Statistics from Altmetric.com
Positron emission tomography/computed tomography (PET/CT scan) represents a new molecular imaging modality using physiological tracers to assess a lesion’s metabolic state. [18F]Fluorodeoxyglucose (FDG) PET appears to be very good for the evaluation of cancer lesions and potentially useful in inflammatory rheumatic disorders, but there is scant information in ankylosing spondylitis (AS).
We evaluated an FDG-PET/CT scan (4 MBq/kg FDG injection, whole body acquisition using a Siemens Biograph Duo device, volumic fixation values are expressed as standardised uptake values (SUVs)), in patients with AS (revised New York criteria) with magnetic resonance imaging (MRI) documented lumbar aseptic spondylodiscitis (discovertebral biopsy in two patients) requiring anti-tumour necrosis factor (TNF) treatment. A second PET evaluation was performed at the time of the anti-TNF response assessment, between 6 and 8 weeks of treatment, in parallel with a second MRI examination, and clinical evaluation.
Short time (6–8 weeks) clinical outcome (tables 1 and 2) showed a good response in patient 2 (>50% improvement in all measures), no response in patient 3 (fig 1), and incomplete response in patient 1 (20% improvement in C reactive protein (CRP), pain, metrology).
MRI evolution, without quantification, showed reduction of T2 weighted hypersignal and reduction of gadolinium enhancement for patients 1 and 2, and absence of modification for patient 3 (without clinical improvement).
FDG-PET/CT imaging showed a reduction of FDG uptake on the spondylodiscitis in all three patients. This reduction was significant in patient 1 (incomplete clinical response), borderline in patient 2 with good clinical response, and not significant in patient 3 (no clinical response). In patient 2, global FDG uptake heterogeneity in the lumbar spine made measurement difficult (very different measurement volumes between the two evaluations).
Aseptic spondylodiscitis is a classic feature in AS.1 An FDG-PET/CT scan may be used to explore several inflammatory disorders: vasculitis,2,3 rheumatoid arthritis,4 peripheral arthritis5 or psoriatic arthritis,6 with a good correlation between disease activity and mean SUV.
In our three cases, FDG uptake was evident at the site of discitis, with mean SUVs ranging from 1.8 to 2.6) and reduction after anti-TNF treatment. This reduced uptake may be due to an impairment of local inflammation, or to a cellular effect of the anti-TNF agent on bone marrow. We did not observe a correlation between reduction in FDG uptake and clinical and MRI evolution in these three cases. Nevertheless, the small number of patients does not allow a firm conclusion to be reached. The localisation of uptake, in the disc or the vertebral plate, should be taken into account and may interfere with the target volume and evaluation of the region of interest, especially when uptake is moderate and diffuse, as in our second case. This may be a limitation of this imaging technique in AS when the various pathophysiological mechanisms leading to aseptic discitis in AS are taken into account.1
In infectious spondylodiscitis, this technique proved its usefulness in topographic and differential diagnosis.7,8 Schmitz et al reported 12 cases of biopsy proven infectious spondylitis, with an initial mean (SD) SUV of 7.5 (3.8), and significant reduction after an 8 week treatment (mean SUV 1.75).8
This preliminary study underlines the potential interests and limitations of an FDG-PET/CT scan in AS.
If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.