Dr Nigel Whittle, head of medical & healthcare at Plextek explores the potential use of microwaves for medicine.
Medical imaging is one of the most important technologies available to doctors and other medical workers, providing critical information for diagnosis and treatment. There is however no single technology for imaging of internal structures that is universally applicable to all tissues, has high resolution, is inexpensive, doesn’t use ionizing radiation and creates images in real-time. An ideal system would also be portable and low-cost, with the potential for use in ambulances and other out-of-hospital environments.
The microwave frequency band (300 MHz – 30 GHz) possesses useful characteristics, including the use of non-ionizing radiation which is harmless at moderate power levels but penetrates biological tissue reasonably well. Compared with more conventional medical imaging systems such as MRI or X-rays, microwave imaging systems produce a lower spatial resolution but a high temporal resolution - the ability to resolve fast-paced events.
Microwave medical imaging has been used for breast cancer screening. In this form it relies on differences in dielectric properties between the constituent tissues of the healthy and cancerous tissues in shallow parts of the body. It has also been investigated for stroke detection, bladder volume control, lung oedema, bone analysis and other possibilities.
A limitation of this approach is that human beings are essentially soft tubes of salty water and therefore quite conductive. This significantly reduces the penetration depth possible with radar, such that deeper screening within the body is not really viable. Lower frequencies (4 - 8 GHz) allow greater penetration but this reduces the spatial resolution of the output image. Even with this limitation it is still possible to perform clinically useful measurements.
Vital Signs
Microwave imaging systems are effectively sophisticated radars, but radars can also be used for other medical applications. An example of this is the ability to measure chest motion without contact. By measuring small changes in distance and looking for periodic oscillations at specific frequencies it is then possible to estimate various physiological parameters. This includes the measurement of both heart rate (0.8 - 2 Hz) and respiration rate (0.1 - 0.5 Hz) simultaneously. Measuring higher frequency oscillations even allows shivering to be measured as an additional diagnostic capability. The general ability to measure motion can also be used to measure activity in bed and produce a sleep tracking sensor.
Fall detection
In more general healthcare situations radar can also be used to detect when people have fallen. This is achieved via measurement of Doppler velocity and the change in position of people over time. This is of particular benefit in elderly assisted living environments where an automatic system that does not intrude on privacy is required. A ceiling mounting radar is ideal for this in contrast to a wearable device that requires manual activation or a camera that removes privacy.
As the medical world turns its attention to long-term, mobile, and even home-based imaging and monitoring for preventive screening and early detection of diseases, radar-based biosensing and imaging applications are likely to play an increasingly important role.
Their advantages include potentially low cost and small size of the required hardware, plus a wide application across fields as diverse as heart rate tracking, sleep monitoring and fall detection for the elderly, right across to screening of the cardiovascular system, breast cancer imaging and stroke detection.
