Musculoskeletal joint, Musculoskeletal soft tissue, Musculoskeletal spine, Ultrasound, CT, MR, Diagnostic procedure, Arthritides, Metabolic disorders
G. Kiernan, C. Azzopardi, J. Teh; Oxford/UK
The presence of monosodium urate crystals in joints and peri-articular soft tissues is the hallmark of gout,
the most common of the crystal arthropathies.
Clinical presentation is typically that of a mono-articular arthritis often leading to a relatively straight forward diagnosis.
However occasionally unusual anatomical locations and atypical presentations can mimic other pathologies and lead to a diagnostic dilemma.
The radiological assessment of these unusual presentations frequently involves multi-modality imaging including radiographs,
MRI and increasingly dual energy CT.
The presence of monosodium urate crystal deposition within the soft tissues is termed tophaceous gout.
Plain films will demonstrate tophaceous gout as increased,
usually peri-articualr soft tissue denity.
Tophaceous gout is also more likely to occur in the setting of renal insufficiency.
While CT and MRI are not specific investigations for gout they add useful information when there is an unclear diagnosis.
CT shows soft tissue densities in keeping with tophi and also excellent characerterisation of bone erosions.
Monosodium urate crystalline deposits have a specific density of approximately 160 to 170 Hounsfield units.
tophi are hypointense on T1 weighted images and vary from hypointense to hyperintense on T2 weighted images.
Dual energy CT is now increasingly becoming the imaging modality of choice in ambiguous cases.
It is advantageous as it can detect the burden of crystal deposition non-invasively.
It also provides shorter scanning times,
the ability to scan multiple joints simultaneously while providing a picture which is reproducible.
In the setting of an acute initial presentation of presumed gout,
dual energy CT may provide a false negative result.
Dual energy CT can also be postive in other pathologies; for example it has been identifed in low volumes in the presence of well establised osteoarthritic joints.
Aspirate of monosodium urate crystals and demonstration of negatively birefringent crystals on polarized light microscopy remains the gold standard for diagnoses.