Musculoskeletal radiology tests - dual energy CT scans

Principle

Dual-energy CT (DECT) acquires images at two different X-ray energy levels simultaneously, allowing material-specific differentiation - in rheumatology, its key application is non-invasive detection of monosodium urate deposits in gout
An emerging, increasingly used tool particularly valuable when joint aspiration is not feasible/technically difficult, or for demonstrating the extent of tophaceous urate deposition throughout the body
Urate crystals and calcium have different attenuation profiles at different X-ray energies - DECT software processing exploits this difference to colour-code and specifically identify urate deposits (typically displayed in green) distinct from calcium-containing structures (bone), across multiple joints simultaneously in a single scan

Can detect subclinical urate deposition even in joints without current symptoms, and quantify total body urate burden - useful for demonstrating the extent of tophaceous disease non-invasively and, in some cases, for monitoring urate burden reduction with treatment over time
Not as sensitive as joint aspiration/polarised light microscopy for an acute, single-joint diagnostic question - DECT is complementary rather than a replacement for synovial fluid analysis when aspiration is feasible

Useful when synovial fluid aspiration is not technically feasible or has been non-diagnostic, or to characterise/quantify overall urate burden (e.g. to support a diagnosis of tophaceous gout, or track burden change with prolonged urate-lowering therapy)
Should not delay standard first-line diagnosis/management (joint aspiration where feasible, urate-lowering therapy once diagnosis is established) given DECT's relative novelty and more limited availability compared with standard imaging

Gout and tophaceous gout, chondrocalcinosis/pseudogout (DECT can help distinguish urate from calcium pyrophosphate deposition, though this specific application is less established than its urate-detection role)

DECT's ability to demonstrate urate deposition throughout the body non-invasively is a genuinely useful complement to (not replacement for) synovial fluid crystal analysis, particularly valuable when aspiration is difficult or when demonstrating total body urate burden specifically matters for management decisions

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