Yeong Yeh-Lee finds a use for the Hall Effect in Medicine

Many of us may have experienced acid reflux symptoms in our lifetime. These symptoms include heartburn, chest discomfort, nausea and a sour taste in the mouth. Also, people can develop cancers of the lower oesophagus as a result of chronic acid reflux. However, in recent decades, doctors are seeing more and more cases of cancer of the lower oesophagus, especially in Scotland, and this is thought to be related to the rising rate in obesity. How obesity affects the mechanics and integrity of the gastro-oesophageal (GE) junction (the guarding door which prevents stomach acid from reaching the lower oesophagus excessively) is unknown.

There are modern tools available to measure acid reflux and other physiological measurements, including the oesophageal wall motility and pressure of the GE junction. However, it is difficult to know where the GE junction is during acid reflux and during the rise in intra-abdominal pressure seen in obese people. Even though it is possible to deploy a metal clip at the GE junction during endoscopy and monitor its movement using fluoroscopy (a form of x-ray machine), this technique is limited by the consequences of radiation exposure. Therefore, we need another method to accurately pinpoint the movement of the GE junction reliably over long periods for research purposes.

The Hall Effect was discovered in 1879 by Edwin Hall, an American physicist who was at the time working on his doctoral thesis in Physics. In his seminal paper published in the American Journal of Mathematics in 1879, he stated: ‘If the current of electricity in a fixed conductor is itself attracted by a magnet, the current should be drawn to one side of the wire, and therefore the resistance experienced should be increased’¹. In simpler terms, a voltage is created when electrons that move within a metal conductor or semiconductor are swayed by the application of a perpendicular magnetic field (pictured above). This created voltage is very small when measured, but modern Hall Effect sensors can amplify the signal and thus allow its detection more easily.

This voltage when detected can allow us to read the position of the magnetic field and also allow us to gauge the strength of the magnetic field. We can deploy a small disc shaped magnet at the GE junction during endoscopy and a linear Hall Effect sensor probe is then passed down the oesophagus. The probe can then detect the position of the magnet and therefore the junction during acid reflux.

Some of the advantages of using Hall Effect sensors include a voltage production that is independent of the rate or velocity of the detected magnetic field. The sensors are also insensitive to ambient conditions (dust, humidity and vibration), show constant characteristics over time and need no mechanical contacts to generate output, making the sensors less predisposed to wear and tear. Finally, the available sensors in the market are small in size and relatively cheap. However, in order for the sensor to work effectively, the distance between the magnet and sensor should not be too large. In addition, the output may change with increasing temperature as a result of an increase in electrical resistance.

Now a common technology, Hall Effect sensors are slowly finding their way into the fields of medicine and dentistry. One of the innovative uses of a Hall Effect sensor is the magnetic-based navigation system for minimally invasive surgery, designed to access many regions of the body via major blood vessels. In ophthalmology, a prosthetic device using a Hall Effect sensor was used in patients suffering from a paralysed eye muscle to detect eyelid closure. In dentistry, Hall Effect sensors were used to record jaw movements for the study of biting and chewing. This technology is useful when precise and accurate measurements are needed in order to guide clinical decision making.

A quest was started 7 years ago to use the Hall Effect sensors with magnets to study the relationship between position of the GE junction and acid reflux. A novel method developed by Kenneth McColl and co-workers from the University of Glasgow is a probe that consists of two overlapping printed circuit boards populated with linear Hall Effect sensors over a range of 120 millimetres. These sensors detect a small disc shaped magnet attached to a metal clip which is deployed at the GE junction during endoscopy (pictured).

The initial prototype underwent extensive bench-top studies and was validated against fluoroscopy (the gold standard) in a group of 10 healthy volunteers. The probe can detect movement of the GE junction as reliably as fluoroscopy, if not better, and there is no risk of radiation exposure. One of the important limitations observed during bench-top studies is the effect of magnet orientation which may reduce the probe’s accuracy. There is also a minor effect on the magnetic field strength by the rise in temperature when the probe is used within the body. Furthermore when the probe is used in combination with a high resolution manometer, which measures the GE junction pressure, the accuracy and signal strength are affected as a result of metallic content within its sensor.

Nevertheless, the bench-top and validation studies proved that the team have a working probe that can assist us in unravelling the mystery behind the effect of obesity on acid reflux, a condition rising in incidence worldwide and associated with cancer of the lower oesophagus.

1. Hall, E.H. On a new action of the magnet on electric currents. 1879. American Journal of Mathematics, 2, 3, p287-92.