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


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:



Detecting and measuring the electrical activity of the brain with Electron Spin Resonance (ESR)

> Use of electromagnetic radiation wavelengths that make the electrons resonate.
> The intensity of the signal is modulated.
> The electrical discharges of the brain cells influence the magnetic field and the spins of the electrons.
> Broadening of resonance lines.
> By measuring the broadening width of the spectrum lines the intensity of neural activation is measured.
Abstract: “Electrical activity of the brain can be detected and measured by measuring the broadening of the width of NMR spectral lines. A discharging neuron in the brain introduces an inhomogeneity in the magnetic field which reveals itself as a broadening of resonance lines which can be measured to determine the neuron discharge current flux.”
“Recently, Holder proposed to use ESR at about 1 GHZ to obtain images of the human head, using a technique derived from NMR imaging; referring to Holder, The Potential Use of ESR or Impedance Measurement to Image Neuronal Activity in the Human Brain” []
“from the viewpoint of neurology, even more useful information can be obtained from the reconstructed images of the electrical firing of nerve cells in the human brain, since this would allow analysis of functional activity of neuroatomical pathways which cannot be achieved with the current techniques. Holder investigated the possibility that neuronal firing can be detected by a form of electromagnetic radiation which can then be reconstructed to form three-dimensional images of this functional activity. Holder teaches the use of ESR and impedance imaging as giving the best results. Holder states that NMR, being well established for spectroscopy and imaging, could be employed to detect neuronal firing, but that current flux from ions moving across the neuronal membrane would be too small to be detectable by NMR.”
“In one embodiment of the present invention, an imaging technique termed Nuclear Electron Double Resonance (NEDOR) starts with a conventional NMR imaging apparatus and includes a high frequency resonator or radiator such as is used in ESR techniques. A sample, such as a human body, is placed inside the main magnetic field coils of the NMR imager. A radio frequency (RF) pulse is applied via RF coils to excite to resonance a plurality of nuclear spins producing an NMR signal. A microwave (MW) pulse is applied via a high frequency resonator or radiator to excite to resonance a plurality of electron spins within the sample. The RF and MW pulses simultaneously resonate the nuclear and electron spins within the sample. The intensity of the MW signal is modulated, which translates into a modulation of the intensity of the NMR signal. The modulation is extracted from the NMR signal to produce an image representative of the local ESR and thereby the concentration of paramagnetic species in the sample. In a preferred embodiment, the RF coil is also used as the MW resonator.”
“In a further embodiment, a plurality of NMR spectral lines having a defined width are derived from the modulated NMR signal. It has been determined by the applicant, that the width of the spectral lines is broadened due to the electric activity in the brain. The discharge of a neuron in the brain introduces an inhomogeneity into the main magnetic field. The contribution of the spectral line width due to the electric activity is measured, which can provide a measurement of the intensity of neuron discharges".

"The Potential Use of Electron Spin Resonance Impedance Measurement to Image Neuronal Electrical Activity in the Human Brain"

Holder, DS; (1985)



Cited at above U.S. Patent 4719425 A ( which refers to the detection and measurement of the electrical activity of the brain with Electron Spin Resonance (ESR) 


"Feasibility of developing a method of imaging neuronal activity in the human brain: a theoretical review"

D. S. Holder


"It is proposed that, for the rapid changes related to the action potential, electron spin resonance using a potential-sensitive spin, label, impedance imaging and NMR are suitable in principle but that only ESR and impedance methods may have sufficient sensitivity and these merit further assessment."








Remote detection of human electroencephalograms using ultrahigh input impedance electric potential sensors


C. J. Harland, T. D. Clark, R. J. Prance
Centre for Physical Electronics, School of Engineering and Information Technology, University of Sussex
University of Sussex link:
Patent (cited above):




"(...) 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.




Remote control brain sensor - BBC News (2002) featuring above study


Excerpt: "From a distance" | "Instead of measuring electric current flow through a fixed-on electrode, the new method takes advantage of the latest developments in sensor technology to measure electric fields from the brain without actually having to make direct contact with the head."

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Detecting electric currents with magnetic resonance
Application: Detection of brain electric currents (of neuronal activity)


Current-induced magnetic resonance phase imaging


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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.


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.




Human in-vivo brain magnetic resonance current density imaging (MRCDI).


"Magnetic resonance current density imaging (MRCDI) and MR electrical impedance tomography (MREIT) are two emerging modalities, which combine weak time-varying currents injected via surface electrodes with magnetic resonance imaging (MRI) to acquire information about the currentflow and ohmic conductivity distribution at high spatial resolution".


"The injected current flow creates a magnetic field in the head, and the component of the induced magnetic field ΔBz,c parallel to the main scanner field causes small shifts in the precession frequency of the magnetization. The measured MRI signal is modulated by these shifts, allowing to determine ΔBz,c for the reconstruction of the current flow and ohmic conductivity".



Current Density component measurement for Humans

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Reconstruction of two-dimensional current density from magnetic image obtained by EPR (Electron Paramagnetic Resonance)


"Nanoscale Imaging of Current Density with a Single-Spin Magnetometer"

Do an Electron Paramagnetic Resonance (EPR) scan using microwaves from 2700 to 2800 MHz.
Based on fluorescence intensity determine the frequency of resonance, the ω for resonance (cf. attached image).

If you know the ω and the gyromagnetic ratio of the electron, you determine the B, the intensity of the magnetic field at a certain point.

You then create a 2D Fourier space and you determine the current density of a structure.


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Comments on the techniques


Electronic magnetic resonance techniques (EMRT)


Electronic magnetic resonance techniques (EMRT) are used in material research and device characterization e.g. diodes, solar cells. The principle of these techniques is the detection of changes in a specific observable e.g. conductivity when the condition for electron spin resonance is reached. The most common EMRT are:
a. Electron spin resonance (ESR)
b. Electrically detected magnetic resonance (EDMR)
c. Optically detected magnetic resonance (OMDR).


Consider a sample or device to be measured i.e. a device on DC current, represented by an RC circuit.



In an EDMR experiment, a magnetic field is modulated in order to create a small alternative current (AC) in the measuring circuit at the resonancecondtion. Note that this is much smaller than the sample DC current.


The magnetic field is swept linearly around a central magnetic field H0 close to the resonant condition. A modulation field with angular frequency ω=2πf, and amplitude Hm is used for phase sensitive detection (PSD).