High Spatial Resolution and Temporally Resolved T2* Mapping of Normal Human Myocardium at 7.0 Tesla: An Ultrahigh Field Magnetic Resonance Feasibility Study

Abstract

Myocardial tissue characterization using T2 * relaxation mapping techniques is an emerging application of (pre)clinical cardiovascular magnetic resonance imaging. The increase in microscopic susceptibility at higher magnetic field strengths renders myocardial T2 * mapping at ultrahigh magnetic fields conceptually appealing. This work demonstrates the feasibility of myocardial T2 * imaging at 7.0 T and examines the applicability of temporally-resolved and high spatial resolution myocardial T2 * mapping. In phantom experiments single cardiac phase and dynamic (CINE) gradient echo imaging techniques provided similar T2 * maps. In vivo studies showed that the peak-to-peak B0 difference following volume selective shimming was reduced to approximately 80 Hz for the four chamber view and mid-ventricular short axis view of the heart and to 65 Hz for the left ventricle. No severe susceptibility artifacts were detected in the septum and in the lateral wall for T2 * weighting ranging from TE = 2.04 ms to TE = 10.2 ms. For TE >7 ms, a susceptibility weighting induced signal void was observed within the anterior and inferior myocardial segments. The longest T2 * values were found for anterior (T2 * = 14.0 ms), anteroseptal (T2 * = 17.2 ms) and inferoseptal (T2 * = 16.5 ms) myocardial segments. Shorter T2 * values were observed for inferior (T2 * = 10.6 ms) and inferolateral (T2 * = 11.4 ms) segments. A significant difference (p = 0.002) in T2 * values was observed between end-diastole and end-systole with T2 * changes of up to approximately 27% over the cardiac cycle which were pronounced in the septum. To conclude, these results underscore the challenges of myocardial T2 * mapping at 7.0 T but demonstrate that these issues can be offset by using tailored shimming techniques and dedicated acquisition schemes.

Myocardial T2* Mapping with Ultrahigh Field Magnetic Resonance: Physics and Frontier Applications

“…Consequently, the delineation of the myocardium in the corresponding T 2 * map ( Figure 7C) is more challenging compared to the RARE based T 2 * map. The average effective transversal relaxation time derived from RARE imaging compares well to values previously reported for MEGRE approaches [39]. The concerns of RF power deposition and RF non-uniformity of RARE imaging were offset in this preliminary study.…”

“…Figure 8 compares B 0 homogeneity in the heart of a healthy subject at 7.0 T after applying a volume shim ( Figure 8A, top), which is focused only on the heart and after applying a global shim ( Figure 8A, bottom) which includes the entire field of view. Volume selective shimming was found to lead to a significant improvement in macroscopic B 0 homogeneity vs. global B 0 shimming ( Figure 8B) [39]. Due to increased susceptibility effects, magnetic field inhomogeneities are pronounced at higher magnetic field strengths [36] (compare Equation 6).…”

“…This reduction in voxel sizes lowers the impact of macroscopic magnetic field gradients on intra-voxel dephasing and hence T 2 * which otherwise can be a concern especially in the vicinity of strong susceptibility transitions. Transitioning to higher magnetic field strengths runs the additional benefit that the in-phase interecho time governed by the fat-water phase shift between the water and main fat peak of about 3.5 ppm is reduced from approximately 4.5 ms (223 Hz) at 1.5 T to 0.96 ms (1,043 Hz) at 7.0 T. This enables rapid acquisition of multiple echoes with different T 2 * sensitization and facilitates high spatio-temporally resolved myocardial CINE T 2 * mapping of the human heart [39]. Taking advantage of this technique, T 2 * mapping at ultrahigh magnetic fields has been suggested as a means to probe myocardial physiology and to advance myocardial tissue characterization.…”

“…This approach permits low inter-echo spacing even for large matrix sizes but results in longer scan times since more than one TR is required to acquire a full T 2 * decay series. While T 2 * mapping at clinical field strengths is limited to single cardiac phase acquisitions, CINE T 2 * mapping covering the entire cardiac cycle is feasible at UHF [39]. This advanced capability is facilitated by two benefits of UHF-MR. First, due to transversal relaxation time shortening at ultrahigh magnetic fields, TE can be limited to a range of approximately TE = 0 ms to TE = 15 ms to properly sample the myocardial T 2 * decay.…”

“…This assumption is justified for typical voxel sizes used in cardiac T 2 * mapping at 7.0 T with an in-plane spatial resolution of about 1 mm and a slice thickness of 2-4 mm [39,84]. | B| in Equation (9) includes microscopic magnetic field perturbations resulting from microstructural changes, blood oxygenation changes, iron accumulation etc.…”