Christian Findeklee

Determination of Electric Conductivity and Local SAR Via B1 Mapping

The electric conductivity can potentially be used as an additional diagnostic parameter, e.g., in tumor diagnosis. Moreover, the electric conductivity, in connection with the electric field, can be used to estimate the local SAR distribution during MR measurements. In this study, a new approach, called electric properties tomography (EPT) is presented. It derives the patients electric conductivity, along with the corresponding electric fields, from the spatial sensitivity distributions of the applied RF coils, which are measured via MRI. Corresponding numerical simulations and initial experiments on a standard clinical MRI system underline the principal feasibility of EPT to determine the electric conductivity and the local SAR. In contrast to previous methods to measure the patients electric properties, EPT does not apply externally mounted electrodes, currents, or RF probes, thus enhancing the practicality of the approach. Furthermore, in contrast to previous methods, EPT circumvents the solution of an inverse problem, which might lead to significantly higher spatial image resolution.

Eight-channel transmit/receive body MRI coil at 3T.

Multichannel transmit magnetic resonance imaging (MR) systems have the potential to compensate for signal‐intensity variations occurring at higher field strengths due to wave propagation effects in tissue. Methods such as RF shimming and local excitation in combination with parallel transmission can be applied to compensate for these effects. Moreover, parallel transmission can be applied to ease the excitation of arbitrarily shaped magnetization patterns. The implementation of these methods adds new requirements in terms of MRI hardware. This article describes the design of a decoupled eight‐element transmit/receive body coil for 3T. The setup of the coil is explained, starting with standard single‐channel resonators. Special focus is placed on the decoupling of the elements to obtain independent RF resonators. After a brief discussion of the underlying theory, the properties and limitations of the coil are outlined. Finally, the functionality and capabilities of the coil are demonstrated using RF measurements as well as MRI sequences. Magn Reson Med 58:381–389, 2007.

B1-based specific energy absorption rate determination for nonquadrature radiofrequency excitation

The current gold standard to estimate local and global specific energy absorption rate for MRI involves numerically modeling the patient and the transmit radiofrequency coil. Recently, a patient‐individual method was presented, which estimated specific energy absorption rate from individually measured B1 maps. This method, however, was restricted to quadrature volume coils due to difficulties distinguishing phase contributions from radiofrequency transmission and reception. In this study, a method separating these two phase contributions by comparing the electric conductivity reconstructed from different transmit channels of a parallel radiofrequency transmission system is presented. This enables specific energy absorption rate estimation not only for quadrature excitation but also for the nonquadrature excitation of the single elements of the transmit array. Though the contributions of the different phases are known, unknown magnetic field components and tissue boundary artifacts limit the technique. Nevertheless, the high agreement between simulated and experimental results found in this study is promising. B1‐based specific energy absorption rate determination might become possible for arbitrary radiofrequency excitation on a patient‐individual basis. Magn Reson Med, 2012.

Specific absorption rate reduction in parallel transmission by k-space adaptive radiofrequency pulse design

The specific absorption rate (SAR) is an important safety criterion, limiting many MR protocols with respect to the achievable contrast and scan duration. Parallel transmission enables control of the radiofrequency field in space and time and hence allows for SAR management. However, a trade‐off exists between radiofrequency pulse performance and SAR reduction. To overcome this problem, in this work, parallel transmit radiofrequency pulses are adapted to the position in sampling k‐space. In the central k‐space, highly homogeneous but SAR‐intensive radiofrequency shim settings are used to achieve optimal performance and contrast. In the outer k‐space, the homogeneity requirement is relaxed to reduce the average SAR of the scan. The approach was experimentally verified on phantoms and volunteers using field echo and spin echo sequences. A reduction of the SAR by 25–50% was achieved without compromising image quality. Magn Reson Med, 2011.

In vivo determination of electric conductivity and permittivity using a standard MR system

The electric properties of human tissue, i.e., the electric conductivity and permittivity, can be used as an additional diagnostic parameter or might be helpful for the prediction of the local SAR during MR measurements. In this study, a new approach “Electric Properties Tomography” (MR-EPT) is presented, which derives the patient’s electric properties using a standard MR system. To this goal, the spatial transmit and receive sensitivity distributions of the applied RF coil are measured. According to the first Maxwell equation, dividing the spatial derivatives of these sensitivities by the corresponding electric field leads to the desired spatial distribution of conductivity and permittivity. The electric field can be estimated using the geometry of the involved RF coil and the patient’s geometry, known from the acquired MR images. Thus, MR-EPT does not apply externally mounted electrodes, currents, or RF probes. The spatial resolution of the reconstructed conductivity and permittivity is of the order of the spatial resolution of the measured MR images. In this study, phantom experiments underline the principle feasibility of MR-EPT.

Electrical conductivity imaging using magnetic resonance tomography

The electrical conductivity of human tissue could be used as an additional diagnostic parameter or might be helpful for the prediction of the local SAR during MR measurements. In this study, the approach “Electric Properties Tomography” (EPT) is applied, which derives the patients electric conductivity using a standard MR system. To this goal, the spatial transmit sensitivity distribution of the applied RF coil is measured. This sensitivity distribution represents the positive circularly polarized component of the magnetic field. It can be post-processed utilizing Faradays and Amperes law, yielding an estimation of the spatial distribution of the patients electric conductivity. Thus, EPT does not apply externally mounted electrodes, currents, or RF probes. In this study, phantom experiments underline the principle feasibility of EPT. Furthermore, initial conductivity measurements in the brain allow distinguishing cerebro-spinal fluid from the surrounding grey and white matter.

Array Noise Matching—Generalization, Proof and Analogy to Power Matching

In array antennas with residual coupling, the signal-to-noise ratio (SNR) can suffer from noise coupling from the inputs of the low noise amplifiers (LNAs) to the other channels. As shown recently, the lost SNR can be retrieved by changing the individual matching circuits. This paper explains the theory behind array noise matching and generalizes it for non-equal LNAs and non-reciprocal sources. It also shows an analogy between noise matching of an array and power matching into the complex conjugate of the optimum noise impedances of the individual LNAs. This turns out to be useful for practical noise matching of mutually coupled arrays. In some cases it becomes impossible to reach the theoretical optimum matching with passive matching networks. Therefore an additional boundary condition will be introduced and investigated.

Dictionary-based electric properties tomography

To develop and validate a new algorithm called “dictionary‐based electric properties tomography” (dbEPT) for deriving tissue electric properties from measured B1 maps.

Imaging Electric Conductivity with MRI

The electric conductivity of human tissue could be used as an additional diagnostic parameter or might be helpful for the prediction of the local SAR during MR measurements. In this study, the approach “Electric Properties Tomography” (MR-EPT) is applied, which derives the patient’s electric conductivity using a standard MR system. To this goal, the spatial transmit sensitivity distribution of the applied RF coil is measured. This sensitivity distribution represents the positive circularly polarized component of the magnetic field. It can be post-processed utilizing Faraday’s and Ampere’s law, yielding an estimation of the spatial distribution of the patient’s electric conductivity. Thus, MR-EPT does not apply externally mounted electrodes, currents, or RF probes. In this study, phantom experiments underline the principle feasibility of MR-EPT. Furthermore, initial conductivity measurements in the brain allow distinguishing cerebrospinal fluid from the surrounding grey and white matter.