In this study, a new approach to measure local electrical conductivity in tissue is presented, which is based on the propagating B In recent years, the used frequency of radio frequency (RF) pulses in MRI has increased considerably as a consequence of the increase in magnetic field strengths. As an increased frequency leads to a shorter wavelength, the electromagnetic fields (EM-fields) induced by the RF pulse should more and more be regarded as waves (1,2) impinging onto a very dielectrically heterogeneous object: the human body. As a wave propagates in the human body, it is being affected by the local dielectric properties which are tissue dependent. The interaction between the applied EM-fields and the body leads to a variation of the EM-fields in amplitude (3,4) as well as in phase (5); this is the origin of B 1 field inhomogeneities. The variations in the B + 1 field can be separated in two categories: body-scale effects and local effects. A recent simulation study has shown that the B + 1 field variations depend predominantly on the global dielectric properties and the contour of the body (6). Small deviations were found; these were attributed to local variations in dielectric properties. Furthermore, it was found that these local B + 1 variations can amount up to 10-20% in magnitude of the average B + 1 field at 3 T (7). These findings indicate that the local B + 1 field variations contain information about the local dielectric environment; in this study we aim to exploit this information. As the B

Electric properties tomography (EPT) is an imaging modality to reconstruct the electric conductivity and permittivity inside the human body based on B1(+) maps acquired by a magnetic resonance imaging (MRI) system. Current implementations of EPT are based on the local Maxwell equations and assume piecewise constant media. The accuracy of the reconstructed maps may therefore be sensitive to noise and reconstruction errors occur near tissue boundaries. In this paper, we introduce a multiplicative regularized CSI-EPT method (contrast source inversion-electric properties tomography) where the electric tissue properties are retrieved in an iterative fashion based on a contrast source inversion approach. The method takes the integral representations for the electromagnetic field as a starting point and the tissue parameters are obtained by iteratively minimizing an objective function which measures the discrepancy between measured and modeled data and the discrepancy in satisfying a consistency equation known as the object equation. Furthermore, the objective function consists of a multiplicative Total Variation factor for noise suppression during the reconstruction process. Finally, the presented implementation is able to simultaneously include more than one B1(+) data set acquired by complementary RF excitation settings. We have performed in vivo simulations using a female pelvis model to compute the B1(+) fields. Three different RF excitation settings were used to acquire complementary B1(+) fields for an improved overall reconstruction. Numerical results illustrate the improved reconstruction near tissue boundaries and the ability of CSI-EPT to reconstruct small tissue structures.

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.

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