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 B ₁⁺ amplitude and phase is required as input. The B ₁⁺ 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.