Quantitative conductivity and permittivity imaging of the human brain using electric properties tomography

Abstract

The electric properties of human tissue can potentially be used as an additional diagnostic parameter, e.g., in tumor diagnosis. In the framework of radiofrequency safety, the electric conductivity of tissue is needed to correctly estimate the local specific absorption rate distribution during MR measurements. In this study, a recently developed approach, called electric properties tomography (EPT) is adapted for and applied to in vivo imaging. It derives the patient's electric conductivity and permittivity from the spatial sensitivity distributions of the applied radiofrequency coils. In contrast to other methods to measure the patient's electric properties, EPT does not apply externally mounted electrodes, currents, or radiofrequency probes, which enhances the practicability of the approach. This work shows that conductivity distributions can be reconstructed from phase images and permittivity distributions can be reconstructed from magnitude images of the radiofrequency transmit field. Corresponding numerical simulations using finite-difference time-domain methods support the feasibility of this phase-based conductivity imaging and magnitude-based permittivity imaging. Using this approximation, three-dimensional in vivo conductivity and permittivity maps of the human brain are obtained in 5 and 13 min, respectively, which can be considered a step toward clinical feasibility for EPT. Magn Reson Med 66:456-466, 2011. V C 2011 Wiley-Liss, Inc.Key words: permittivity; conductivity; electric properties tomography; quantitative MRI; patient-specific SAR MR provides a vast variety of possible image contrasts. Because of reasons of reproducibility and comparability, contrasts comprising quantitative parameters are of particular clinical interest. Current examples of quantitative MRI techniques are diffusion, perfusion, and permeability imaging; however, electric conductivity and permittivity are also possible candidates for quantitative parameters. The idea of extracting these electric properties from MR images was already proposed in 1991 (1). However, only recently, the electric properties of the human body have been introduced as a quantitative image contrast in standard MRI via electric properties tomography (EPT) (2). EPT allows the determination of the conductivity and permittivity using the radiofrequency (RF) transmit field map of a standard MR scan (3).The task of imaging electric properties has been addressed by a variety of imaging modalities. Among these modalities, electric impedance tomography is probably the most prominent one. It is performed by applying low frequency currents through multiple electrodes and reconstructing electric properties by solving the resulting inverse problem (4-7). Magnetic induction tomography is a similar approach, however, using RF coils for current induction and reception of the resulting fields (8,9). MR electric impedance tomography is a method initially based on electric impedance tomography, including electrode mounting, however, taking advantage of the spatial encodin...

Non-Invasive in vivo Loss Tangent Imaging: Thermal Sensitivity Estimation at the Larmor Frequency

“…The MREPT process is sensitive to noise due to the Laplacian operator. Recently, various post-processing methods, such as weighted polynomial fitting, median fitting, and bilateral filtering, have been developed to obtain more accurate electrical property maps (12,25). Smoothing filters such as the Gaussian filter have also frequently been used to reduce the noise from acquired data.…”

“…Recently, a novel non-invasive technique called MREPT (Magnetic resonance electric properties tomography) was introduced for the measurement of electrical properties (12)(13)(14). MREPT yields quantitative values for conductivity and permittivity, determined at the MR Larmor frequency.…”

“…In reality, conductivity can be calculated to a reasonable accuracy using only the phase of B 1 + , while permittivity can be determined from the magnitude of B 1 + alone (12). Therefore, both the magnitude and phase of B 1 + should be acquired.…”

A Boundary-Value-Free Reconstruction Method for Magnetic Resonance Electrical Properties Tomography Based on the Neumann-Type Integral Formula over a Circular Region

“…A primitive approach to solve Eq. (3) is to reduce it into an algebraic equation by neglecting the spatial gradient of κ. Haacke et al [12] and later Wen [13] introduced a formula κ = −(ΔH + )/(ω 2 μ 0 H + ) by assuming that ∇κ = 0; stabilized versions of this approach were also proposed [14], [15]. However, the assumption that ∇κ 0 results in severe artifacts in the tissue transition regions where EPs vary [16], [17].…”

Magnetic Resonance-Based Electrical Property Tomography (MR-EPT) for Prostate Cancer Grade Imaging

“…This program builds off of our extensive experience in using electrical properties of prostate to distinguish malignant from benign tissues [1][2][3][4][5] and specifically stems from exciting new data published in The Prostate [6] in which we demonstrated significant electrical property differences between high-and low-grade prostate cancer. These electrical properties are influenced by a tissue's intra-and extra-cellular composition, morphology, and cellular constituency, and we have hypothesize that it is possible to use these properties to discriminate between normal, low-grade, and high-grade malignant formations in a clinical setting.…”