Electric Current Density Imaging

Electric Current Density Imaging (CDI) is a new modality of magnetic resonance imaging that enables electric current distribution imaging in conductive samples containing water. CDI techniques may be further divided to: DC-CDI, a technique designed to image direct electric current density distribution, AC-CDI technique enabling imaging of alternating electric current distribution of the frequency up to a few kHz and RF-CDI that operates at the RF Larmor frequency. The principle of CDI is based on mapping magnetic field changes caused by electric currents flowing through the sample. Once maps of the magnetic field changes are obtained, the electric current density can be calculated from magnetic field change maps using Ampere's law.

Direct electric current density MR imaging sequence.

CDI sequences utilize electric currents applied in pulses that are well synchronized with the imaging sequence. Electric pulses produce temporal shifts of the precession frequency and consequently phase shifts proportional to the magnetic field change. These shifts can be added cumulatively by an appropriate arrangement of electric and RF pulses in the CDI sequence and later detected by a phase sensitive MRI.


The image of the real MR signal component (left) of the sample consisting of two concentric cylinders: inner field with saline and outer with water (right).

In one sample orientation, only the component of magnetic field change along the static magnetic field can be measured. Therefore, the sample has to be rotated in remaining perpendicular spatial orientations to map other components of magnetic field change. Once this is done the electric current density can be calculated as the curl of magnetic field maps.



Conventional magnitude MR image (left) and calculated electric current density image of the same sample (right). The electric current of 20 mA was flowing through the inner cylinder of the sample.

References:
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