Radio-frequency (RF) antennas are commonly used in MRI to transmit and receive signals from which the MR images are reconstructed. Alternatively, RF antennas can be used as external non-contact sensors to sense physiological motion inside the human body. This principle is useful in the MRI scanner to detect subject motion, but can also be useful outside of the MRI as a wearable device to measure cardiac or respiratory function. When an RF antenna is placed on the body and electromagnetic waves are emitted into the body, a tiny fraction of those waves is scattered back into the antenna. The amount of backscattering depends on the properties of the tissue close to the antenna. When these properties change due to cardiorespiratory motion, the backscattered RF waves change1–3. With a network analyzer, a weak harmonic signal is transmitted into the RF antenna and the backscattering can be quantified. An RF antenna can thus be used as a non-invasive motion sensor, the detection principle does not require skin contact and even works when the antenna is in the same room as the moving subject.
The aim of the RF Sensing (RFS) project is to measure and assess cardiac biomarkers such as stroke volume and filling pressure with wearable RF antennas. By using the same type of RF antennas that are used in MRI, useful physiological information can be derived from MRI scans to improve the precision of our measurements. Figure1 shows a prototype of the RFS with a wearable miniature network analyzer and preliminary validation data.
Figure 1: 1a. Current RFS prototype (1a and 1b) and preliminary data showing sensitivity to cardiac volume (1c) and atrial pressure (1d).
RFS is mainly sensitive to mechanical deformation of local tissue properties. This differentiates RFS from ECG where neurological impulses are measured. With RFS, we measure changes in the actual contractile motion of the heart. RFS can measure changes in stroke volume and ventricular pressure, which are highly relevant parameters for heart failure monitoring.
One of our main goals is the development of wearable home diagnostics for patients with chronic heart failure. By measuring cardiac function once per day, it can be determined if heart failure is becoming worse (decompensation). When decompensation is detected, the medication of the patient can be adapted to prevent hospitalization. This feedback loop is currently possible with invasive sensors, we are investigating the potential of RF sensing as a non-invasive alternative.
This work was funded by an internal Circulatory Health Seeding Fund, NWO Open Mind grant 18802, NWO Take off grant 19609, an Utrecht Holdings Innovation Voucher, a DCVA Valorization Voucher and the Dutch heart Foundation Dekker Postdoc grant 03-006-2022-0024.