Magnetomyography

Magnetomyography (MMG) is a technique for mapping muscle activity by recording magnetic fields produced by electrical currents occurring naturally in the muscles, using arrays of SQUIDs (superconducting quantum interference devices). It has a better capability than electromyography for detecting slow or direct currents. The magnitude of the MMG signal is in the scale of pico (10−12) to femto (10−15) Tesla (T). Miniaturizing MMG offers a prospect to modernize the bulky SQUID to wearable miniaturized magnetic sensors ^[1].

History

At the early 18th century, the electric signals from living tissues have been investigated. These researchers have promoted many innovations in healthcare especially in medical diagnostic. Some example is based on electrical signals produced by human tissues, including Electrocardiogram (ECG), Electroencephalography (EEG) and Electromyogram (EMG). Besides, with the development of technologies, the biomagnetic measurement from the human body, consisting of Magnetocardiogram (MCG), Magnetoencephalography (MEG) and Magnetomyogram (MMG), provided clear evidence that the existence of the magnetic fields from ionic action currents in electrically active tissues can be utilized to record activities. For the first attempt, Cohen et al. used a point-contact superconducting quantum interference device (SQUID) magnetometer in a shielded room to measure the MCG. They reported that the sensitivity of the recorded MCG was orders of magnitude higher than the previously recorded MCG. The same researcher continued this MEG measurement by using a more sensitive SQUID magnetometer without noise averaging. He compared the EEG and alpha rhythm MEG recorded by both normal and abnormal subjects. It is shown that the MEG has produced some new and different information provided by the EEG. Because the heart can produce a relatively large magnetic field compared to the brain and other organs, the early biomagnetic field research originated from the mathematical modelling of MCG. Early experimental studies also focused on the MCG. In addition, these experimental studies suffer from unavoidable low spatial resolution and low sensitivity due to the lack of sophisticated detection methods. With advances in technology, research has expanded into brain function, and preliminary studies of evoked MEGs began in the 1980s. These studies provided some details about which neuronal populations were contributing to the magnetic signals generated from the brain. However, the signals from single neurons were too weak to be detected. A group of over 10,000 dendrites is required as a group to generate a detectable MEG signal. At the time, the abundance of physical, technical, and mathematical limitations prevented quantitative comparisons of theories and experiments involving human electrocardiograms and other biomagnetic records. Due to the lack of an accurate micro source model, it is more difficult to determine which specific physiological factors influence the strength of MEG and other biomagnetic signals and which factors dominate the achievable spatial resolution. In the past three decades, a great deal of research has been conducted to measure and analyze the magnetic field generated by the flow of ex vivo currents in isolated axons and muscle fibers. These measurements have been supported by some complex theoretical studies and the development of ultra-sensitive room temperature amplifiers and neuromagnetic current probes. Nowadays, cell-level magnetic recording technology has become a quantitative measurement technique for operating currents.

Miniaturized MMG

Very recently CMOS compatible magnetic sensors have been proposed for future of MMG in form of wearable devices.

See also

• Magnetoencephalography
• Electromyography

References