterization of multiple tissue processes in OA. In addition, an increasing effort in AI techniques is undertaken to enhance OA imaging acquisition and analysis.

Gadolinium-free assessment of synovitis using diffusion tensor imaging

The dynamic contrast-enhanced (DCE)-MRI parameter Ktrans can quantify the intensity of synovial inflammation (synovitis) in knees with osteoarthritis (OA), but requires the use of gadolinium-based contrast agent (GBCA). Diffusion tensor imaging (DTI) measures the diffusion of water molecules with parameters mean diffusivity (MD) and fractional anisotropy (FA), and has been proposed as a method to detect synovial inflammation without the use of GBCA. The purpose of this study is to (1) determine the ability of DTI to quantify the intensity of synovitis in OA by comparing MD and FA with our imaging gold standard Ktrans within the synovium and (2) compare DTI and DCE-MRI measures with the semi-quantitative grading of OA severity with the Kellgren-Lawrence (KL) and MRI Osteoarthritis Knee Score (MOAKS) systems, in order to assess the relationship between synovitis intensity and OA severity. Within the synovium, MD showed a significant positive correlation with Ktrans (r = 0.79, p < 0.001), while FA showed a significant negative correlation with Ktrans (r = -0.72, p = 0.0026). These results show that DTI is able to quantify the intensity of synovitis within the whole synovium without the use of exogenous contrast agent. Additionally, MD, FA, and Ktrans values did not vary significantly when knees were separated by KL grade (p = 0.15, p = 0.32, p = 0.41, respectively), while MD (r = 0.60, p = 0.018) and Ktrans (r = 0.62, p = 0.013) had a significant positive correlation and FA (r = -0.53, p = 0.043) had a negative correlation with MOAKS. These comparisons indicate that quantitative measures of the intensity of synovitis may provide information in addition to morphological assessment to evaluate OA severity. Using DTI to quantify the intensity of synovitis without GBCA may be helpful to facilitate a broader clinical assessment of the severity of OA.

[18F]NaF PET-MRI provides direct in-vivo evidence of the association between bone metabolic activity and adjacent synovitis in knee osteoarthritis: a cross-sectional study

OBJECTIVE

Synovitis is hypothesized to play a role in the development and growth of osteophytes. Our objectives were to use hybrid positron emission tomography-magnetic resonance imaging (PET-MRI) to (1) determine whether synovitis adjacent to peripheral bone subregions with increased metabolic activity is greater than adjacent to regions without increased metabolic activity and (2) assess the association between subregional bone metabolic activity and adjacent synovitis.

DESIGN

We recruited 11 participants (22 knees) with a diagnosis of OA in at least one knee. Simultaneous bilateral knee PET-MRI was performed. We quantified bone metabolic activity using the radiotracer [18F]sodium fluoride ([18F]NaF) with calculation of maximum standardized uptake values (SUVmax). Synovitis was quantified using dynamic contrast-enhanced MRI with calculation of Ktrans. Bone subregions were coded as osteophyte (OP), focal increased [18F]NaF uptake without osteophyte (FIU), or normal (no osteophyte or FIU). We used robust linear mixed effects models to assess differences in adjacent Ktrans between different subregion types and to assess association between Ktrans and adjacent SUVmax.

RESULTS

94 OPs were detected (59 MOAKS grade 1, 30 grade 2, 5 grade 3), along with 28 FIU and 18 normal subregions. Ktrans was higher adjacent to FIU (adjusted mean [95% CI] = 0.06 [0.03,0.09]) and OPs (0.08 [0.05,0.11]) when compared to normal bone subregions (0.03 [0.00,0.09]). PET SUVmax was positively associated with adjacent Ktrans (β[95% CI] = 0.018 [0.008,0.027]).

CONCLUSIONS

Synovitis is more intense adjacent to peripheral bone regions with increased metabolic activity than those without, although there is some overlap. Subregional bone metabolic activity is positively associated with intensity of adjacent synovitis.

Imaging of Synovial Inflammation in Osteoarthritis, From the AJR Special Series on Inflammation

Synovitis, inflammation of the synovial membrane, is a common manifestation in osteoarthritis (OA) and is recognized to play a role in the complex pathophysiology of OA. Increased recognition of the importance of synovitis in the OA disease process and potential as a target for treatment has increased the need for non-invasive detection and characterization of synovitis using medical imaging. Numerous imaging methods can assess synovitis involvement in OA with varying sensitivity and specificity as well as complexity. This article reviews the role of contrast-enhanced MRI, conventional MRI, novel unenhanced MRI, gray-scale ultrasound (US), and power Doppler US in the assessment of synovitis in patients with OA. The role of imaging in disease evaluation as well as challenges in conventional imaging methods are discussed. We also provide an overview into the potential utility of emerging techniques for imaging of early inflammation and molecular inflammatory markers of synovitis, including quantitative MRI, superb microvascular imaging, and PET. The potential development of therapeutic treatments targeting inflammatory features, particularly in early OA, would greatly increase the importance of these imaging methods for clinical decision making and evaluation of therapeutic efficacy.