Synopsis

Description of Practical Strategies for Planning, Acquiring and Reconstructing Tissue Electrical Properties Using MRI

Target Audience

The target audience of this presentation is those who are interested in testing methods for Acquiring and Reconstructing Electrical Property Images. It would be appropriate for graduate students beginning in this area.

Objectives

The objective of the presentation is to prepare attendees to make practical measurements and evaluate and process data. These objectives include:

• Preparing Appropriate Phantoms
• Programming current sources/scanner interaction.
• Experimental considerations for Magnetic Resonance Electrical Impedance Tomography (MREIT)
• Pulse sequences used for measuring Electrical Properties
• Exporting and validating acquired data.
• How to correct data before processing
• Construction of Computational Models
• How to validate Conductivity reconstruction methods.

Purpose

The electrical properties of the human body are closely linked to many important determinants of physiological state, including ionic concentrations, ionic mobility, cell density, cell type, and pathological condition. Electrical properties are also sensitive to temperature. Amongst other things, electrical property mapping methods are increasingly being used in neuroimaging to non-invasively determine conductivities at frequencies similar to those experienced by endogenous brain activity. These conductivity maps have application in numerous areas, including to the improvement of EEG-based source localization methods. Electrical property mapping methods that involve current to be administered to the body, such as MREIT, can also be used to perform current density imaging, which are of interest in determining the fields caused by neuromodulation methods such as transcranial DC and AC stimulation and deep brain stimulation. High frequency (Larmor frequency) images of conductivity and permittivity provide useful indications of ionic concentrations in tumors.

Topics Covered

• Phantom preparation: Phantoms must be electrically conductive, and contain conductivity contrasts. This is difficult to achieve because conductivity contrasts caused by varying ionic concentrations will quickly reduce. Methods for construction of more stable conductivity phantoms will be described. Anisotropic phantoms are also challenging to produce. Some methods for construction of their constructions will be given.

• Programming current source/scanner interaction: In methods of mapping electrical properties that involve current administration, current sources must be safe for MR use and positioned correctly inside the scanner. Methods for ensuring this will be described. These current sources must also be triggered by the scanner. Tips for programming trigger pulses, locating spectrometer outputs and for modifying current sources to accept trigger pulses will be given.

• Experimental considerations for methods involving current administration: Safety must always be considered when combining current administration and imaging equipment. Current amplitudes and densities and charge loads considered safe for human and animal imaging will be outlined. Electrode types will be discussed. Waveforms used in MREIT will be demonstrated.
• Pulse Sequences used for Electrical Properties Imaging: Pulse sequences must be chosen to maximize magnitude SNR and phase quality but minimize time. Examples of pulse sequences used for MREIT will be given showing current waveforms and example images. Examples of other pulse sequences and strategies used for measuring high-frequency electrical properties will be given. Parallel imaging methods will be discussed and demonstrated.
• Phase data export and quality checking: For MREIT, methods for converting phase to current-created B_z will be shown. Calculation of standard deviation in B_z and validation of data using complementary methods will be demonstrated. Denoising methods (e.g. removal of salt and pepper noise, filling in phase data obscured by air pockets) will also be discussed, as well as how to correct for stray fields caused by current source wires.
• Computational Models Construction: Reconstruction of electrical properties typically involves reference calculations, performed using a computational models based on MR magnitude images. Examples of pipelines used to construct and simulate data using these models will be shown.
• Conductivity Reconstruction Methods: There are a large number of methods that can be used to reconstruct electrical property data. Examples of practical methods for implementing them will be shown.

Acknowledgements

References

1. Seo JK, Woo, EJ, Katscher U, Wang Y. Electro-magnetic tissue properties MRI: Imperial College Press; 2014.
2. Holder DS, editor. Electrical Impedance Tomography. Methods, history and applications. 1st ed. Bristol and Philadelphia: Institute of Physics Publishing; 2005.
3. Sadleir, RJ, Minhas, A. Quantitative Magnetic Resonance Mapping of Tissue Electrical Properties: A Handbook for Researchers, in preparation Springer; 2020.