Monitoring foetal health using optically pumped magnetometers
Supervisors: Matt Brookes, Richard Bowtell, Penny Gowland, Nia Jones
Research proposal
Measurements of the magnetic fields generated outside the human body can be used to non-invasively assess electrophysiological activity in the adult brain (magnetoencephalography - MEG), heart (magnetocardiography-MCG) and muscles (magnetomyography-MMG). Measurements of the magnetic fields generated outside the abdomen in pregnancy can potentially be used to monitor the health and wellbeing of the fetus, through detection of the fields generated by electrical activity in the fetal heart and brain, along with those produced by the muscle of the uterine wall. These measurements are difficult because extra-corporeal magnetic fields are very weak (~1 pT for foetal heart and ~10 fT for foetal brain) and dominated by the signals from the maternal heart (~50 pT). In addition, the magnitude of these fields depends on the position of the fetus in utero and will be strongly modulated by fetal movement. However, fetal magnetocardiography offers significant advantages over fetal electrocardiography (in which the voltages are monitored using electrodes attached to the mother’s abdomen), because it is much easier to separate the magnetic fields from the fetal heart from other sources of interference, than is the case when measuring electrical potentials. A small number of systems for measuring the magnetic fields generated by the fetus have been developed. These use cryogenically-cooled SQUID sensors for measuring the weak magnetic fields, which means that it is difficult to bring sensors close to the womb, and the fields can only be mapped on one surface of the body. This reduces the sensitivity and accuracy of measurements, and along with the high purchase and running costs of such cryogenic systems, has limited the take-up of fetal MCG/MEG. The recent development of small optically pumped magnetometers (OPMs) which have a similar sensitivity to SQUIDs, but which can be flexibly placed to measure the magnetic fields within a few mm of the body surface, opens up the opportunity to develop a powerful (and affordable) obstetric scanning modality for assessing fetal health and which can also be used in the investigation of fetal brain development.
This PhD project will focus on designing, constructing and evaluating an OPM-based system for obstetric scanning. Initial effort will focus on simulation of the extracorporeal magnetic fields produced by current flow in the fetal brain and heart, and in the uterine wall, and these simulations will be used to evaluate and optimise the locations of the sensors and to investigate the efficacy of methods for source localisation. Subsequent work will involve devising, constructing and testing an experimental set-up for holding the sensors in position around the abdomen, while comfortably positioning the mother inside our magnetically shielded room. Experimental work will first focus on fetal magnetocardiography and will involve implementing signal separation methods for picking out the fetal MCG from other sources of interference. Extension to fetal MEG measurements, will require the development of appropriate stimulation methods building on our previous work in fetal fMRI. The proposed project links to a wider effort on electrophysiological measurements using OPMs for MEG in adults and infants.