- Oral presentation
- Open Access
The MR-stethoscope: safe cardiac gating free of interference with electro-magnetic fields at 1.5 T, 3.0 T and 7.0 T
© Tobias et al; licensee BioMed Central Ltd. 2009
- Published: 28 January 2009
- Cardiac Gating
- Patient Burn
- Acoustic Wave Guide
- Acoustic Cardiac Trigger
- Acoustic Trigger
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.
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.
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.
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.
This article is published under license to BioMed Central Ltd.