Scientists and clinicians interested in fast cardiac MRI in rodents.The $$$T_1$$$ relaxation time of myocardial tissue is an important parameter for the investigation of various cardiac diseases. The $$$T_1$$$ measurement in small animals, however, is challenging. When using an initial inversion of the magnetization, it is usually necessary to acquire data within several repetitions/inversions to adequately cover the relaxation process and to account for cardiac and breathing motion. In [1], a method was presented, which allows determining real time cardiac images in mice at a temporal resolution of ~12 ms, by using highly undersampled radial acquisitions reconstructed by a combination of through-time radial GRAPPA [2] and CS [3]. This technique was applied to an inversion recovery (IR) prepared acquisition in the mouse heart to enable the determination of a 2D $$$T_1$$$-map by using a single inversion only.

Data were acquired in healthy C57BL/6 mice (n=5) on a horizontal 9.4T MR system comprising a VNMRS DDR2 console (Agilent, Santa Clara, US), a 1000 mT/m gradient system and a four-channel cardiac array (Rapid Biomedical, Germany). A radial IR-Look-Locker FLASH sequence (TE/TR=0.99/1.98ms, FOV=30x30mm, 1mm short-axis slice, matrix size 128x128, flip angle 5°) was applied using a slice selective, adiabatic inversion pulse. The undersampled datasets (six projections per timeframe) were then subjected to the reconstruction algorithm described in [1], while a low-rank plus sparse model [4] was applied in the compressed sensing part instead of the temporal TV operator of the original implementation. This yielded a fully sampled series of images at a temporal resolution of 11.88 ms corresponding to ~10 timeframes per cycle. The ECG, recorded during the measurement, in combination with the DC-signal (i.e. center of k-space) of the series allowed to identify a diastolic timeframe per RR-interval and to exclude frames corrupted by respiratory motion (see Fig. 1). The chosen frames and their timestamps were finally used to perform a 3-parameter fit to the temporal course of every voxel: $$$M(t) = M_0^* - (M_0 + M_0^*) \cdot exp(-t/T_1^*)$$$, with $$$T_1 = T_1^* \cdot [(M_0 + M_0^*)/M_0^* -1]$$$.

Fig. 2a shows the resulting $$$T_1$$$-map in full field-of-view for one mouse. Voxels containing low signal in the real-time images were masked. The image quality was confirmed in the maps obtained for the remaining four mice (Fig. 2b-e). The $$$T_1$$$ values in myocardial tissue agreed well between the different animals and a ROI-analysis within the left ventricle yielded a mean value across all mice of $$$T_1$$$ = 1.12 $$$\pm$$$ 0.15 s which is in accordance with the values found earlier [5].The initial results demonstrate that the proposed technique provides accurate myocardial $$$T_1$$$ maps in mice using a single inversion only. The acquisition time of only ~ 5s per slice would significantly shorten the duration of the measurement and therefore simplify and speed up comprehensive studies of the murine heart. Next steps comprise of the validation of the technique in a larger cohort as well as the investigation of disease models.