Measurement of biological electromagnetic fields using a transistor circuit placed in an MRI scanner

 

https://www.nature.com/articles/s41551-018-0309-8*

 

A tiny transistor circuit sensitive to millivolt electric fields is placed into a MRI scanner and tunes to the MRI coil.

As a result it amplifies the MRI signal in its proximity.

Upon presentation of an electric field, the transistor closes to a variable degree, is detuned from the MRI coil and the local signal enhancement is diminished.

 

Additional reading notes:
Use inside an MRI scanner a microscopic coil with a transistor, a system termed ImpACT for "implantable active coil-based transducer". Specifically, it consists of an RLC circuit connected in parallel to a field-effect transistor (FET)**.

The ImpACT coil is inductively coupled to the MRI scanner coil. As a result it harvests radiofrequency energy and creates a large drain-source voltage (VDS) at the FET. This sets the device in open mode. We say that the microscopic coil is tuned to the MRI coil. If for instance the MRI frequency is 400 MHz, the ImpACT will have the same frequency. Additionally, it amplifies the MRI signal in its proximity (ratio BImpACT/B1).

Upon the presentation of an electric field to the FET, the gate-source voltage (VGS) is altered and the FET closes to a variable degree depending on the strength of the electric field.
This results to shunting of current to or from the coil and capacitor and the microscopic coil is detuned from the MRI coil to a variable degree. This means that it no longer has the same frequency as the MRI coil. This results in a decrease in MRI signal enhancement.

 

Experiment with water:

The gyromagnetic frequency of protons (H+) is 42.5781 MHz/T. If you use a 9.4 T MRI scanner what would be the radiofrequency needed for magnetic resonance of protons (e.g. water)?
Answer: 42.5781 MHz/T * 9.4 T = 400 MHz approximately

Place a volume of water in an MRI scanner and measure the signal of the protons using 400 MHz.

 

Then place in the scanner the ImpACT. In the absence of an electric field, the FET will be closed and the microscopic coil will be fully tuned to the MRI coil. As a result, there will be an amplification of the MRI signal by the ImpACT at the volume of water that is near it. There will be a local intensification of radiofrequency flux by the ImpACT in its tuned state.

 

Then apply a photonic signal of 10^11 photons per second (p/s). The MRI signal enhancement near the ImpACT will be diminished by approximatively 6%.

 

* article rented for 48h ($10.79 - includes tax)

 

** Information on the transistor: http://www.information-book.com/science-tech-general/circuits/

 

 

 

C. J. Harland, T. D. Clark, R. J. Prance

Centre for Physical Electronics, School of Engineering and Information Technology, University of Sussex

http://www.sussex.ac.uk/ei/

 

University of Sussex link: http://sro.sussex.ac.uk/19534/

Patent (cited above): https://www.google.com/patents/WO2003048789A2?cl=en

 

Publication: https://www.researchgate.net/publication/237612253_Remote_Detection_of_Human_Electroencephalograms_Using_Ultrahigh_Input_Impedance_Electric_Potential_Sensors

 

Excerpts:

"(...) for conventional clinical analysis, the EEG is recorded using electrodes that are in real charge current contact with the scalp tissue. In practice, this is provided by an Ag/AgCl electrode used in conjunction with an electrolytic paste. These act together to form an electrical transducer to convert the ionic current flow in the skin into an electron flow which can then be detected by an electronic amplifier [3]."

 

"This sensor operates using electric displacement, rather than real charge, current. It therefore does not require real electrical contact to the signal source (e.g., the human body) in order to function. This overcomes the disadvantages of conventional Ag/AgCl electrodes while still maintaining sufficient sensitivity ~70 nV/AHz at 1 Hz noise floor) [6] to detect body electrical signals off body."

 

"Furthermore, using similar techniques, we have been able to record the human heartbeat at distance up to 1 m off body with no electrical connection between the sensor and the body [6]. In this letter, we demonstrate that we now have the sensitivity to detect EEGs through the scalp hair with no electrical contact to the scalp. We also show that it is now possible to detect brain electrical activity with an air gap between the scalp hair and the sensor."

 

"Given that there is still much scope for improvements in sensitivity, input impedance, and size of electric potential sensors, we anticipate rapid development and exploitation of these devices in the field of brain research and diagnostics in the near future."

 

Many thanks to Skizit Gesture's YouTube channel for mentioning the paper in one of her videos and to Neal Chevrier for sharing the video.

Reading notes
Introduction and experimentation

Magnetic resonance current-density imaging has been used to create images of electric current density in homogenous and heterogenous media (2,3).

 

Human brain mapping technology has made significant progress based on integrated anatomical and functional measurements. It would be ideal to directly detect neuronal electric firing. Ionic currents flow across the neuron cell membrane, the axon and the surrounding medium. These ionic currents produce a magnetic flux density Bc that if superimposed with the Bo field*, will alter the phase of the surrounding water protons.

 

*external magnetic flux (presumably from the ambient magnetic field cf. Earth magnetic field and other local events)

 

It is hypothesized that electric current-induced phase alterations could be imaged by fast magnetic resonance imaging (MRI) technology.

 

A thin copper wire (60 mm diameter) was formed into a 3-inch long and 2-inch wide rectangle (figure). A pulse electric current was applied.

A pulse generator was used to provide a 0-, 10-, 20-, 30-,50-, 70-, or 100-mA current with 2 s on and 2 s off cycle through the wire.

The induced magnetic field Bc, and specifically its Z component (parallel to Bo) was calculated.

The induced positive and negative phase changes occurred mainly in the left or right direction relative to the wire position corresponding to parallel and antiparallel components of the induced field (Bc) (figure).

 

Attached figured 2 shows the current-induced (70 mA) phase images (A) and time series of induced phase changes (B).

The induced change of magnetic field was calculated to be 1.7 +- 0.3 nT.

 

Discussion
The magnetic field due to an action potential propagating down a crayfish axon was calculated to have a maximal strength of 1 nT for the distance of 0.12 mm from the axon (15, 16). That corresponds to ionic current flow of the order of microamperes. 

The ionic current in a single neuron is of the order of picoamperes and results in a magnetic field strength of the order of 10^-15 T (16). 

The field strength from magnetocardiogram is 10^-10 T and from the brain 10^-12 T for spontaneous (a wave) activity, and 10^-13 T for evoked response (17, 18). These magnetic field changes are far beyond MRI detection capabilities (note: paper published in 1999).

The magnetic field strength produced by a single neuron is too small to be measured by MRI; however, if, for example, 10^-6 parallel-oriented neurons act together then the resulting magnetic field could be detectable.

 

The authors believe that future work will result in the direct in vivo detection of electric neuronal activity.