Foraminal [F]FDG uptake on PET/MRI is associated with radiculopathy and symptom reduction after image-guided nerve root block.

Background And Purpose: There is an imperfect correlation between morphological MRI findings and radiating low back pain. Nerve irritation, visualized as glucose hypermetabolism on [F]FDG-PET/MRI, has the potential to identify symptomatic segments. This study aimed to investigate the association of foraminal [F]FDG uptake on PET/MRI, radiological abnormalities and patient outcomes.

Materials And Methods: Prospectively recruited patients with radiating low back pain underwent [F]FDG PET/MRI of the lumbar spine in this observational study. Back pain and leg/buttock pain were assessed using the visual analog scale (0-10). Foraminal stenosis, facet joint arthropathy, and annular fissures of the disc were graded by radiologists. As part of the standard clinical care, a subset of patients received image-guided nerve root blocks, using a steroid/anesthetic mixture, and pain on visual analog scale was noted before and after injection. Standardized tracer uptake was quantitatively assessed in all neural foramina, facet joints, and discs. Generalized estimating equations were used to investigate associations between the maximum standardized uptake value of [F]FDG in the neural foramina, degree of stenosis (none, mild, moderate, severe), and pain, additionally adjusted for tracer uptake in the adjacent tissues, age, sex, and body mass index.

Results: A total of 110 lumbar neural foramina in 11 patients were included in the analysis. Generalized estimating equations revealed significant associations between foraminal [F]FDG uptake and degree of foraminal stenosis (beta 0.18, 95%CI 0.03, 0.33, p=0.02). In patients with unilateral radicular symptoms but bilateral stenoses on MRI, [F]FDG uptake was significantly higher on the symptomatic side (1.64 vs. 1.88, p=0.002). In segments treated with image-guided nerve root block, change in pain was positively associated with foraminal [F]FDG uptake before injection (beta 2.24, 95%CI 0.03, 4.45, p=0.05) but negatively associated with degree of stenosis (beta -1.27, 95%CI -2.24, -0.31, p=0.01).

Conclusions: Foraminal [F]FDG uptake on PET/MRI as a surrogate marker of nerve irritation may improve differentiation between painful vs. non-painful foraminal stenosis.

Abbreviations: LBP = Low back pain, SUVmax = Maximum standardized uptake value.