From Breath‐Hold to Free‐Breathing: Possibilities in High‐Resolution Quantitative Susceptibility Mapping ( QSM ) of the Kidney With Respiratory Gating

PURPOSE: This study aimed to assess the feasibility of QSM in the human kidney during free-breathing using respiratory gating for motion compensation. METHODS: Renal imaging of 15 healthy volunteers was performed using an axial multi-echo GRE sequence on a 3 T MRI scanner. Each subject was scanned once during breath-hold and once during free-breathing with respiratory gating based on a respiratory cushion. Five subjects were repositioned and scanned again to test for reproducibility. Additionally, one subject was measured in axial and coronal direction maximizing the feasible resolution during free-breathing. All phase data was unwrapped, the background field removed, and the inverse problem solved to create susceptibility maps. For statistical analysis, the mean susceptibility values of one kidney were analyzed across three consecutive slices and referenced to the susceptibility of the paravertebral muscle. RESULTS: for the free-breathing. Bland-Altman analysis demonstrated good reproducibility for both methods. The high-resolution measurements enabled by gated breathing showed renal detail previously unseen in QSM. CONCLUSION: Free-breathing QSM protocols in the human kidney using respiratory gating show equal susceptibility results compared to breath-hold acquisition while increasing clinical feasibility. A free-breathing protocol allows for the measurement of a larger subject pool and enables an increase in critical sequence parameters, such as echo-time and resolution. This could improve the accuracy of susceptibility calculations in the human abdomen.