Age- and level-dependent glucose metabolism in the lumbar spine and preliminary associations with severe low back pain: A retrospective FDG PET imaging study

Low back pain is the world's most prevalent disability and lumbar disc degeneration is implicated as a pain driver. The discs are large avascular tissues and consequently fluid transport from adjacent vertebrae is required to perfuse the disc tissues with the necessary nutrients to maintain disc health. Deficiencies in disc transport may be predictive of spine disease and low back pain. Positron emission tomography (PET) usingFluorine - Fluorodeoxyglucose (FDG) is one method to measure metabolism of glucose, a key nutrient, and there is evidence of altered FDG uptake in the lumbar vertebrae with age, with degenerative disc/facet joint disease, and with disc/vertebrae infection. Prior studies demonstrate that transport depends on age and spinal level, decreasing from cranial to caudal, and we hypothesize that the transport of glucose also depends on age and spinal level. Our primary objective was to determine how FDG uptake in the lumbar discs and vertebrae is affected by age and spinal level. Our secondary objective was to validate FDG PET as a diagnostic imaging tool for spine disease. We identified patients in the Duke electronic health record (EHR) aged 18-70 years who received FDG PET CT imaging for any medical indication over a 10-year period, excluding patients with history of spine surgery or congenital disease, systemic rheumatic disease, or primary or metastatic malignancies of the spine. We further excluded individuals with severe comorbidities and poor-quality scans, leaving N=102 for analysis. Using custom 3D segmentation tools, we measured FDG uptake in vertebral bodies and discs across the lumbar spine, and subsequently evaluated relationships between FDG uptake, spinal level, age, and low back pain. Mean FDG uptake was greater in the vertebral bodies than the discs (mean/std. dev.: vertebral body SUV, 1.74±0.48; disc SUV, 1.02±0.33). Vertebral body FDG uptake was marginally associated with spinal level and was not associated with age. Disc FDG uptake was significantly affected by spinal level, where peak FDG uptake occurred at the T12-L1 and L5-S1 discs and was minimum at the L3-L4 and L5-S1 discs. Furthermore, lower lumbar discs demonstrated increased FDG uptake with age, and in a preliminary analysis of low back pain severity, disc and vertebral body FDG uptake were associated with severe pain at older ages. BMI was associated with vertebral body and disc FDG uptake as well. Lumbar disc FDG uptake is associated with age, spinal level, and higher BMI, where discs are more metabolically active at lower lumbar levels and with age. We propose the disc FDG uptake is a biomarker for abnormal metabolism and warrants further investigation as a biomarker of pain.