Cardiac Magnetic Resonance Imaging at 7 Tesla

Summary

The sensitivity gain inherent to ultrahigh field magnetic resonance holds promise for high spatial resolution imaging of the heart. Here, we describe a protocol customized for functional cardiovascular magnetic resonance (CMR) at 7 Tesla using an advanced multi-channel radio-frequency coil, magnetic field shimming and a triggering concept.

Abstract

CMR at an ultra-high field (magnetic field strength B₀ ≥ 7 Tesla) benefits from the signal-to-noise ratio (SNR) advantage inherent at higher magnetic field strengths and potentially provides improved signal contrast and spatial resolution. While promising results have been achieved, ultra-high field CMR is challenging due to energy deposition constraints and physical phenomena such as transmission field non-uniformities and magnetic field inhomogeneities. In addition, the magneto-hydrodynamic effect renders the synchronization of the data acquisition with the cardiac motion difficult. The challenges are currently addressed by explorations into novel magnetic resonance technology. If all impediments can be overcome, ultra-high field CMR may generate new opportunities for functional CMR, myocardial tissue characterization, microstructure imaging or metabolic imaging. Recognizing this potential, we show that multi-channel radio frequency (RF) coil technology tailored for CMR at 7 Tesla together with higher order B₀ shimming and a backup signal for cardiac triggering facilitates high fidelity functional CMR. With the proposed setup, cardiac chamber quantification can be accomplished in examination times similar to those achieved at lower field strengths. To share this experience and to support the dissemination of this expertise, this work describes our setup and protocol tailored for functional CMR at 7 Tesla.

Introduction

Cardiovascular magnetic resonance (CMR) is of proven clinical value with a growing range of clinical indications^1,2. In particular, the evaluation of cardiac morphology and function is of major relevance and typically realized by tracking and visualizing the heart motion throughout the entire cardiac cycle using segmented breath-held two-dimensional (2D) cinematograpic (CINE) imaging techniques. While a high spatio-temporal resolution, high blood-myocardium contrast and high signal-to-noise ratio (SNR) are required, the data acquisition is highly constrained by the cardiac and respiratory motion and the use of....

Protocol

The study is approved by the ethics committee of the University of Queensland, Queensland, Australia and informed consent has been obtained from all subjects included in the study.

1. Subjects

1. Recruit volunteer subjects over 18 years of age internally at the University of Queensland.
2. Informed consent
   1. Inform each subject about potential risks of undergoing the examination before entering the magnetic resonance imaging (MRI) safety zone. Specifically, discuss the ultra-high magnetic field exposure and possible contraindications for undergoing an MRI examination. Inform the subject that participating in the ex....

Results

Representative results of cardiac CINE examinations derived from volunteers are depicted in Figure 4. Shown are diastolic and systolic time-frames of short axis and a four-chamber long axis views of the human heart. The significantly higher spatial resolution for the short axis views (Figure 4a, 4b, 4e, 4f) compared to the long axis views (Figure 4c,.......

Discussion

Functional CMR examinations could be conducted successfully at 7 Tesla. Based on the field strength driven SNR gain, CINE images of the human heart could be acquired with significantly higher spatial resolution compared to 1.5 or 3 T. While a slice thickness of 6 to 8 mm and in-plane voxel edge lengths of 1.2 to 2.0 mm are commonly used at lower clinical field strengths^1,30, the measurements at 7 Tesla could be conducted with a slice thickness of 4 mm and an isot.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
7 Tesla MRI system	Siemens		Investigational Device
32-Channel -1H-Cardiac Coil	MRI.Tools GmbH		Transmit/Receive RF Coil for MR Imaging and Spectroscopy at 7.0 Tesla
ECG Trigger Device	Siemens
Pulse Trigger Device	Siemens