Background/Purpose: The gold standard for the diagnosis of crystal-related arthritis requires aspiration and visualization of crystals from synovial fluid or tophus material. However, crystal identification may be suboptimal particularly for crystals other than monosodium urate (MSU) which are associated with gout. New imaging modalities such as ultrasound and dual energy CT are increasingly being used to aid in the diagnosis of gout but their role is less well defined in other crystal deposition diseases. The ability to characterise calcium pyrophosphate (CPP) or hydroxyapatite (HA) crystals using non-invasive imaging would be an advance. The aim of this study was to determine whether we could detect and differentiate between MSU, CPP and HA crystals using novel multi-energy spectral CT (MARS) imaging.

Methods: A finger with a gouty tophus from the Christchurch Cancer Tissue bank and a calcified medial meniscus excised at the time of joint replacement surgery were obtained. The gouty finger was imaged using plain X-ray, DECT and a preclinical MARS scanner. Images of the whole meniscus using plain X-ray and MARS were obtained. MARS imaging of biological specimens and calibration phantoms containing known concentrations of MSU, CPP and HA crystals was performed using energy thresholds of 20, 30, 40 and 50 keV at 80 kVp. Material decomposition was applied to the imaging data to distinguish MSU, CPP and HA crystals. For validation purposes, samples of the crystals were obtained from the meniscus and tophus and examined by x-ray diffraction (XRD) and polarized microscopy respectively.

Results: Plain X-ray of the gouty finger revealed erosions and changes consistent with a tophus (Figure 1a). DECT and MARS both identified MSU crystals, however, MARS was able to detect finer detail (Figure 1b). Within the meniscus plain X-ray identified chondrocalcinosis consistent with CPP crystals while MARS showed a predominance of CPP crystals in the calcified regions in the excised meniscus and in some parts there was a mixture of HA and CPP (Figure 1d). MARS imaging findings of the meniscus were validated using x-ray diffraction of the crystal scrapings.

Conclusion: MARS spectral CT can not only detect and differentiate MSU crystals in a gouty finger but also specifically detect and identify CPP within an osteoarthritic human knee meniscus and distinguish them from HA crystals. There is potential for MARS to become useful in the diagnosis of crystal-related arthropathies including CPPD, gout and calcific tendonitis.

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