· · · Repository hosted by TU Delft Library

Noninvasive MR-based Electric Properties Tomography (EPT) forms a framework for an accurate determination of local SAR, and may providea diagnostic parameter in oncology. 3D SSFP sequences were found tobe a promising candidate for fast volumetric conductivity imaging. In this work, an in vivo study has been performed to assess the reproducibility of phase-based conductivity imaging with 3D SSFP.

MR-based Electric Properties Tomography (EPT) provides a noninvasivemeans to assess electric tissue properties, such as conductivity and permittivity, and provides a framework for an accurate determination of local SAR. Furthermore, it may provide a diagnostic parameterin oncology and cardiology. Recently, simplified EPT reconstructionmethods based on the pure image phase information were introduced.In these studies, spin echo (SE) sequences were employed due to their low susceptibility to B0 variations, or fast field echo (FFE) sequences in concert with B0 mapping were performed. However, these scans are prohibitively long for applications which require breath-holding. In the present study, we have employed a fast balanced SSFP sequence, which has similar properties as SE in terms of B0-independendphase accuracy, but provides increased imaging speed and allows forabdominal imaging in a single breathhold. Phantom experiments and first in vivo conductivity scans in the liver of healthy adults are shown.

According to ex vivo studies, breast tumors exhibit a significantlyaltered electric conductivity. This feature opens the chance to increase the specificity of breast tumor characterization with MRI. Theelectric conductivity can be measured in vivo using “Electric Properties Tomography (EPT). EPT has shown its potential in phantom, volunteer, and initial clinical studies. However, the complex frayed structure of fat and ductile tissue in the breast hampers the straight-forward application of EPT, based on the second derivative of the RFTX phase. In this study, a new EPT reconstruction algorithm, based on fitting local parabolic functions on the TX phase, is developed and applied to example breast tumors.

A central issue of parallel RF transmission is the SAR management toensure patient safety. The additional degrees of freedom availablein parallel transmission hamper straight-forward SAR estimations asapplied for single channel transmission. As an alternative to the usually applied model-based SAR estimation, a new method has been proposed to estimate SAR from the acquired B1 maps. This B1-based SAR determination has been successfully tested for quadrature (single channel) excitation in vivo and non-quadrature (multi-channel) excitation in a phantom study. This study adapts B1-based SAR determination for non-quadrature excitation in vivo. To this goal, the local SAR inthighs and pelvis of a volunteer is investigated and compared withresults of corresponding FDTD simulations based on the same volunteer.

Transmit SENSE is typically applied to improve spatially selective RF pulses in two or three dimensions. This study investigates the application of Transmit SENSE to one-dimensional RF pulses. For these RF pulses, Transmit SENSE is applicable in case of large B1 variations across the slice or slab to be excited. Typically, such large B1 variations are found across the slabs excited in 3D volume imaging orin the framework of the REgional Saturation Technique (REST). 1D Transmit SENSE can improve the excitation slab profile, and particularly can result in a significant reduction of RF power and the relatedspecific absorption rate (SAR). Since the involved RF pulses have the same duration as standard slice-selection pulses, they can easilybe incorporated in standard sequences without changing sequence timing. The approach was tested using synthetic and realistic coil sensitivity profiles.

In the framework of “Electric Properties Tomography” (EPT), approximate conductivity imaging is possible by analyzing the B1 phase, assuming constant B1 amplitude. The more this assumption is violated, the less accurate the reconstructed conductivity. This study optimizesthe B1 amplitude via parallel RF transmission with respect to the error expected for phased-based conductivity imaging. It turns out that an optimal B1 amplitude homogeneity, as obtained with RF shimming, is also beneficial for phase-based EPT.

Recently, it was shown that human dielectric properties can be mapped using MRI. This method, named electrical properties mapping (EPT),relies on measurements of the B1+ amplitude and its phase. This phase, however, cannot be measured directly; therefore, the assumptionthat the transceive phase is twice the B1+ phase is used. This assumption is acceptable at low field strengths, however, leads to significant reconstruction errors with increasing field strength. Here, opposing errors were observed for the permittivity and conductivity mapping, which can be explained by the effect of the transceive phaseerror on the error in the reconstructed conductivity and permittivity. This abstract investigates if the wave-number, combining the conductivity and permittivity, benefits from these opposing errors, andcan be determined more accurately at 7T than the separate dielectricproperties.

To measure the complex permittivity or only the conductivity non-invasively, several methods were proposed. In MR current density imaging (MR-CDI), an external current source is connected to the skin to inject current into the tissue. Using some post-processing steps, theconductivity can be derived from the obtained current density map,this is called MR electrical impedance mapping (MR-EIT). MR-CDI andMR-EIT is generally performed using a DC current for a few ms, i.e.,the obtained complex permittivity belongs to the “low” frequency range (below ~1kHz). Recently, a new method was developed, for which no external source was needed anymore. Using this method, the complexpermittivity is measured at the Larmor frequency. Both MR-CDI and MR-EIT require an external current source, which is not present on astandard MR machine. This current source should be connected to theskin to inject a current. A major concern of these methods is the high current needed to obtain sufficient SNR, and the consequential pain induced in the subject. For MR-EPT (in the following called “RF-EPT”), no additional equipment is needed besides an MR scanner, and no current is injected. However, the applied frequency is limited tothe Larmor frequency of the MR scanner. The newly proposed method, called “gradient-EPT”, uses the EM fields induced by the gradients applied for MR spatial encoding. This combines the advantages of bothMR-EIT/MR-CDI (frequency of particular biologic interest) and RF-EPT(no current injection). This study is based on the insight that thedistribution of eddy currents induced in the body by gradient switching depends on the local dielectric properties of tissue. During the ramping of the gradients, currents will be induced through the process of induction. The current will be affected by the local conductivity of the tissue. This induced current will locally alter the main magnetic field, to be measured using phase imaging.