PET-CT for Spine Infection — What It Shows and Where It Falls Short

MRI is the first investigation most doctors reach for when they suspect spondylodiscitis or osteomyelitis, because it shows the disc, the vertebra and any surrounding tissue in close detail. PET-CT is requested when MRI has not given a complete answer, or when it cannot be done at all.

If you have a metal spinal implant, a pacemaker, or another device that makes MRI unsafe or unreadable, PET-CT is often the practical alternative. It is also used when the question is how far the infection has spread — PET-CT surveys the whole skeleton in a single session, which MRI cannot.

ESNR and SNMMI both recognise whole-body FDG PET-CT as appropriate for suspected multifocal bone and joint infection, and for guiding biopsy when there is more than one potential site.

MRI remains the reference standard for spondylodiscitis because it shows soft-tissue detail that PET-CT does not — including how close the infection is to the spinal cord and whether there is an epidural abscess that may need urgent drainage. If there is any question of cord compression, MRI is the priority.

PET-CT does things MRI cannot. It covers the whole skeleton in one scan. Metal implants do not interfere with it. Because it measures metabolic activity rather than anatomy, it can sometimes detect active infection earlier, and it can assess whether treatment is working before the anatomy on MRI has visibly changed.

ESNR guidance describes the two as complementary rather than competing. Your treating doctor — not the imaging team — decides which combination of scans is right for your situation.

PET-CT cannot tell your doctor what organism is causing the infection. The scan shows metabolic activity — it does not culture bacteria or confirm whether the infection is bacterial, fungal or mycobacterial. A biopsy is still needed for that, and the imaging can help choose where to take it.

PET-CT also cannot always distinguish infection from tumour or from post-surgical inflammation. An area that has been operated on recently will show increased activity for months. The imaging team will read the result in the context of your clinical history, but your referring specialist retains responsibility for the clinical judgement.

If your blood glucose is poorly controlled on the day, image quality can be affected. You will be asked to fast before the scan and your glucose will be checked on arrival. Tell the team about any recent surgery, fracture, or joint injection near the site in question.