INTRODUCTION:

MRI has become a reference standard for the assessment of cardiac disease due to its excellent soft-tissue contrast. To utilize the increased signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) at ultra-high field strength (7T), development of new cardiac imaging techniques is necessary surpassing the limitations imposed by magnetic-field inhomogeneity, SAR constraints and also the coil-depth¹ issues of surface-coils.^2–6 The increased transmit field (B₁⁺) heterogeneity in the body at 7T leads to an inhomogeneous flip angle (FA) distribution including regions of complete signal cancelation within the volume of interest^6,7. So far only a few 3D Coronary Magnetic Resonance Angiography (CMRA) studies at 7T were reported^2,4 employing thin slabs (without whole-heart coverage) using two channel quadrature surface-coils.

For non-contrast enhanced CMRA, T₂-preparation is used to improve the contrast between the coronary arteries and myocardium. To our best knowledge, no numerical optimization method has been proposed for T₂-preparation refocusing pulses in cardiovascular imaging. We therefore sought to develop an optimization toolbox for the adiabatic hyperbolic secant (HS) pulse used in T₂-preperation (T₂-prep). We incorporated the T₂-prep in a free-breathing 3D dual-echo CMRA pulse-sequence (Figure 1) for imaging a 7T. The T₂-prepared CMRA sequence has been validated in phantom and healthy subjects employing a low-excitation flip-angle scheme.

METHODS:

1. Pulse optimization toolbox:
The pulse optimization toolbox (Figure 2) is written in MATLAB (v9.4, The MathWorks, Massachusetts, USA) and consists of the following four modules: Bloch simulation tool, numerical optimization tool for RF pulses, numerical simulation of the optimized pulse performance, RF pulse-shape generator for Siemens scanners. The actual rotation angle provided by a RF refocusing pulse with nominal flip angle (α) of π radians can be represented as α. B_1ratio . Here is the ratio of effective field to the nominal RF field strength. All optimizations were carried out on discrete grids of B_1ratio and ΔB₀ values with respective ranges of [0.2, 1.5] and ±500 Hz.

HS pulses^10,11 with the following pulse-profile variant were used

B₁ (t)=A(t)e^iϕ(t)

with amplitude modulation:
A(t)=A₀ sech(βt) ---(1)

and phase modulation:
ϕ(t)=µln⁡[sech⁡(βt) ]+ µln⁡(A0)

Where A₀ is the peak B₁ amplitude, µ and β control the shape of the amplitude and phase modulation functions and therefore are optimized.

3D Bloch-simulation tool: was written to efficiently handle pulse profiles and generate the magnetization components (M_x, M_y, M_z) separately.

Numerical optimization of the HS pulse: was performed for the optimal suppression of the human myocardium using the 8Tx/16Rx surface-coil from Rapid (Rapid Biomedical GmbH, Rimpar, Germany) in single-transmit (1Tx) mode for 7T using the abovementioned optimization grid. The numerical optimizations and simulations were performed in a cartesian magnetization space (M_x, M_y, M_z).⁸ To obtain the optimum inversion from the HS pulse we solved a minimization problem utilizing the interior-point algorithm to minimize the cost function

f = 1/(p.q) ∑_(i,j=1)^(p,q) (1/(B_1ratio)_i,j ) . | M_x,ij + M_y,ij | ---(2)

with the following patient and hardware safety constraints

P_l ≤ V_max
A₀ ≤ (B₁)_max
SAR ≤ (SAR)_10g,max

with
SAR = (1/T) . (σ/ρ) ∫₀^T |A(t)|² dt

here V_max is the maximum transmit voltage of RF pulse and B_1max (18 µT, empirically determined) is the maximum B₁ amplitude. σ is the tissue conductivity, ρ is the volumetric mass density, T is the pulse duration and A(t) is the amplitude modulation function of the HS pulse eq.(1). i,j are indices on the p × q dimensional B_1ratio - ∆B₀ optimization grid.

Simulation: Refocusing performance of the HS pulse could be best represented with the simulated values of | M_x+ iM_y | on the inhomogeneity grid of B₁⁺ and ∆B₀ after optimization.

2. Image acquisition:
We employed a free‐breathing iNAV motion corrected 3D dual-echo CMRA pulse-sequence (IDEA version E12U-SP01) and combined it with the new optimized T₂-prep on a 7T scanner (MAGNETOM Terra, Siemens Healthcare, Erlangen, Germany) (Figure 1). To circumvent the magnetohydrodynamic effect, which is highly prominent at 7T, we employed a pulse oximeter (Siemens Healthcare, Erlangen, Germany) for cardiac synchronization instead of ECG. All the acquisitions were carried out with a body 8Tx/16Rx surface-coil in Trueform (1Tx) mode. The motion-corrected image reconstruction framework used in this work has been described earlier.^12–15

RESULTS:

Figure 3 represents the amplitude, frequency and phase modulation waveforms for optimized HS pulse as well as the real and imaginary parts of the HS pulse. The simulated performance of the HS pulse on the B₁⁺ and ∆B₀ grids is demonstrated in Figure 4 for cardiac imaging at ultra-high field (7T) with the 8Tx/16Rx surface coil in Trueform (1Tx) mode.

Figure 5 represents the phantom images obtained with T₂-prep containing conventional HS pulse and our optimized HS pulse. The artefacts at phantom-air interface with the standard T₂-prep are greatly reduced in the image obtained with the optimized HS pulse. In the volunteer image, artifacts from magnetic field inhomogeneities (marked with blue arrows) are visible with the standard T₂-prep (Figure 5c) but not with the optimized T₂-prep (Figure 5d).

CONCLUSION:

We present an optimization algorithm for adiabatic refocusing pulses, that resulted in improved T₂-preparation performance at 7T with reduced sensitivity to magnetic field inhomogeneities. Further we demonstrate the successful initial use of a motion-corrected free-breathing 3D dual-echo CMRA pulse-sequence incorporating this T₂-prep in low flip-angle phantom and in vivo measurements.

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]).

References

1. Terekhov, M. et al. New commercial 8Tx / 16RX array for Clinical 7T Cardiac MRI : initial experience. Proc. Intl. Soc. Mag. Reson. Med. 29 (2021), 2–4.
2. Van Elderen, S. G. C. et al. Initial results on in vivo human coronary MR angiography at 7 T. Magn. Reson. Med. 62, 1379–1384 (2009).
3. Snyder, C. J. et al. Initial results of cardiac imaging at 7 Tesla. Magn. Reson. Med. 61, 517–524 (2009).
4. Van Elderen, S. G. C. et al. Right coronary MR angiography at 7 T: A direct quantitative and qualitative comparison with 3 T in young healthy volunteers. Radiology 257, 254–259 (2010).
5. Schmitter, S. et al. Cardiac imaging at 7 tesla: Single- and two-spoke radiofrequency pulse design with 16-channel parallel excitation. Magn. Reson. Med. 70, 1210–1219 (2013).
6. Aigner, C. S., Dietrich, S. & Schmitter, S. Three-dimensional static and dynamic parallel transmission of the human heart at 7 T. NMR Biomed. 34, 1–15 (2021).
7. Schmitter, S., Schnell, S., Uğurbil, K., Markl, M. & Van de Moortele, P. F. Towards high-resolution 4D flow MRI in the human aorta using kt-GRAPPA and B1+ shimming at 7T. J. Magn. Reson. Imaging 44, 486–499 (2016).
8. Moore, J., Jankiewicz, M., Anderson, A. W. & Gore, J. C. Evaluation of non-selective refocusing pulses for 7 T MRI. J. Magn. Reson. 214, 212–220 (2012).
9. Moore, J., Jankiewicz, M., Zeng, H., Anderson, A. W. & Gore, J. C. Composite RF pulses for B1+-insensitive volume excitation at 7 Tesla. J. Magn. Reson. 205, 50–62 (2010).
10. Silver, M. S., Joseph, R. I. & Hoult, D. I. Selective spin inversion in nuclear magnetic resonance and coherent optics through an exact solution of the Bloch-Riccati equation. Phys. Rev. A 31, 2753–2755 (1985).
11. Silver, M. S., Joseph, R. I. & Hoult, D. I. Highly selective π 2 and π pulse generation. J. Magn. Reson. 59, 347–351 (1984).
12. Henningsson, M., Smink, J., Razavi, R. & Botnar, R. M. Prospective respiratory motion correction for coronary MR angiography using a 2D image navigator. Magn. Reson. Med. 69, 486–494 (2013).
13. Bustin, A. et al. 3D whole-heart isotropic sub-millimeter resolution coronary magnetic resonance angiography with non-rigid motion-compensated PROST. J. Cardiovasc. Magn. Reson. 22, 1–16 (2020).
14. Prieto, C. et al. Highly efficient respiratory motion compensated free-breathing coronary MRA using golden-step Cartesian acquisition. J. Magn. Reson. Imaging 41, 738–746 (2015).
15. Cruz, G., Atkinson, D., Henningsson, M., Botnar, R. M. & Prieto, C. Highly efficient nonrigid motion-corrected 3D whole-heart coronary vessel wall imaging. Magn. Reson. Med. 77, 1894–1908 (2017).