Electrical properties tomography in the human brain at 1.5, 3, and 7T: a comparison study

Purpose: To investigate the effect of magnetic field strength on the validity of two assumptions (namely, the "transceive phase assumption" and the "phase-only reconstruction") for electrical properties tomography (EPT) at 1.5, 3, and 7T.

Theory: Electrical properties tomography is a method to map the conductivity and permittivity using MRI; the B1 (+) amplitude and phase is required as input. The B1 (+) phase, however, cannot be measured and is therefore deduced from the measurable transceive phase using the transceive phase assumption. Also, earlier studies showed that the B1 (+) amplitude is not always required for a reliable conductivity reconstruction; this is the so-called "phase-only conductivity reconstruction."

Methods: Electromagnetic simulations and MRI measurements of phantoms and the human head.

Results: Reconstructed conductivity and permittivity maps based on B1 (+) distributions at 1.5, 3, and 7T were compared to the expected dielectric properties. The noise level of measurements was also determined.

Conclusion: The transceive phase assumption is most accurate for low-field strengths and low permittivity and in symmetric objects. The phase-only conductivity reconstruction is only applicable at 1.5 and 3T for the investigated geometries. The measurement precision was found to benefit from a higher field strength, which is related to increased signal-to-noise ratio (SNR) and increased curvature of the B1 (+) field.