COMPARISON OF ULTRASOUND BEAM ATTENUATION BY CALCIUM PYROPHOSPHATE, HYDROXYAPATITE AND MONOSODIUM URATE CRYSTALS: A PROOF-OF-CONCEPT STUDY

BackgroundUltrasound (US) demonstrated to be reliable and accurate for the diagnosis of crystal induced arthropathies, especially gout and calcium pyrophosphate deposition disease (CPPD) and validated definitions for uric acid and calcium pyrophosphate deposition in joints were released by the Outcome Measures in Rheumatology (OMERACT) US group. Less is known regarding hydroxyapatite (HA) deposition disease (HAAD) and the role of US in the assessment of HA crystal deposition.It is general belief that HA crystals create posterior acoustic shadowing, monosodium urate could create or not US beam attenuation while CPP crystals generally do not attenuate the US beam.ObjectivesAim of this proof-of-concept study was to investigate the US appearance in terms of beam attenuation due to increasing concentrations of MSU, CPP and HA crystals.MethodsSixteen synthetic crystal suspensions with known concentrations of CPP (26-109 mg/mL), HA (31-153 mg/mL) and MSU (90-500 mg/mL) were prepared. These specific concentrations were selected to replicate the X-ray attenuation characteristics of those crystals when imaged by conventional radiography, computed tomography (CT) and dual-energy CT (DECT) in vivo[1]. The density of the agar-based lipogel background was intentionally increased to mimic the X-ray attenuation of hyaline cartilage (i.e., 100-120 HU at 120 kVp). Each crystal suspension was placed in a plastic container filled with US gel, next to the control (i.e., crystal-free background) calibration phantom. We acquired all US images using a Samsung RS80A system equipped with a high-frequency linear array transducer (4-18 MHz) set at the maximum frequency, by applying the same settings. US scans were performed by a single experienced sonographer, blinded to the crystal type and concentration. For each of the 16 crystal suspensions, at least two images were recorded both in the long- and short-axis views, the latter including the control phantom in the field of view. Interpretation of US images for the extent of US beam attenuation and the presence of acoustic shadowing was performed in consensus with a second experienced sonographer.ResultsNone of the five CPP phantoms generated posterior acoustic shadowing or US beam attenuation regardless of CPP concentration. HA 31mg/mL did not generate US beam attenuation, while HA 62, 92 and 123 mg/mL generated a progressively increasing US beam attenuation with posterior acoustic shadow clearly generated by HA 153 mg/mL. Similarly, MSU 90 mg/mL did not generate US beam attenuation. MSU 195 mg/mL generated only a very faint US beam attenuation that became progressively more visible at 270 and 345 mg/mL even if a clear posterior acoustic shadow was detectable only with MSU 420 and 500 mg/mL (Figure 1).Figure 1.Short-axis views comparing the US attenuation characteristics, including acoustic shadowing, of increasing concentrations (in mg/mL) of CPP, HA and MSU crystals. The left column shows the US attenuation of the crystal-free (0 mg/mL) agar-based lipogel background for referenceConclusionThis proof-of-concept study confirmed that in the concentrations of crystals encountered in vivo, CPP do not generate posterior shadowing, while MSU and HA determine US beam attenuation proportionally to the concentration of the crystals. Being this a proof-of-concept study, attenuation of the US beam was assessed empirically and not in a quantitative or semi-quantitative way. However, this study highlights the potential of US to differentiate between CPP, MSU and HA crystals based on their appearance on gray scale imaging. Future studies should be carried out with different crystal concentrations, different US equipment and settings, in order to create a scoring system for US beam attenuation that is actually lacking.References[1]Pascart T et al. Dual-energy computed tomography in calcium pyrophosphate deposition: initial clinical experience. Osteoarthritis Cartilage 2019;27:1309–14. doi:10.1016/j.joca.2019.05.007Disclosure of InterestsNone declared