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 T(2)(*) 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 T(2)(*) mapping at ultrahigh magnetic fields conceptually appealing. This work demonstrates the feasibility of myocardial T(2)(*) imaging at 7.0 T and examines the applicability of temporally-resolved and high spatial resolution myocardial T(2)(*) mapping. In phantom experiments single cardiac phase and dynamic (CINE) gradient echo imaging techniques provided similar T(2)(*) maps. In vivo studies showed that the peak-to-peak B(0) 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 T(2)(*) 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 T(2)(*) values were found for anterior (T(2)(*) = 14.0 ms), anteroseptal (T(2)(*) = 17.2 ms) and inferoseptal (T(2)(*) = 16.5 ms) myocardial segments. Shorter T(2)(*) values were observed for inferior (T(2)(*) = 10.6 ms) and inferolateral (T(2)(*) = 11.4 ms) segments. A significant difference (p = 0.002) in T(2)(*) values was observed between end-diastole and end-systole with T(2)(*) 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 T(2)(*) mapping at 7.0 T but demonstrate that these issues can be offset by using tailored shimming techniques and dedicated acquisition schemes.

Figure 1. Synopsis of multi-echo gradient echo strategies used for T₂ ^* mapping at 7.0 T.

A). Conventional multi-echo (ME) gradient echo for single cardiac phase myocardial T₂ ^* mapping. Multiple echoes are acquired after excitation to obtain a set of T₂ ^* weighted images. The competing constraints of inter echo time and spatial resolution inherent to the ME approach are addressed by the B) interleaved multi-shot multi-echo (MS) gradient echo technique. In MS a set of excitations is employed together with echo interleaving echoes to acquire a set of T₂ ^* weighted images. C) The multi-breath-hold multi-echo (MB CINE) gradient echo technique allows myocardial CINE T₂ ^* mapping by interleaving the echoes over several breath-holds. For benchmarking D) multi-echo CINE (ME CINE) gradient echo and E) multi-shot multi-echo CINE (MS CINE) were applied for T₂ ^* mapping in phantom studies. To guide the eye vertical dashed lines refer to k-space lines. Vertical solid lines refer to cardiac phases. A unipolar readout using gradient flyback was applied for all strategies.

Figure 2. Survey of T₂ ^* maps derived from phantom studies.

T₂ ^* maps obtained for all imaging strategies using a long T₂ ^* (A) and a medium T₂ ^*phantom (B). Slice thicknesses ranging from 8 mm to 2.5 mm (top to bottom) were applied. T₂ ^* analysis revealed similar results for all T₂ ^* mapping strategies. For a slice thickness of 8 mm T₂ ^* varied substantially across both phantoms. The uniformity in T₂ ^* was improved for a slice thickness of 6 mm and even further enhanced for a slice thickness of 4 mm or 2.5 mm.

Figure 3. B₀ distribution for global and volume selective B₀ shimming of a four chamber view of the heart.

A) Four chamber view of the heart illustrating the positioning of the volume (marked in red) used for global (left) and volume selective (right) shimming. B) B₀ field variation derived from global and volume selective shimming. For this subject the global shim provided a peak-to-peak field variation of about 400 Hz across the entire heart. After volume selective shimming peak-to-peak B₀ variation across the heart was reduced to approximately 80 Hz. The direction of the maximal B₀ gradient is illustrated by the dashed black line in B) and the corresponding profile of B₀ field distribution is plotted in C). To guide the eye the epicardial borders are marked in B) and C) by two triangles. The histogram of the field distribution over the left ventricle is shown in D). The full width at half maximum is approximately 200 Hz for the globally shimmed B₀ field map and was reduced to about 80 Hz after volume selective shimming.

Figure 4. B₀ distribution for global and volume selective shimming of a mid-ventricular short axis view of the heart.

A) Mid-ventricular short axis view of the heart illustrating the positioning of the volume (marked in red) used for volume selective shimming. B) B₀ field maps. C) B₀ profile along the direction of the strongest B₀ gradient which is highlighted by the dashed black line in B). To guide the eye the epicardial borders are marked in B) and C) by two triangles. D) Frequency histogram across the left ventricle. After volume selective shimming a strong susceptibility gradient at the inferior region of the heart could be reduced. The full width at half maximum is approximately 300 Hz for the globally shimmed field map and was reduced to about 80 Hz after volume selective shimming.

Figure 5. Short axis views derived from single cardiac phase and dynamic CINE T₂ ^* weighted imaging of the heart.

Echo times ranging from 2.04 ms to 10.20 ms were used for MS and MB CINE acquisitions. A low nominal flip angle of 20° was used to preserve myocardial signal. Image quality observed for MS and MB CINE acquisitions is comparable. No severe susceptibility artifacts were detected in the septum and in the lateral wall for TEs ranging between 2.04 ms to 10.20 ms. For anterior and inferior myocardial areas encompassing major cardiac veins susceptibility weighting related signal void was observed for TE >7 ms as highlighted by white arrows.

Figure 6. T₂ ^* maps derived from single cardiac phase and dynamic CINE mapping of a four chamber and short axis view of the heart at end-diastole and end-systole.

Four chamber (top) and short axis view T₂ ^* colour maps obtained from MS and MB CINE superimposed to anatomical 2D CINE FLASH gray scale images. For MB CINE a systolic and diastolic phase was chosen to match the cardiac phase with the end-systolic and end-diastolic phase derived from MS. T₂ ^* maps deduced from MS and MB CINE showed no significant differences between both methods in the segmental analysis of T₂ ^* values. When comparing systolic and diastolic T₂ ^* maps significant differences were found with p = 0.002 for MS and p = 0.01 for MB CINE.

Figure 7. CINE T₂ ^* maps over the cardiac cycle.

Short axis view T₂ ^* colour maps derived from MB CINE acquisitions across the cardiac cycle overlaid to conventional 2D CINE FLASH images. T₂ ^* values are increasing from diastole to systole, especially for endocardial layers. Macroscopic susceptibility induced T₂ ^* reduction effects were present at the epicardium at inferior regions.

Figure 8. Analysis of T₂ ^* across the cardiac cycle.

Synopsis of the evolution of mean T₂ ^* averaged over all subjects for standard mid-ventricular segments of the heart. T₂ ^* derived from each cardiac segment are plotted versus the cardiac cycle. T₂ ^* changes over the cardiac cycle. Averaging T₂ ^* over all mid-ventricular myocardial segments revealed that T₂ ^* increases approximately 27% between systole and diastole. Myocardial T₂ ^* was derived from MB CINE acquisitions. Prospective triggering was used which resulted in a gap at end-diastole of approximately 100 ms depending on the heart rate. For this reason the cardiac cycle is normalized for all subjects without including this gap.

Figure 9. High spatial resolution four chamber and short axis view T₂ ^* maps derived from T2* weighted CINE imaging.

For MB CINE slice thickness was reduced to 2.5 mm while maintaining the in-plane spatial resolution of (1.1×1.1) mm². Compared to the results obtained with MB CINE using a slice thickness of 4 mm, changes in T₂ ^* from epicardial to endocardial septal myocardial layers are more pronounced, in particular during systole.