Synopsis

Radiofrequency (RF) field inhomogeneities affect image quality while performing body and cardiac imaging especially at high field strengths. RF shimming and quantitative MR applications such as T1 mapping can potentially benefit from an accurate knowledge of variations of the transmitted RF field (B₁⁺).

The purpose of this work is to investigate spatial homogeneity of the B₁⁺ in the myocardium at 3.0T. For this study, we developed a prospective ECG-gated method based on DREAM (Dual Refocusing Echo Acquisition Mode).

The presented approach allows free breathing multi-shot acquisition B₁⁺ mapping and its feasibility is shown in vivo.

PURPOSE

Clinically relevant applications of cardiac MRI like quantitative tissue characterization of the myocardium require an accurate knowledge of the uniformity of the transmitted RF (B₁⁺) field, especially at high field strengths. Several studies have reported that B₁⁺ inhomogeneities can lead to image artifacts and thereby compromise diagnosis ^1-3.

However, quantification of cardiac B₁⁺ maps remains challenging due to cardiac and respiratory motion and, moreover the need of time-efficient and motion-robust sequences.

In the present work, we propose a fast multi-single shot B₁⁺ mapping method, based on the DREAM sequence ⁴ and prospectively synchronized with the cardiac cycle to measure and characterize static and dynamic field inhomogeneities within the myocardium.

As reported in other cardiac B₁⁺ studies ⁵ the specific absorption rate (SAR) could be a limiting factor when using the Bloch-Siegert Shift. The DREAM method derives B₁⁺ maps from SPGR images acquired with a low flip angle train, therefore the SAR burden is low making it a promising B₁⁺ mapping solution for ultrahigh-field systems.

MATERIALS AND METHODS

In-vivo experiments were performed on a 3.0T system GE MR750w (GE Healthcare, Milwaukee, WI) using the standard birdcage body coil in quadrature mode for RF transmission and 20-channel torso array coil for signal reception. B₀ shimming was applied using the default techniques supplied by the vendor.

B₁⁺ maps were obtained using the modified DREAM sequence for a short axis orientation according to the cardiac imaging standards. For each slice, prospective ECG-gating allowed imaging during the diastolic quiescence of a single heartbeat. The average heart rate for the subject was 50 bpm.

The virtual stimulated DREAM echo scheme (STE*) was used. Figure 1 shows the basic pulse sequence scheme and acquisition window. Scans were acquired with the following imaging parameters: T_R/T_S/T_D =6.2ms/0.8ms/3.9ms; STEAM flip angle α= 60°, imaging flip angle β= 15°, matrix size= 96x96; slice thickness=10mm; Phase FOV=0.8; BW=15kHz; Partial Fourier (NEX=0.5) was performed to minimize total acquisition time (380 ms) and centric order was chosen to reduce intra-acquisition motion artefacts.

Ten repeated single-shot B₁⁺ maps were acquired to study reproducibility in free breathing and breath-held positions (inhale and exhale). Native STE and FID echoes which are used to derive B₁⁺maps ⁴, were registered and combined to obtain motion insensitive B₁⁺ maps with higher SNR. All computations were performed using MATLAB (The Mathworks, Natick, MA, USA).

RESULTS

In vivo relative B₁⁺ maps, calculated as a fraction of intended flip angle are shown in Figure 2 for different respiratory positions and reconstruction methods. Mean values and standard deviations in myocardium were calculated within six ROI’s manually drawn as defined by AHA ⁶. High consistency of B₁⁺ values is observed between free-breathing and breath hold (inhale/exhale) states which indicates motion robustness of this approach (see Figure 3).

As depicted in Figure 4 the standard deviation of B₁⁺ values within a representative ROI is demonstrating the impact of registration and combination of multiple shots compared to single shot B₁⁺ maps.

Variations between basal anteroseptal and anterolateral segments (segment2/segment 6) of 34% (free breathing), 25 % (inhale), 28% (exhale) were observed (Figure 3) showing strong agreement with previous studies ⁷.

DISCUSSION AND CONCLUSION

Here we demonstrated in vivo feasibility of a cardiac gated multi single shot B₁⁺ mapping, which allows motion artifact-free mapping.

Cardiac motion is addressed by synchronizing acquisition with the cardiac cycle using a physiological trigger device (prospective ECG gating).

Furthermore, multiple single shot acquisitions are expected to improve robustness to respiratory motion which is a clear advantage compared to previously published studies where acquisition of two separate images and breath-holding are required ^7,8.

Moreover, the proposed method performs a SPGR Cartesian k-space readout, as commonly used in clinical CMR sequences. This confers robustness against acquisition specific artifacts, e.g. radial/spiral while relying on standard reconstruction techniques available in most MRI systems and could be easily integrated into the clinical workflow.

The proposed method is particularly interesting for cardiac B₁⁺ mapping at high fields due to its low SAR burden.

Although studying B₀ changes was beyond the scope of this study the DREAM approach provides simultaneous B₀ maps and could be beneficial for other applications.

Due to flow effects, reliable values in the blood pool of the left ventricle are still challenging. However, flow compensation, a smart undersampling strategy across shots and further validation in a larger group may help and will be investigated in future research.

Acknowledgements

This publication was supported by the European Commission, through Grant Number 605162. The content is solely the responsibility of the authors and does not necessarily represent the official views of the EU.

The authors gratefully thank the assistance and fruitful discussions with Prof. Burschka, Prof. J. Felblinger, Dr. F. Odille and Dr. P.-A. Vuissoz.

References

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