[18 F]-Sodium Fluoride PET/MR Imaging for Bone-Cartilage Interactions in Hip Osteoarthritis: A Feasibility Study

Role of Positron Emission Tomography (PET) in the Diagnosis of Musculoskeletal Disorders

Musculoskeletal disorders (MSDs) represent a broad spectrum of diseases and injuries that significantly affect the musculoskeletal system and impose a considerable burden on global public health. This review focuses on the landscape of MSD diagnoses and emphasizes the high prevalence of these conditions. Additionally, it recognizes the inadequacies of conventional evaluation methods, including radiography and subjective assessments, when addressing their complex pathophysiology. It also attempts to highlight the promise of positron emission tomography (PET), which offers quantitative insights into metabolic and molecular activities before structural changes become evident. The review focuses on key radiotracers, specifically, fluorodeoxyglucose ([18F]-FDG) and sodium fluoride ([18F]-NaF), discussing their efficacy in assessing inflammatory processes and bone metabolism. By exploring the abilities of these advanced imaging modalities, we aim to identify the potential of using PET in the early detection and more accurate assessment of MSDs. Furthermore, we provide a brief outline of directions for future research, advocating for the development of novel radiotracers, the integration of multiple imaging modalities, and the application of artificial intelligence in imaging analysis. This review contributes to a deeper understanding of MSDs and underscores the urgent need for innovative diagnostic strategies to improve patient care and outcomes in musculoskeletal health.

Validation of quantitative [18F]NaF PET uptake parameters in bone diseases: a systematic review

PURPOSE

[18F]NaF PET has become an increasingly important tool in clinical practice toward understanding and evaluating diseases and conditions in which bone metabolism is disrupted. Full kinetic analysis using nonlinear regression (NLR) with a two-tissue compartment model to determine the net rate of influx (Ki) of [18F]NaF is considered the gold standard for quantification of [18F]NaF uptake. However, dynamic scanning often is impractical in a clinical setting, leading to the development of simplified semi-quantitative parameters. This systematic review investigated which uptake parameters have been used to evaluate bone disorders and how they have been validated to measure disease activity.

METHODS

A literature search (in PubMed, Embase.com, and Clarivate Analytics/Web of Science Core Collection) was performed up to 28th November 2023, in collaboration with an information specialist. Each database was searched for relevant literature regarding the use of [18F]NAF PET/CT to measure disease activity in bone-related disorders. The main aim was to explore whether the reported semi-quantitative uptake values were validated against full kinetic analysis. A second aim was to investigate whether the chosen uptake parameter correlated with a disease-specific outcome or marker, validating its use as a clinical outcome or disease marker.

RESULTS

The initial search included 1636 articles leading to 92 studies spanning 29 different bone-related conditions in which [18F]NaF PET was used to quantify [18F]NaF uptake. In 12 bone-related disorders, kinetic analysis was performed and compared with simplified uptake parameters. SUVmean (standardized uptake value) and SUVmax were used most frequently, though normalization of these values varied greatly between studies. In some disorders, various studies were performed evaluating [18F]NaF uptake as a marker of bone metabolism, but unfortunately, not all studies used this same approach, making it difficult to compare results between those studies.

CONCLUSION

When using [18F]NaF PET to evaluate disease activity or treatment response in various bone-related disorders, it is essential to detail scanning protocols and analytical procedures. The most accurate outcome parameter can only be obtained through kinetic analysis and is better suited for research. Simplified uptake parameters are better suited for routine clinical practice and repeated measurements.

18F-Fluoride PET/CT-Updates

Sodium Fluoride-18 production started in the 1940s and was described clinically for the first time in 1962 as a bone-imaging agent. However, its use became dormant with the development of conventional bone scintigraphy, especially due to its low cost. Conventional bone scintigraphy has been the most utilized Nuclear Medicine technique for identifying osteoblastic bone metastases, especially in prostate and breast cancers for decades and is also employed to identify benign bone disease, especially in the orthopedic setting. While bone scintigraphy is highly sensitive, it lacks adequate specificity. With the advent of high-quality 3D Whole-Body Positron Emission Tomography combined with computed tomography (PET/CT), images, Sodium Fluoride-18 imaging with PET/CT (Fluoride PET/CT) re-emerged. This PET/CT bone-imaging agent provides higher sensitivity and specificity to detect bone lesions in both the oncological scenario as well as to identify benign bone and joint disorders. PET/CT bone-imaging provides a precise view of the bone metabolism remodeling processes at a molecular level, throughout the skeleton, and combines anatomical information, enhancing diagnostic specificity and accuracy. This article review will explore the updates on clinical applications of Fluoride PET/CT in oncology and benign conditions encompassing orthopedic, inflammatory and cardiovascular conditions and treatment response assessment.

Multimodal positron emission tomography (PET) imaging in non-oncologic musculoskeletal radiology

Musculoskeletal (MSK) disorders are associated with large impacts on patient's pain and quality of life. Conventional morphological imaging of tissue structure is limited in its ability to detect pain generators, early MSK disease, and rapidly assess treatment efficacy. Positron emission tomography (PET), which offers unique capabilities to evaluate molecular and metabolic processes, can provide novel information about early pathophysiologic changes that occur before structural or even microstructural changes can be detected. This sensitivity not only makes it a powerful tool for detection and characterization of disease, but also a tool able to rapidly assess the efficacy of therapies. These benefits have garnered more attention to PET imaging of MSK disorders in recent years. In this narrative review, we discuss several applications of multimodal PET imaging in non-oncologic MSK diseases including arthritis, osteoporosis, and sources of pain and inflammation. We also describe technical considerations and recent advancements in technology and radiotracers as well as areas of emerging interest for future applications of multimodal PET imaging of MSK conditions. Overall, we present evidence that the incorporation of PET through multimodal imaging offers an exciting addition to the field of MSK radiology and will likely prove valuable in the transition to an era of precision medicine.

Imaging of joint response to exercise with MRI and PET

Imaging of the joint in response to loading stress may provide additional measures of joint structure and function beyond conventional, static imaging studies. Exercise such as running, stair climbing, and squatting allows evaluation of the joint response to larger loading forces than during weight bearing. Quantitative MRI (qMRI) may assess properties of cartilage and meniscus hydration and organization in vivo that have been investigated to assess the functional response of these tissues to physiological stress. [18F]sodium fluoride ([18F]NaF) interrogates areas of newly mineralizing bone and provides an opportunity to study bone physiology, including perfusion and mineralization rate, as a measure of joint loading stress. In this review article, methods utilizing quantitative MRI, PET, and hybrid PET-MRI systems for assessment of the joint response to loading from exercise in vivo are examined. Both methodology and results of various studies performed are outlined and discussed. Lastly, the technical considerations, challenges, and future opportunities for these approaches are addressed.

18F-FDG and 18F-NaF PET/CT Global Assessment of Large Joint Inflammation and Bone Turnover in Rheumatoid Arthritis

Rheumatoid arthritis (RA) involves chronic inflammation of synovial joints, causing pain, stiffness, and limited mobility. ¹⁸F-sodium fluoride (NaF) is a PET tracer whose uptake reflects bone turnover, while ¹⁸F-fludeoxyglucose (FDG) shows glucose metabolism and can serve as a marker for inflammation. The aim of this study is to determine the feasibility of calculating the FDG and NaF mean standardized uptake value (SUVmean) in the knee joint, hip joint, and sacroiliac (SI) joint of RA patients and to determine their association with patient characteristics. Prospective FDG-PET/CT as well as NaF-PET/CT imaging was performed on 18 RA patients. The global SUVmean was calculated on FDG-PET/CT and NaF-PET/CT images using a semiautomated CT-based method of segmentation. FDG and NaF uptake were found to be significantly correlated in the knee (r = 0.77, p < 0.001), but not in the hip and SI joints. In the knee, both NaF SUVmean and FDG SUVmean were significantly correlated with body weight, BMI, leptin, and sclerostin levels (p < 0.05). NaF SUVmean was significantly positively correlated with BMI and leptin for both the hip and SI joints (p < 0.05). No significant correlation was observed between either PET parameter and age, height, erythrocyte sedimentation rate (ESR), and interleukins 1 and 6 (IL-1 and IL-6); however, FDG was correlated with inflammatory markers such as C-reactive protein (CRP) and patient global visual analogue scale (VAS-PtGlobal) in some joints. In this study, both FDG and NaF uptake were quantified in large joints of patients with RA using CT segmentation. NaF and FDG SUVmean were correlated with clinical variables related to body weight and adiposity, suggesting that degenerative joint disease may play a larger role in influencing the uptake of these tracers in large joints than RA disease activity. FDG and its correlation with markers of inflammation such as CRP and VAS-PtGlobal suggests that this tracer may serve as a more specific marker for RA disease activity than NaF. Larger prospective and longitudinal data are necessary to gain a better understanding of the roles of FDG and NaF in evaluating RA joint activity in these joints.

Hybrid PET/MRI in Infection and Inflammation: An Update About the Latest Available Literature Evidence

PET/MRI has been reported to be promising in the diagnosis and evaluation of infection and inflammation including brain disorders, bone and soft tissue infections and inflammations, cardiovascular, abdominal, and systemic diseases. However, evidence came out manly from anecdotal cases or small cohorts. The present review aimed to update the latest available evidence about the role of PET/MRI in infection and inflammation. The search (January, 1 2018-July, 8 2022) on PubMed produced 504 results. Sixty-five articles were selected and included in the qualitative synthesis. The number of publications on PET/MRI in the 3 years 2018-2020 was comparable, while it increased in 2021 and 2022 (from 11 to 17 and 15, respectively). [¹⁸F]FDG and ⁶⁸Ga-DOTA-FAPI-04 were the most frequently used (42/65) and innovative radiopharmaceuticals, respectively. [¹⁸F]fluoride (9/65), translocator protein (TSPO)-targeted PET agents (6/65), CXCR4 receptor targeting tracer and β-amyloid plaques binding radiopharmaceuticals (2/65 and 2/65, respectively) were also used. Most PET/MRI studies in the period 2018-2022 focused on inflammation (55/65), and cardiovascular diseases represented the most frequent field of interest (30/65), also when considering each year singularly. An increasing trend in bone and joint publications was observed in the considered period (12/65). Other topics included neurology (11/65), inflammatory bowel disease (8/65), and other (4/65). PET/MRI technology demonstrated to be useful in infection and inflammation, being superior to each single modality and/or facilitating diagnosis in a number of conditions (eg, cardiac sarcoidosis, myocarditis, endocarditis), and/or allowing to provide insightful information about disease biology and apply innovative radiopharmaceuticals (eg, neurology, atherosclerosis). Publications focused on PET/MRI in large vessel vasculitis and aortic diseases include both diagnostic and discovery objectives. The current review corroborates the potential of PET/MRI - combining in a single examination the high soft tissue contrast, high resolution, and functional information of MRI, with molecular data provided by PET technology - to positively impact on the management of infectious diseases and inflammatory conditions.

Latest advancements in imaging techniques in OA

The osteoarthritis (OA) research community has been advocating a shift from radiography-based screening criteria and outcome measures in OA clinical trials to a magnetic resonance imaging (MRI)-based definition of eligibility and endpoint. For conventional morphological MRI, various semiquantitative evaluation tools are available. We have lately witnessed a remarkable technological advance in MRI techniques, including compositional/physiologic imaging and automated quantitative analyses of articular and periarticular structures. More recently, additional technologies were introduced, including positron emission tomography (PET)-MRI, weight-bearing computed tomography (CT), photon-counting spectral CT, shear wave elastography, contrast-enhanced ultrasound, multiscale X-ray phase contrast imaging, and spectroscopic photoacoustic imaging of cartilage. On top of these, we now live in an era in which artificial intelligence is increasingly utilized in medicine. Osteoarthritis imaging is no exception. Successful implementation of artificial intelligence (AI) will hopefully improve the workflow of radiologists, as well as the level of precision and reproducibility in the interpretation of images.

Imaging articular cartilage in osteoarthritis using targeted peptide radiocontrast agents

Background

Established MRI and emerging X-ray contrast agents for non-invasive imaging of articular cartilage rely on non-selective electrostatic interactions with negatively charged proteoglycans. These contrast agents have limited prognostic utility in diseases such as osteoarthritis (OA) due to the characteristic high turnover of proteoglycans. To overcome this limitation, we developed a radiocontrast agent that targets the type II collagen macromolecule in cartilage and used it to monitor disease progression in a murine model of OA.

Methods

To confer radiopacity to cartilage contrast agents, the naturally occurring tyrosine derivative 3,5-diiodo-L-tyrosine (DIT) was introduced into a selective peptide for type II collagen. Synthetic DIT peptide derivatives were synthesised by Fmoc-based solid-phase peptide synthesis and binding to ex vivo mouse tibial cartilage evaluated by high-resolution micro-CT. Di-Iodotyrosinated Peptide Imaging of Cartilage (DIPIC) was performed ex vivo and in vivo 4, 8 and 12 weeks in mice after induction of OA by destabilisation of the medial meniscus (DMM). Finally, human osteochondral plugs were imaged ex vivo using DIPIC.

Results

Fifteen DIT peptides were synthesised and tested, yielding seven leads with varying cartilage binding strengths. DIPIC visualised ex vivo murine articular cartilage comparably to the ex vivo contrast agent phosphotungstic acid. Intra-articular injection of contrast agent followed by in vivo DIPIC enabled delineation of damaged murine articular cartilage. Finally, the translational potential of the contrast agent was confirmed by visualisation of ex vivo human cartilage explants.

Conclusion

DIPIC has reduction and refinement implications in OA animal research and potential clinical translation to imaging human disease.

Positron emission tomography/magnetic resonance imaging in musculoskeletal benign conditions

A wide spectrum of benign musculoskeletal (orthopedic and rheumatological) conditions affect the general population. Collectively, these are common, and they can inflict significant morbidity with resultant negative impact on the quality of life of patients. For many of these conditions, there is established evidence for research and clinical use of PETCT and MRI for assessment of disease. Introduction of integrated PET/MRI around a decade ago brought optimism that combining the strength of PET and MRI techniques on a single platform could have synergistic effect to benefit imaging assessment of patients, including in the context of benign musculoskeletal conditions. This review specifically focuses on the progress that has been made. This aims to showcase clinical studies derived primarily from the integrated PET/MRI platforms for the evaluation of common orthopedic and rheumatological conditions. Despite enthusiasm and progress by early adopters of the PET/MRI technology, significant barriers are present for its wider adoption, validation, and translation to routine clinical practice. Attenuation correction is a particular challenge which affects regions close to the skeleton and impacts PET/MRI assessment of musculoskeletal disorders. Continued effort on research and validation, as well as promotion of its multimodal multiparametric capability to clinical and pharmaceutical stakeholders, and increased availability through increased adoption of PET/MRI scanners internationally, may accelerate its translation into routine clinical practice in this domain.

[18F] Sodium Fluoride Dose Reduction Enabled by Digital Photon Counting PET/CT for Evaluation of Osteoblastic Activity

The aim of the study was to assess the quality and reproducibility of reducing the injected [¹⁸F] sodium fluoride ([¹⁸F]NaF) dose while maintaining diagnostic imaging quality in bone imaging in a preclinical skeletal model using digital photon counting PET (dPET) detector technology. Beagles (n = 9) were administered three different [¹⁸F]NaF doses: 111 MBq (n = 5), 20 MBq (n = 5), and 1.9 MBq (n = 9). Imaging started ≃45 min post-injection for ≃30 min total acquisition time. Images were reconstructed using Time-of-Flight, ultra-high definition (voxel size of 1 × 1 × 1 mm³), with 3 iterations and 3 subsets. Point spread function was modeled and Gaussian filtering was applied. Skeleton qualitative and quantitative molecular image assessment was performed. The overall diagnostic quality of all images scored excellent (61%) and acceptable (39%) by all the reviewers. [¹⁸F]NaF SUV_mean showed no statistically significant differences among the three doses in any of the region of interest assessed. This study demonstrated that a 60-fold [¹⁸F]NaF dose reduction was not significantly different from the highest dose, and it had not significant effect on overall image quality and quantitative accuracy. In the future, ultra-low dose [¹⁸F]NaF dPET/CT imaging may significantly decrease PET radiation exposure to preclinical subjects and personnel.

Emerging role of integrated PET-MRI in osteoarthritis

Osteoarthritis (OA) is a common degenerative disorder of the articular cartilage, which is associated with hypertrophic changes in the bone, synovial inflammation, subchondral sclerosis, and joint space narrowing (JSN). Radiography remains the first line of imaging till now. Due to the lack of soft-tissue depiction in radiography, researchers are exploring various imaging techniques to detect OA at an early stage and understand its pathophysiology to restrict its progression and discover disease-modifying agents in OA. As the OA relates to the degradation of articular cartilage and remodeling of the underlying bone, an optimal imaging tool must be sensitive to the bone and soft tissue health. In that line, many non-invasive imaging and minimally invasive techniques have been explored. Out of these, the non-invasive compositional magnetic resonance imaging (MRI) for evaluation of the integrity of articular cartilage and positron emission tomography (PET) scan with fluorodeoxyglucose (FDG) and more specific bone-seeking tracer like sodium fluoride (18F-NaF) for bone cartilage interface are some of the leading areas of ongoing work. Integrated PET-MRI system, a new hybrid modality that combines the virtues of the above two individual modalities, allows detailed imaging of the entire joint, including soft tissue cartilage and bone, and holds great potential to research complex disease processes of OA. This narrative review attempts to signify individual characteristics of MRI, PET, the fusion of these characteristics in PET-MRI, and the ongoing research on PET-MRI as a potential tool to understand the pathophysiology of OA.

T2 mapping of the acetabular cartilage in infants and children with developmental dysplasia of the hip

BACKGROUND

T2 mapping is useful for evaluating the cartilage matrix.

PURPOSE

To determine the variations in the acetabular cartilage T2 relaxation values between healthy individuals and those with developmental dysplasia of the hip (DDH).

MATERIAL AND METHODS

Thirty-three patients with unilateral DDH underwent 3-T magnetic resonance imaging (MRI) between January 2018 and February 2019. Fifteen volunteers (30 hips) were enrolled as controls. T2 values were measured with the T2 mapping sequence in all layers and were equally divided into three layers (deep, middle, and superficial) with equal thickness. We calculated the mean T2 relaxation values for the full thickness, deep, middle, and superficial layers and compared the values between the different groups. In addition, the inter- and intra-observer agreements were calculated.

RESULTS

The T2 relaxation values in the DDH arm were significantly lower in the middle, superficial, and full thickness layers compared with those of the volunteers and contralateral hips. The T2 relaxation values of the deep layers showed no significant difference between the different groups. The acetabular cartilage T2 relaxation values increased from the deep layer to the superficial layer in the control and contralateral groups. Both inter- and intra-observer agreements were good.

CONCLUSION

MRI T2 mapping may help to diagnose developmental disorders of the acetabular cartilage matrix in infants and children with DDH. Abnormal acetabular cartilage T2 relaxation values may be due to the extraordinary stress load of the femoral head.

[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.

Novel Musculoskeletal and Orthopedic Applications of 18F-Sodium Fluoride PET

PET imaging with ¹⁸F-sodium fluoride (NaF), combined with computed tomography or magnetic resonance, is a sensitive method of assessing bone turnover. Although NaF-PET is gaining popularity in detecting prostate cancer metastases to bone marrow, osseous changes represent secondary effects of cancer cell growth. PET tracers more appropriate for assessing prostate cancer metastases directly portray malignant activity and include ¹⁸F-fluciclovine and prostatic specific membrane antigen ligands. Recent studies investigating NaF-PET suggest utility in the assessment of benign musculoskeletal disorders. Emerging applications in assessing traumatic injuries, joint disease, back pain, orthopedic complications, and metabolic bone disease are discussed.

Principal Component Analysis of Simultaneous PET-MRI Reveals Patterns of Bone-Cartilage Interactions in Osteoarthritis

BACKGROUND

Bone-cartilage interactions have been implicated in causing osteoarthritis (OA).

PURPOSE

To use [18 F]-NaF PET-MRI to 1) develop automatic image processing code in MatLab to create a model of bone-cartilage interactions and 2) find associations of bone-cartilage interactions with known manifestations of OA.

STUDY TYPE

Prospective study aimed to evaluate a data analysis method.

POPULATION

Twenty-nine patients with knee pain or joint stiffness.

FIELD STRENGTH/SEQUENCE

3T MRI (GE), 3D CUBE FSE, 3D combined T1 ρ/T2 MAPSS, [18F]-sodium fluoride, SIGNA TOF (OSEM).

ASSESSMENT

Correlation between MRI (cartilage) and PET (bone) quantitative parameters, bone-cartilage interactions model described by modes of variation as derived by principal component analysis (PCA), WORMS scoring on cartilage lesions, bone marrow abnormalities, subchondral cysts.

STATISTICAL TESTS

Linear regression, Pearson correlation.

RESULTS

Mode 1 was a positive predictor of the bone abnormality score (P = 0.0003, P = 0.001, P = 0.0007) and the cartilage lesion score (P = 0.03, P = 0.01, P = 0.02) in the femur, tibia, and patella, respectively. For the cartilage lesion scores, mode 5 was the most important positive predictor in the femur (P = 3.9E-06), and mode 2 were predictors, significant negative predictor in the tibia (P = 0.007). In the patella, mode 1 was a significant positive predictor of the bone abnormality score (P = 0.0007).

DATA CONCLUSION

By successfully building an automatic code to create a bone-cartilage interface, we were able to observe dynamic relationships between biochemical changes in the cartilage accompanied with bone remodeling, extended to the whole knee joint instead of simple colocalized observations, shedding light on the interactions that occur between bone and cartilage in OA. Evidence Level: 3 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2020;52:1462-1474.