18F-Fluorodeoxyglucose Positron Emission Tomography-Computed Tomography in Large Vessel Vasculitis: Standardization is the Way Forward.

Large vessel vasculitis (LVV) is defined as inflammatory pathology predominantly affecting the large arteries. The most common subtypes are Takayasu arteritis and giant cell arteritis (GCA). GCA in patient is often associated with polymyalgia rheumatica (PMR), as both belong to the same disease spectrum.[1]18 F-fluorodeoxyglucose (FDG) positron-emission tomography-computed tomography (PET-CT) is a functional hybrid imaging method which is an established tool in oncology and has also demonstrated its potential in various inflammatory pathologies such as LVV. It has been employed in LVV for diagnosis in patients with clinical suspicion as well as in fever of unknown origin, for monitoring response to immunosuppressive therapy and detecting relapse.[23] The underlying mechanism is based on the ability of FDG PET-CT to detect enhanced glucose uptake from high glycolytic activity of inflammatory cells in inflamed arterial walls and synovia/bursa.[4]

The main hindrance to FDG PET-CT becoming a standardized diagnostic tool in LVV is the lack of internationally accepted visual and quantitative parameters. Several visual and semiquantitative methods have been proposed, from simple standardized uptake value metrics to target-to-background ratios.[5] A joint EANM, SNNMI, PIG, and ASNC working committee have addressed this issue and proposed two criterions for better standardization of PET-CT interpretation.[6] These include the use of a standardized 0–3 grading system: 0 = no uptake (≤mediastinum); 1 = low-grade uptake (liver), with Grade 2 possibly indicative and Grade 3 considered positive for active LVV. Another parameter, the total vascular score (TVS) can also be used, at seven different vascular regions (thoracic aorta, abdominal aorta, subclavian arteries, axillary arteries, carotid arteries, iliac arteries, and femoral arteries) as negative (0) or positive, further scored semiquantitatively as 1 (minimal but not negligible FDG uptake), 2 (clearly increased FDG uptake), or 3 (very marked FDG uptake). Therefore, a TVS could be calculated ranging from 0 (no vascular FDG uptake in any of the seven vascular regions) to 21 (vascular FDG uptake scored 3 in all seven territories). In one prospective study, by Blockmans and Bley, FDG PET-CT images were acquired at baseline and after 3 and 6 months of steroid treatment in LVV. It was seen that the TVS decreased significantly at 3 months (P < 0.0005), but no further decrease was seen at 6 months. As PMR and GCA frequently overlap, typical FDG joint uptake patterns should be reported, including uptake in glenohumeral synovia, subacromial-subdeltoid bursa, supraspinatus tendinitis and biceps synovitis (shoulder), trochanteric/ischial bursa, hip synovia, interspinous regions of the cervical and lumbar vertebrae, or the synovial tissue of the knees if present, including the use of a standardized 0–3 grading system.

In the current issue of Indian Journal of Nuclear Medicine, Malik et al. have addressed the standardization of FDG PET-CT in LVV. They have employed the TVS proposed previously and validated its use in the Indian population. In their retrospective study with FDG PET-CT images of 106 patients with LVV, not only TVS was seen to clinically useful, it also correlated with other markers of disease activity such as ESR. Furthermore, TVS was found to be very useful for the assessment of response to immunosuppressive therapy. Interestingly, associated joint inflammation due to PMR was picked up in 17% of this cohort of patients. Through their study, Malik et al. have addressed the gap in our knowledge regarding this important topic and must be congratulated for that. Further prospective studies with the goal toward standardizing FDG PET-CT for evaluation of LVV are warranted to decipher this complex but crucial subject.