INTRODUCTION:

Cardiovascular MR imaging (CMRI) at ultra-high magnetic field (7 Tesla) has the potential to provide images with spatiotemporal resolution at mesoscopic level due to the improved signal-to-noise ratio (SNR). However, at 7T, the increased magnetic-filed inhomogeneity, SAR constraints and also the coil-depth¹ issues for transmit and receive of surface array coils make cardiac imaging extremely challenging.^2–6 The increased transmit field (B₁⁺) heterogeneity in the body at 7T leads to an inhomogeneous flip angle (FA) distribution.^6,7 Different parallel transmission techniques (pTx)^3,5,7–9 have been used to address this problem and improve cardiac imaging. So far at 7T only a few 3D CMRI⁹ studies with pTx and Coronary Magnetic Resonance Angiography (CMRA) studies employing thin slabs using two channel quadrature surface coils have been reported^2,4. Because of the SAR constraints, it is also not straightforward to use T₂-preparation pulses¹⁰ at 7T with standard adiabatic pulses. Therefore, an acquisition technique with low SAR preparation pulses or without the need of any preparation pulse is highly desired.

Here we present for the first time ultra-high resolution (0.7mm³ isotropic) free-breathing translational motion compensated 3D CMRA images obtained without any preparation pulses (e.g. T₂-prep) with Trueform B₁-shim mode at 7T without using any contrast agent. We further have investigated the effect of magnetization transfer (MT) on the contrast behavior between blood and myocardium by employing FA variation.

METHODS:

A free‐breathing 3D CMRA gradient-echo sequence with variable density Cartesian spiral-like trajectory¹¹, image navigator (iNAV)¹² and chemical-shift selective fat-suppression (fatsat) was implemented on a MAGNETOM Terra 7T MRI scanner (Siemens Healthcare, Erlangen, Germany) (Figure 1). All the acquisitions were carried out with a body 8Tx/16Rx surface coil (Rapid Biomedical GmbH, Rimpar, Germany) in single transmit (1Tx) mode (Trueform). To circumvent the highly prominent magnetohydrodynamic effect at 7T, which disturbs the ECG typically used for cardiac synchronization, we employed a pulse oximeter (Siemens, Erlangen, Germany) instead. The motion-corrected image reconstruction framework used in this work has been described earlier.^10–13 Foot-head (FH) and right-left (RL) translational respiratory motion is estimated from the iNAVs acquired at each heartbeat before the 3D CMRA readout and used to correct the acquired k-space data.

To avoid SAR constraints imposed by the use of T₂-preparation pulses and to address transmit field (B₁⁺) heterogeneity across the thorax, the effect of high excitation flip-angles (FA) on contrast between the coronary arteries and myocardium was investigated employing free-breathing 3D CMRA for flip angles of 6°, 10°, 20°, 30° and 40°.

A healthy volunteer (male, 25 years old) was scanned in three different sessions to observe the reproducibility of the proposed approach. Acquisitions were performed in-vivo with 0.7mm³ (TR/TE = 4.8/2.17) and 0.9 mm³ (TR/TE = 4.6/2.1) isotropic resolution and 3-fold undersampling with FOV=320mm.

RESULTS:

A comparison between the CMRA images obtained with different excitation FAs (6°- 40°) is presented in Figure 2 for two different slice positions. For low flip-angles (6° and 10°), contrast between blood and myocardium is almost absent. The best contrast was obtained for a FA of 40°. A gradual increase in contrast and signal intensity from low towards high FA is apparent.

CMRA images with the proposed approach and ultra-high resolution (0.7mm³ isotropic) are shown in Figure 3. Reformatted CMRI views are shown in Figure 3c-e. Figure 3f presents a tracked right coronary artery (RCA) and three cross-sectional images of the RCA.

CONCLUSION:

We demonstrated a new technique to obtain contrast-free ultra-high-resolution free-breathing 3D CMRA images at 7T with high contrast between the coronary arteries and myocardium. The dependency of magnetization transfer (MT) on FA and repetition time (TR) was reported earlier in human brain studies.¹⁴ The observed contrast could be due to MT and/or inflow at high FA and low TR. This method may be useful for 7T clinical applications for high resolution cardiac imaging and further evaluation in healthy subjects and patients is warranted.

Acknowledgements

This work was supported by EPSRC (EP/L015226/1, EP/P032311/1, EP/P007619/1 and EP/P001009/1), the Wellcome/EPSRC Centre for Medical Engineering (NS/A000049/1) and 7T grant (Wellcome Trust Collaboration in Science grant [WT201526/Z/16/Z]).

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