Quantitative Susceptibility Mapping of Kidney Stones: An Ex Vivo MRI Phantom Study

To visualize and characterize the five most common kidney stone types based on their magnetic susceptibilities in MRI using QSM.

Three water-based agar phantoms were constructed, containing a total of 53 ex vivo kidney stones of varying types and sizes. Seven different MRI multi-echo gradient echo sequences were employed at field strengths of 1.5 T and 3 T. Each acquisition was repeated three times. Susceptibility maps were reconstructed and mean susceptibility values calculated for the individual kidney stones (normalized to the agar gel). Five of the most common kidney stone types-carbonate apatite, calcium oxalate, uric acid, cystine, and struvite-were investigated with respect to different sequence parameters and field strengths.

All examined kidney stones were reliably visualized as diamagnetic using QSM. Overall, good repeatability was observed for the mean susceptibility values and standard deviations of the individual kidney stones. Mean susceptibility values were highly consistent across acquisitions at both field strengths for kidney stones belonging to the five major types with diameters larger than 3.0 mm (66% of all stones). The overall mean susceptibility values (± SD) were - 1.25 ± 0.16 ppm for carbonate apatite, -1.08 ± 0.13 ppm for calcium oxalate, -0.94 ± 0.17 ppm for uric acid, -1.32 ± 0.16 ppm for cystine, and -1.28 ± 0.28 ppm for struvite.

Our data indicate that QSM is well-suited to reliably visualize kidney stones and distinguish certain types among the five most common kidney stone compositions using MRI. This represents a crucial step toward in vivo imaging of suspected urolithiasis in clinical practice.