Evaluation of the Magnetic Susceptibility in Cardiac Tissues using Magnetic Resonance Imaging. Quantitative Susceptibility Mapping for the Quantification of Heart Valve Calcification using a Calcium Phantom

Heart valve calcification is a chronic, degenerative condition of high clinical relevance, resulting from an abnormal buildup of calcium salts.

This preliminary work aimed to study it using Quantitative Susceptibility Mapping (QSM), a novel MRI technique that measures tissue magnetic susceptibility. It is sensitive to calcium, like Computed Tomography (CT), the gold standard for calcification diagnosis and assessment. However, it is ionizing-radiation-free.

To replicate the calcified tissue, an agarose phantom containing calcium carbonate (CC) and hydroxyapatite (HA) was made. By adapting existing sequences and reconstruction software, we obtained an acquisition and post-processing protocol suitable for the phantom, based on prior cardiac MR studies.

We managed to detect calcifications measuring 2 mm, close to our spatial resolution. Furthermore, the data suggests that calcium concentrations of 0.30 M and below 0.25 M can be detected, for CC and HA, respectively. Our results indicate that susceptibility decreases linearly with calcium content and that HA is more diamagnetic than CC, coherent with the fact that it is the primary culprit in calcification and has been shown to be a biomarker of malignancy.

However, this appears to be attenuated in the presence of other calcium salts, where the total calcium content becomes more relevant. Finally, we investigated the relationship between the CT attenuation value and susceptibility, seemingly linear for identical composition. It also hints at the diverse composition of calcifications.

Although it does not enable a complete biochemical analysis, QSM is a promising technique in the detection and quantification of calcium burden.