Basic imaging - MRI

Role

MRI provides superior soft tissue contrast compared with CT, without ionising radiation, at the cost of longer acquisition time, higher cost, and more limited availability - particularly valuable in infectious diseases practice for spinal infection (discitis/osteomyelitis) and deep soft tissue/musculoskeletal infection where CT is comparatively insensitive
Increasingly used as a second-line/problem-solving modality once CT or ultrasound has been inconclusive, or as first-line for specific indications (spinal infection) where its sensitivity is well recognised to exceed CT

Different sequences (T1, T2, STIR, contrast-enhanced/gadolinium, diffusion-weighted) exploit different tissue characteristics - T2/STIR sequences are particularly sensitive to oedema (an early marker of infection/inflammation), while diffusion-weighted imaging helps distinguish an abscess (restricted diffusion) from other fluid collections

First-line for suspected spinal infection (discitis-osteomyelitis, epidural abscess) given substantially higher sensitivity than CT or plain X-ray, particularly important given the risk of missed epidural abscess causing cord compression
Contraindications: certain implanted devices (older pacemakers, some metallic foreign bodies) and gadolinium contrast caution in significant renal impairment (nephrogenic systemic fibrosis risk)

Confirmed spinal infection/epidural abscess -> urgent neurosurgical/infectious diseases involvement given the risk of neurological compromise
Deep soft tissue infection characterisation guides the extent of surgical debridement required

Discitis-osteomyelitis, epidural abscess, necrotising fasciitis extent assessment, prosthetic joint infection (with limitations from metal artefact)

MRI's sensitivity advantage over CT is most clinically important for spinal infection specifically - a normal CT spine does not exclude discitis-osteomyelitis or epidural abscess, and MRI should be pursued when this diagnosis is clinically suspected despite a reassuring CT

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