Non-contact cardiac gating with ultra-wideband radar sensors for high field MRI

Cardiac MR (CMR) imaging in a defined phase of the heart cycle requires triggered data acquisition. Systems with B0 up to 1.5 T utilize ECG signals for triggering. However, due to the increasing perturbing influence of the magneto-hydrodynamic effect on the ECG signal, triggering is additional hampered in high and ultrahigh field MRI systems. The necessity to attach ECG electrodes to the human body is an additional obstacle if patients are unable to cooperate. Therefore, we have developed an alternative, contact-less approach for the detection of cardiac mechanics by means of ultra-wideband (UWB) radar. The basic idea of UWB sensing is to determine the position and/or displacement of tissue interfaces by measuring the two-way propagation time of the transmitted electromagnetic UWB signal [1]. Due to the different propagation times of signals from different interfaces and different dielectric contrast, the mechanical displacement of a selected interface can be monitored. The raw UWB radar data, however, contain a linear combination of all simultaneously occurring physiological signatures, including the dominating respiratory and cardiac displacements. To extract beneficial information from UWB radar for cardiac MRI the decomposition of respiratory and cardiac displacements is mandatory, since the MRI data have to be unambiguously assigned to the state of the individual respiratory and cardiac cycle. Due to higher harmonics from the highly nonlinear respiratory motion, the extraction of the cardiac motion from the composite signal just by common frequency-domain signal filtering is limited. We applied blind-source separation (BSS) based on several time-delayed second-order correlation matrices to decompose the UWB signals and extract a pure cardiac component. Within this component, we are able to identify landmarks indicating the point of maximal contraction of the myocardium. This article is to present our novel imaging approach, the application of retrospective cardiac triggering based on UWB signals acquired concurrently with a clinical cardio MR sequence.