The MR-stethoscope: safe cardiac gating free of interference with electro-magnetic fields at 1.5 T, 3.0 T and 7.0 T

Introduction

In clinical CVMR, cardiac motion is commonly dealt with using ECG-gating. ECG, being an inherently electrical measurement, is prone to lead and patient burns. Furthermore, ECG is corrupted by interferences with electromagnetic fields and by magneto-hydrodynamic effects. Consequently, artifacts in the ECG trace and T-wave elevation might be mis-interpreted as R waves resulting in erroneous triggering together with motion corrupted image quality – an issue which is pronounced at (ultra)high fields.

Purpose

Overcome the challenges of conventional ECG-gating by developing an acoustic cardiac gating approach, which offers (i) no risk of high voltage induction and patient burns, (ii) immunity to electromagnetic interferences, (iii) suitability for all magnetic field strengths and ease of use for the pursuit of robust and safe clinical CVMR. For this purpose, this study examines and demonstrates the suitability, efficacy and robustness of acoustic cardiac triggering (ACT) in CVMR applications at 1.5 T, 3.0 T and 7.0 T including prospective gating and retrospective triggering regimes.

Methods

The acoustic gating device consists of three main components: (i) an acoustic sensor, (ii) a signal processing unit and (iii) a coupler unit to the MRI system. An acoustic wave guide was used for signal transmission while accomplishing galvanic decoupling. Signal conditioning and conversion were conducted outside of the scanner room using dedicated electronic circuits. All scanner and gradient coil noise contributions to the acoustic signal were cancelled using a 3rd order inverse Chebychev filter. The final waveform was delivered to the internal physiological signal controller circuitry of a clinical MR scanner. The current implementation connects the trigger signal with the MR-scanner's standard ECG-signal input. Hence, no changes to the MR system's hardware and software are required. Volunteer studies (n = 10) were performed on 1.5 T, 3.0 T and 7.0 T whole body MR systems (Achieva, Philips, Best, The Netherlands). The acoustic sensor was positioned at the anterior left side of the torso to obtain acoustic cardiograms. For comparison, ECG was recorded for all subjects. A retrospectively triggered 2D CINE SSFP technique was used to examine acoustic gating for reliable tracking of myocardial contractions over entire R-R intervals. Black blood prepared gradient echo imaging, 3D phase contrast MRA and free breathing 3D coronary MRA were employed to evaluate acoustic triggering in a prospective gating regime.

Results

The acoustic MR-stethoscope provided cardiograms at 1.5 T, 3.0 T and 7.0 T free of interferences from electromagnetic fields or magneto-hydraulic effects and hence is suitable for synchronization (Fig 1). In comparison, ECG waveforms were susceptible to T-wave elevation and other distortions which were pronounced at (ultra)high fields (Fig 1). Acoustically triggered 3D SSFP coronary MRA imaging produced images free of motion artifacts (Fig. 2a). Conversely, R-wave mis-registration occurred in ECG-triggered acquisitions due to T-wave elevation, which made 3D coronary MRA prone to motion artefacts (Fig. 2a). Motion artifacts were not present in full R-R interval coverage, acoustically triggered CINE imaging (Fig. 2b). The merits of acoustic triggering were further explored in prospectively gated, blood suppressed anatomic imaging, which provided image quality competitive or even superior to that obtained from the ECG-gated approach as indicated by Fig. 2c. Acoustically triggered 3D PC MRA acquisitions resulted in MR angiographies of superb quality free of motion artifacts even at (ultra)high magnetic field strengths, as shown in Fig. 2d.

Figure 1

Figure 2

Discussion and conclusion

The proposed acoustic approach was found to fully meet the demands of cardiac gated/triggered MRI. Its superior robustness has been demonstrated by eliminating the frequently-encountered difficulty of mis-triggering due to ECG-waveform distortions. ACT-MR substantially reduces the complexity of patient preparation by obviating the need to set up ECG-electrodes and position ECG-leads, and hence serves to streamline clinical CVMR.