The idea of using parallel imaging to shorten the acquisition time by the simultaneous use of multiple receive coils can be adapted for the parallel transmission of a spatially-selective multidimensional RF pulse. As in data acquisition, a multidimensional RF pulse follows a certain k-space trajectory. Shortening this trajectory shortens the pulse duration. The use of multiple transmit coils, each with its own time-dependent waveform and spatial sensitivity, can compensate for the missing parts of the excitation k-space. This results in a maintained spatial definition of the pulse profile, while its duration is reduced. This work introduces the concept of parallel transmission with arbitrarily shaped transmit coils (termed "Transmit SENSE"). Results of numerical studies demonstrate the theoretical feasibility of the approach. The experimental proof of principle is provided on a commercial MR scanner. The lack of multiple independent transmit channels was addressed by combining the excitation patterns from two separate subexperiments with different transmit setups. Shortening multidimensional RF pulses could be an interesting means of making 3D RF pulses feasible even for fast T* 2 relaxing species or strong main field inhomogeneities. Other applications might benefit from the ability of Transmit SENSE to improve the spatial resolution of the pulse profile while maintaining the transmit duration. Key words: spatially-selective RF pulses; 2D RF pulses; SENSE; parallel imagingMultidimensional spatially-selective RF pulses (1-5) have found a number of useful applications in MRI. These RF pulses are able to generate or to refocus transverse magnetization within arbitrarily shaped, spatially restricted areas (2) in up to three spatial dimensions. They can be employed to perform volume selective excitation, outer volume suppression (2,6), and curved slice imaging (7), and they can serve as navigators for motion sensing (8).The basic principle of a spatially-selective RF pulse consists in a definite deposition of RF energy in the excitation k-space spanned by appropriate gradients applied simultaneously (1). Spatial definition using such multidimensional RF pulses is limited by the performance of the gradient system, as well as the finite lifetime of the transverse magnetization caused by T* 2 effects. Moreover, strong main-field inhomogeneities hinder clinical applications of 3D spatially-selective RF pulses, with typical durations of 20 -30 ms (5,9). The ability to shorten such RF pulses without losing spatial definition of the excited area is a prerequisite for exploring their use.Parallel imaging techniques, such as simultaneous acquisition of spatial harmonics (SMASH) (10) and sensitivity encoding (SENSE) (11), recently have been developed to accelerate MR image acquisition. These methodological breakthroughs triggered the question of whether it is possible to benefit from similar concepts in RF pulse design. In that respect, it is important to note that the physics of spatially-selective RF pulses show strong sim...

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 patient's 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 patient's 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.

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...

The specific absorption rate (SAR) is a limiting factor in high-field MR. SAR estimation is typically performed by numerical simulations using generic human body models. However, SAR concepts for single-channel radiofrequency transmission cannot be directly applied to multichannel systems. In this study, a novel and comprehensive SAR prediction concept for parallel radiofrequency transmission MRI is presented, based on precalculated magnetic and electric fields obtained from electromagnetic simulations of numerical body models. The application of so-called Q-matrices and further computational optimizations allow for a real-time estimation of the SAR prior to scanning. This SAR estimation method was fully integrated into an eight-channel whole body MRI system, and it facilitated the selection of different body models and body positions. Experimental validation of the global SAR in phantoms demonstrated a good qualitative and quantitative agreement with the predictions. An initial in vivo validation showed good qualitative agreement between simulated and measured amplitude of (excitation) radiofrequency field. The feasibility and practicability of this SAR prediction concept was shown paving the way for safe parallel radiofrequency transmission in high-field MR.

Electric properties tomography (EPT) derives the patient's electric properties, i.e. conductivity and permittivity, using standard magnetic resonance (MR) systems and standard MR sequences. Thus, EPT does not apply externally mounted electrodes, currents or radiofrequency (RF) probes, as is the case in competing techniques. EPT is quantitative MR, i.e. it yields absolute values of conductivity and permittivity. This review summarizes the physical equations underlying EPT, the corresponding basic and advanced reconstruction techniques and practical numerical aspects to realize these reconstruction techniques. MR sequences which map the field information required for EPT are outlined, and experiments to validate EPT in phantom and in vivo studies are described. Furthermore, the review describes the clinical findings which have been obtained with EPT so far, and attempts to understand the physiologic background of these findings.

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Following the development of parallel imaging, parallel transmission describes the use of multiple RF transmit coils. Parallel transmission can be applied to improve RF excitation, in particular, multidimensional, spatially selective RF excitation. For instance, parallel transmission is able to shorten spatially selective RF pulses in two or three dimensions, or to minimize the occurring SAR. One potential major application might be the compensation of patient-induced B 1 inhomogeneities, particularly at high main fields. This paper provides an overview of selected aspects of this new transmission approach. The basic principles of parallel transmission are discussed, initial experimental proofs are described, and the impact of error propagation on coil design for parallel transmission is outlined.

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.