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Dual Energy CT in the evaluation of Gout

by Roberto Russo

We are again in the midst of another festive season, where our days are often immersed in celebration, which invariably involves the consumption of good food and drink!! Whilst for the most of us that brings a few extra kilos for some it could herald an attack of a disease that in a bygone era was termed the ‘king of diseases and the disease of kings”. It is generally accepted that a fluctuation in the serum urate level, from any cause, is the trigger to an attack of gout. Purine amino acids, which are found in abundance in shellfish, red meat, and beer, are metabolized to Uric acid and hence would definitely cause such a fluctuation.

GoutTherefore, it is not uncommon for patients to present during or following this season with a painful joint. In the scenario when this is their first presentation the diagnosis can be elusive. Gout is often considered on the basis of the joint involved, such as the 1st MTP joint, and the demographics of the patient (middle aged male with the metabolic syndrome). However, infection can present in exactly the same manner and since these cannot be differentiated on clinical grounds alone, the traditional approach is to obtain synovial fluid. If no bacteria are seen and monosodium urate crystals are present (especially if they are seen engulfed by neutrophils) then the diagnosis of acute gouty arthritis can be made confidently.

It would be ideal if it was always that simple, but the joint fluid can at times be very difficult to aspirate, even with ultrasound guidance, or there is concern for overlying cellulitis which you would not want to track into the joint. The other circumstance is when the patient presents in the subacute phase and there is not enough fluid to aspirate. Consequently, there remains the dilemma of whether the patient actually had an attack of gout or whether their joint disease was due to another cause. Previously this would have required longer term follow up to resolve, with investigations aimed at assessing for other inflammatory joint diseases. The serum uric acid level is not a diagnostic test for gout but is only a predictor of the frequency and severity of their attacks. Frequently, the serum Urate level could be in the normal range during an attack since the level often drops during that period.

This is the situation where dual energy CT can be of great clinical assistance, which marks a recent advance in the diagnosis of Gout arthritis. A dual energy scanner allows the acquisition of image data sets at two different energy levels, which allows for the separation of calcium from monosodium urate crystals by software that performs complex algorithmic processing. This is possible because the material specific attenuation of the energy emitted by the CT provides information about the chemical composition of the tissue. For visualization, these different tissues are displayed in different colours, whereby calcium is usually blue and monosodium urate crystals are either green or red.

So how does it perform?
  • In a group of patients known to have gout, dual energy CT confirmed the presence of MSU crystals in four times the number of locations compared to clinical examination and history, highlighting the ability of this technique to detect subclinical disease.
  • In aspiration-proven sites of gout arthritis, dual energy CT proved to be reliable in confirming the presence of MSU crystals.
  • Furthermore, there is preliminary investigation suggesting this method can accurately determine the overall volumetric burden of crystal deposition and hence be of value as a monitoring tool to assess response to treatment.

Therefore, dual energy CT presents itself as a unique and clinically relevant modality in the diagnosis and management of gout. It should be noted that not all radiology practices have invested in the software packages or the equipment capable of performing this technique, so it is worth exploring which of your radiology practices provides this service. All that is then required is a request form that specifies dual energy CT for the evaluation of gout.

Reference:

1. Desai MA, Peterson JJ, Garner HW, Kransdorf MJ. Clinical utility of dual-energy CT for evaluation of tophaceous gout. Radiographics. 2011;31(5):1365–75– discussion 1376–7. (Image is taken from this article)