Radial MP2RAGE sequence for rapid 3D T${}_1$ mapping of mouse abdomen: application to hepatic metastases

The T${}_1$ longitudinal recovery time is regarded as a biomarker of cancer treatment efficiency. In this scope, the Magnetization Prepared 2 RApid Gradient Echo (MP2RAGE) sequence relevantly complies with fast 3D T${}_1$ mapping. Nevertheless, with its Cartesian encoding scheme, it is very sensitive to respiratory motion. Consequently, a radial encoding scheme was implemented for the detection and T${}_1$ measurement of hepatic metastases in mice at 7T. A 3D radial encoding scheme was developed using a golden angle distribution for the k-space trajectories. As in that case, each projection contributes to the image contrast, the signal equations had to be modified. Phantoms containing increasing gadoteridol concentrations were used to determine the accuracy of the sequence in vitro. Healthy mice were repetitively scanned to assess the reproducibility of the T${}_1$ values. The growth of hepatic metastases was monitored. Undersampling robustness was also evaluated. The accuracy of the T${}_1$ values obtained with the radial MP2RAGE sequence was > 90% compared to the Inversion-Recovery sequence. The motion robustness of this new sequence also enabled repeatable T${}_1$ measurements on abdominal organs. Hepatic metastases of less than 1-mm diameter were easily detected and T${}_1$ heterogeneities within the metastasis and between the metastases within the same animal were measured. With a twofold acceleration factor using undersampling, high-quality 3D T${}_1$ abdominal maps were achieved in 9 min. The radial MP2RAGE sequence could be used for fast 3D T${}_1$ mapping, to detect and characterize metastases in regions subjected to respiratory motion.