Rotating frame RF current density imaging

Demonstration of full tensor current density imaging using ultra-low field MRI

Direct imaging of impressed dc currents inside the head can provide valuable conductivity information, possibly improving electro-magnetic neuroimaging. Ultra-low field magnetic resonance imaging (ULF MRI) at μT Larmor fields can be utilized for current density imaging (CDI). Here, a measurable impact of the magnetic field B_J, generated by the impressed current density J, on the MR signal is probed using specialized sequences. In contrast to high-field MRI, the full tensor of B_J can be derived without rotation of the subject in the scanner, due to a larger flexibility in the sequence design. We present an ULF MRI setup based on a superconducting quantum interference device (SQUID), which is operating at a noise level of 380 aT Hz^-1/2 and capable of switching all imaging fields within a pulse sequence. Thereby, the system enables zero-field encoding, where the full tensor of B_J is probed in the absence of other magnetic fields. 3D CDI is demonstrated on phantoms with different geometries carrying currents of approximately 2 mA corresponding to current densities between 0.45 and 8 A/m². By comparison to an in vivo acquired head image, we provide insights to necessary improvements in signal-to-noise ratio.

Electric properties tomography: Biochemical, physical and technical background, evaluation and clinical applications

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.

Magnetic resonance imaging of ionic currents in solution: the effect of magnetohydrodynamic flow

PURPOSE

Reliably detecting MRI signals in the brain that are more tightly coupled to neural activity than blood-oxygen-level-dependent fMRI signals could not only prove valuable for basic scientific research but could also enhance clinical applications such as epilepsy presurgical mapping. This endeavor will likely benefit from an improved understanding of the behavior of ionic currents, the mediators of neural activity, in the presence of the strong magnetic fields that are typical of modern-day MRI scanners.

THEORY

Of the various mechanisms that have been proposed to explain the behavior of ionic volume currents in a magnetic field, only one-magnetohydrodynamic flow-predicts a slow evolution of signals, on the order of a minute for normal saline in a typical MRI scanner.

METHODS

This prediction was tested by scanning a volume-current phantom containing normal saline with gradient-echo-planar imaging at 3 T.

RESULTS

Greater signal changes were observed in the phase of the images than in the magnitude, with the changes evolving on the order of a minute.

CONCLUSION

These results provide experimental support for the MHD flow hypothesis. Furthermore, MHD-driven cerebrospinal fluid flow could provide a novel fMRI contrast mechanism.

Current-density imaging using ultra-low-field MRI with zero-field encoding

Electric current density can be measured noninvasively with magnetic resonance imaging (MRI). Determining all three components of the current density, however, requires physical rotation of the sample or current injection from several directions when done with conventional methods. However, the emerging technology of ultra-low-field (ULF) MRI, in which the signal encoding and acquisition is conducted at a microtesla-range magnetic field, offers new possibilities. The low applied magnetic fields can even be switched off completely within the pulse sequence, increasing the flexibility of the available sequences. In this article, we present a ULF-MRI sequence designed for obtaining all three components of a current-density pattern without the need of sample rotations. The sequence consists of three steps: prepolarization of the sample, signal encoding in the current-density-associated magnetic field without applying any MRI fields, and spatial encoding in a microtesla-range field using any standard ULF-MRI sequence. The performance of the method is evaluated by numerical simulations. The method may find applications, e.g., in noninvasive conductivity imaging of tissue.

High-resolution MRI encoding using radiofrequency phase gradients

Although MRI offers highly diagnostic medical imagery, patient access to this modality worldwide is very limited when compared with X-ray or ultrasound. One reason for this is the expense and complexity of the equipment used to generate the switched magnetic fields necessary for MRI encoding. These field gradients are also responsible for intense acoustic noise and have the potential to induce nerve stimulation. We present results with a new MRI encoding principle which operates entirely without the use of conventional B0 field gradients. This new approach--'Transmit Array Spatial Encoding' (TRASE)--uses only the resonant radiofrequency (RF) field to produce Fourier spatial encoding equivalent to conventional MRI. k-space traversal (image encoding) is achieved by spin refocusing with phase gradient transmit fields in spin echo trains. A transmit coil array, driven by just a single transmitter channel, was constructed to produce four phase gradient fields, which allows the encoding of two orthogonal spatial axes. High-resolution two-dimensional-encoded in vivo MR images of hand and wrist were obtained at 0.2 T. TRASE exploits RF field phase gradients, and offers the possibility of very low-cost diagnostics and novel experiments exploiting unique capabilities, such as imaging without disturbance of the main B0 magnetic field. Lower field imaging (<1 T) and micro-imaging are favorable application domains as, in both cases, it is technically easier to achieve the short RF pulses desirable for long echo trains, and also to limit RF power deposition. As TRASE is simply an alternative mechanism (and technology) of moving through k space, there are many close analogies between it and conventional B0 -encoded techniques. TRASE is compatible with both B0 gradient encoding and parallel imaging, and so hybrid sequences containing all three spatial encoding approaches are possible.

Current-density imaging using ultra-low-field MRI with adiabatic pulses

Magnetic resonance imaging (MRI) allows measurement of electric current density in an object. The measurement is based on observing how the magnetic field of the current density affects the associated spins. However, as high-field MRI is sensitive to static magnetic field variations of only the field component along the main field direction, object rotations are typically needed to image three-dimensional current densities. Ultra-low-field (ULF) MRI, on the other hand, with B0 on the order of 10-100 μT, allows novel MRI sequences. We present a rotation-free method for imaging static magnetic fields and current densities using ULF MRI. The method utilizes prepolarization pulses with adiabatic switch-off ramps. The technique is designed to reveal complete field and current-density information without the need to rotate the object. The method may find applications, e.g., in conductivity imaging. We present simulation results showing the feasibility of the sequence.

Recent progress and future challenges in MR electric properties tomography

MR Electric Properties Tomography (EPT) is a lately developed medical imaging modality capable of visualizing both conductivity and permittivity of the patient at the Larmor frequency using B 1 maps. The paper discusses the development of EPT reconstructions, EPT sequences, EPT experiments, and challenging issues of EPT.

Physics of MRI: a primer

This article is based on an introductory lecture given for the past many years during the "MR Physics and Techniques for Clinicians" course at the Annual Meeting of the ISMRM. This introduction is not intended to be a comprehensive overview of the field, as the subject of magnetic resonance imaging (MRI) physics is large and complex. Rather, it is intended to lay a conceptual foundation by which magnetic resonance image formation can be understood from an intuitive perspective. The presentation is nonmathematical, relying on simple models that take the reader progressively from the basic spin physics of nuclei, through descriptions of how the magnetic resonance signal is generated and detected in an MRI scanner, the foundations of nuclear magnetic resonance (NMR) relaxation, and a discussion of the Fourier transform and its relation to MR image formation. The article continues with a discussion of how magnetic field gradients are used to facilitate spatial encoding and concludes with a development of basic pulse sequences and the factors defining image contrast.

RF field visualization of RF ablation at the Larmor frequency

Radio-frequency ablation (RFA) is an effective minimally invasive treatment for tumors. One primary source of difficulty is monitoring and controlling the ablation region. Currently, RFA is performed at 460 kHz, for which magnetic resonance imaging (MRI) could play a role given its capability for temperature monitoring and tumor visualization. If instead the ablation were to be performed at the MRI Larmor frequency, then the MR capability for B(1) field mapping could be used to directly visualize the radio-frequency (RF) fields created by the ablation currents. Visualizing the RF fields may enable better control of the ablation currents, enabling better control of lesion shape and size and improving repeatability. We demonstrate the feasibility of performing RFAs at 64 MHz and show preliminary results from imaging the RF fields from the ablation. The post-ablation RF fields show an increase in current density in the ablated region, consistent with an increase in conductivity of the ablated tissue.

Magnetic resonance driven electrical impedance tomography: a simulation study

Magnetic resonance electrical impedance tomography (MREIT) is a method for reconstructing a three-dimensional image of the conductivity distribution in a target volume using magnetic resonance (MR). In MREIT, currents are applied to the volume through surface electrodes and their effects on the MR induced magnetic fields are analyzed to produce the conductance image. However, current injection through surface electrodes poses technical problems such as the limitation on the safely applicable currents. In this paper, we present a new method called magnetic resonance driven electrical impedance tomography (MRDEIT), where the magnetic resonance in each voxel is used as the applied magnetic field source, and the resultant electromagnetic field is measured through surface electrodes or radio-frequency (RF) detectors placed near the surface. Because the applied magnetic field is at the RF frequency and eddy currents are the integral components in the method, a vector wave equation for the electric field is used as the basis of the analysis instead of a quasi-static approximation. Using computer simulations, it is shown that complex permittivity images can be reconstructed using MRDEIT, but that improvements in signal detection are necessary for detecting moderate complex permittivity changes.

Radio-frequency current density imaging based on a 180 (°) sample rotation with feasibility study of full current density vector reconstruction

Radio-frequency current density imaging (RF-CDI) is a technique that noninvasively measures current density distributions at the Larmor frequency utilizing magnetic resonance imaging. Previously implemented RF-CDI methods reconstruct the applied current density component J(z) along the static magnetic field of the imager [(B)\vec](0) (the z direction) based on the assumption that the z-directional change of the magnetic field component H(z) can be ignored compared to J(z). However, this condition may be easily violated in biomedical applications. We propose a new reconstruction method for RF-CDI, which does not rely on the aforementioned assumption. Instead, the sample is rotated by 180 (°) in the horizontal plane to collect magnetic resonance data from two opposite positions. Using simulations and experiments, we have verified that this approach can fully recover one component of current density. Furthermore, this approach can be extended to measure three dimensional current density vectors by one additional sample orientation in the horizontal plane. We have therefore demonstrated for the first time the feasibility of imaging the magnitude and phase of all components of a radio-frequency current density vector field.

Magnetic resonance imaging of oscillating electrical currents

Functional MRI has become an important tool of researchers and clinicians who seek to understand patterns of neuronal activation that accompany sensory and cognitive processes. However, the interpretation of fMRI images rests on assumptions about the relationship between neuronal firing and hemodynamic response that are not firmly grounded in rigorous theory or experimental evidence. Further, the blood-oxygen-level-dependent effect, which correlates an MRI observable to neuronal firing, evolves over a period that is 2 orders of magnitude longer than the underlying processes that are thought to cause it. Here, we instead demonstrate experiments to directly image oscillating currents by MRI. The approach rests on a resonant interaction between an applied rf field and an oscillating magnetic field in the sample and, as such, permits quantitative, frequency-selective measurements of current density without spatial or temporal cancellation. We apply this method in a current loop phantom, mapping its magnetic field and achieving a detection sensitivity near the threshold required for the detection of neuronal currents. Because the contrast mechanism is under spectroscopic control, we are able to demonstrate how ramped and phase-modulated spin-lock radiation can enhance the sensitivity and robustness of the experiment. We further demonstrate the combination of these methods with remote detection, a technique in which the encoding and detection of an MRI experiment are separated by sample flow or translation. We illustrate that remotely detected MRI permits the measurement of currents in small volumes of flowing water with high sensitivity and spatial resolution.

Polar decomposition radio-frequency current density imaging

Polar Decomposition Radio-frequency Current Density Imaging (PD-RFCDI) is an imaging technique that non-invasively measures RF current density components inside a sample using MRI. Previous PD-RFCDI implementations suffer from the strict constraint on the amount of applied current as well as severe interference from the unwanted induced current. This work proposes solutions to both problems which successfully remove the current constraints of PD-RFCDI. Both simulation and experiment were used to verify the validity of PD-RFCDI on a clinical MRI scanner.

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 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.

Multislice radio-frequency current density imaging

Radio-frequency current density imaging (RF-CDI) is an imaging technique that noninvasively measures current density distribution at the Larmor frequency utilizing magnetic resonance imaging (MRI). Previously implemented RF-CDI techniques were only able to image a single slice transverse to the static magnetic field B(0) . This paper describes the first realization of a multislice RF-CDI sequence on a 1.5 T clinical imager. Multislice RF current density images have been reconstructed for two phantoms. The influence of MRI random noise on the sensitivity of the multislice RF-CDI measurement has also been studied by theoretical analysis, simulation and phantom experiments.

Electrical conductivity imaging using MRI measurement of the magnetic field vector

Current density and electrical conductivity imaging research at the University of Toronto is reviewed. Methods for imaging live animals at low frequency are described and contrasted with EIT and other MRI based techniques. New work on imaging at radio frequencies is presented and future work directions are discussed. It is concluded that low frequency methods are mature and ready for application in small animals and that radio frequency methods will soon have application in small animals.

MR current density and conductivity imaging: the state of the art

Current density imaging (CDI) is an imaging technique that measures electrical current density distributions in a volume of material or tissue, which can be imaged using magnetic resonance imaging (MRI). Measurements of current density are obtained by applying an external current to the material/tissue during an MRI acquisition. The magnetic fields produced by the applied current are mapped onto the phase image of the MRI acquisition. The phase images are processed to compute the current density distribution. Performing CDI requires an MRI system, additional hardware, a modified pulse sequence (PSD) and data processing software. Greig C. Scott, Michael L.G. Joy and R. Mark Henkelman developed CDI in 1988 at the University of Toronto (Canada). The CDI Research Group is presently based at the University of Toronto and is supervised by the author. This paper describes the CDI technique, its applications by this and other groups and recently proposed methods for electrical conductivity imaging based on the technique.

A study of tablet dissolution by magnetic resonance electric current density imaging

The electric current density imaging technique (CDI) was used to monitor the dissolution of ion releasing tablets (made of various carboxylic acids and of sodium chloride) by following conductivity changes in an agar-agar gel surrounding the tablet. Conductivity changes in the sample were used to calculate spatial and temporal changes of ionic concentrations in the sample. The experimental data for ion migration were compared to a mathematical model based on a solution of the diffusion equation with moving boundary conditions for the tablet geometry. Diffusion constants for different acids were determined by fitting the model to the experimental data. The experiments with dissolving tablets were used to demonstrate the potential of the CDI technique for measurement of ion concentration in the vicinity of ion releasing samples.

Investigating direct detection of axon firing in the adult human optic nerve using MRI

The aim of this study was to directly detect spectral components of the magnetic fields of ionic currents caused by firing of the axons in the optic nerve in response to visual strobe stimulation. The magnetic field parallel to the main B0 field can potentially alter the local phase and magnitude of the MR signal which can cause signal loss due to intravoxel dephasing. Measured frequency spectra showed evidence of the strobe stimulus localized to regions containing the optic nerve, not thought to be due to motion artifacts, in 30 out of 52 experiments in 5 adult human subjects. The effect was (0.15 +/- 0.05)% of the mean magnitude equilibrium signal from the voxel in the frequency range 0.7-3.3 Hz, corresponding to an estimated field of (1.2 +/- 0.4) nT, at an echo time of TE = 32.4 ms using a 1.5 T MRI scanner. Only 1 of 12 phase image experiments showed effects. These findings provide preliminary evidence for direct detection of axonal firing in the optic nerve.

Investigation of MR signal modulation due to magnetic fields from neuronal currents in the adult human optic nerve and visual cortex

Neuronal currents produce weak transient magnetic fields, and the hypothesis being investigated here is that the components of these parallel to the B0 field can potentially modulate the MR signal, thus providing a means of direct detection of nerve impulses. A theory for the phase and amplitude changes of the MR signal over time due to an external magnetic field has been developed to predict this modulation. Experimentally, a fast gradient-echo EPI sequence (TR = 158 ms, TE = 32.4 ms) was employed in an attempt to directly detect these neuronal currents in the adult human optic nerve and visual cortex using a 280-mm quadrature head coil at 1.5 T. A symmetrical intravoxel field distribution, which can be plausibly hypothesized for the axonal fields in the optic nerve and visual cortex, would result in phase cancellation within a voxel, and hence, only amplitude changes would be expected. On the other hand, an asymmetrical intravoxel field distribution would produce both phase and amplitude changes. The in vivo magnitude image data sets show a significant nerve firing detection rate of 56%, with zero detection using the phase image data sets. The percentage magnitude signal changes relative to the fully relaxed equilibrium signal fall within a predicted RMS field range of 1.2-2.1 nT in the optic nerve and 0.4-0.6 nT in the visual cortex, according to the hypothesis that the axonal fields create a symmetrical Lorentzian field distribution within the voxel.

Noise analysis for multi-slice radio frequency current density imaging

Radio frequency current density imaging (RF-CDI) is an imaging technique that measures current density distribution at the Larmor frequency utilizing magnetic resonance imaging (MRI). The multi-slice RF-CDI sequence has extended the ability of RF-CDI to image multiple slices and thus has enhanced its capacity for biomedical applications. In this paper, the influence of MRI random noise on the sensitivity of multi-slice RF-CDI measurement is studied. The formula of current noise is derived, which is verified by both simulation and phantom experiments. A 3-D finite-difference time-domain (FDTD) model is employed to compute the electromagnetic fields in the simulation.

MR-Guided Interventions of the Breast

Techniques and instrumentation are now widely available that enable interventional MR-guided preoperative needle localization and lesion marking. Minimally invasive MR-guided core biopsy techniques have been demonstrated but remain limited for small lesions and will be facilitated by the development of biopsy instruments that can be directly visualized using MR imaging. MR-guided tumor ablation is beginning to be evaluated in a few centers. It holds promise as new treatment modality in the continuing trend toward greater breast conservation in the local therapy of breast cancer. Further studies are needed to document the ability of MR-guided ablation to control the margins of a tumor as effectively as surgery. Patients with an extensive in situ intra-ductal component may pose a significant hurdle because the extent of ductal carcinoma in situ maybe underestimated on breast MR images. Ultimately, the success of MR-guided thermal ablation depends on the ability of MR imaging to map the extent of heating during the procedure so that the procedure can be performed to achieve complete control of the tumor margins. It is unfortunate that the conventional method for MR thermometry--the proton resonance frequency shift method--does not work in fat or in voxels with a mix of fat and glandular tissue and, hence, has limited applicability in the breast. Other methods, including measurement of T1 and T2, are being investigated as alternatives.

Equipotential projection-based magnetic resonance electrical impedance tomography and experimental realization

In this study, a direct, fast image reconstruction algorithm, based on the fact that equipotential lines are perpendicular to current lines in a volume conductor, is proposed for magnetic resonance electrical impedance tomography (MR-EIT). The proposed technique is evaluated both on simulated and measured data for conductor and insulator objects.

Echo-planar rotating-frame imaging

A new rotating-frame imaging method that produces a complete cross section of an object in a single experiment is reported. The echo planar rotating frame imaging (EPROFI) technique uses two perpendicular RF gradients for two-dimensional spatial encoding and fully exploits the formation of rotary echoes for fast sampling of spatial frequencies. The acquisition scheme yields the Fourier transform of the spin distribution on Cartesian coordinates for straightforward image reconstruction. Implementation of the technique on a low-field portable NMR probe is described and results are presented for test objects with different geometries.

Current constrained voltage scaled reconstruction (CCVSR) algorithm for MR-EIT and its performance with different probing current patterns

Conventional injected-current electrical impedance tomography (EIT) and magnetic resonance imaging (MRI) techniques can be combined to reconstruct high resolution true conductivity images. The magnetic flux density distribution generated by the internal current density distribution is extracted from MR phase images. This information is used to form a fine detailed conductivity image using an Ohm's law based update equation. The reconstructed conductivity image is assumed to differ from the true image by a scale factor. EIT surface potential measurements are then used to scale the reconstructed image in order to find the true conductivity values. This process is iterated until a stopping criterion is met. Several simulations are carried out for opposite and cosine current injection patterns to select the best current injection pattern for a 2D thorax model. The contrast resolution and accuracy of the proposed algorithm are also studied. In all simulation studies, realistic noise models for voltage and magnetic flux density measurements are used. It is shown that, in contrast to the conventional EIT techniques, the proposed method has the capability of reconstructing conductivity images with uniform and high spatial resolution. The spatial resolution is limited by the larger element size of the finite element mesh and twice the magnetic resonance image pixel size.

Magnetic resonance imaging of alternating electric currents

Electric Current Density Imaging (CDI) is a new modality of magnetic resonance imaging that enables electric current distribution imaging in conductive samples containing water. So far, two CDI techniques have been in use: DC-CDI operating at zero frequency and RF-CDI operating at the RF Larmor frequency. In this paper we present a new CDI technique, which extends the CDI frequency range to alternating electric currents (AC-CDI). First, a theoretical model for the electric current response to the alternating voltage is presented. Later, this model is used for the frequency analysis of the AC-CDI sequence. Additionally, the effect of off-resonance spins and imperfect refocusing RF pulses on the stability of the AC-CDI sequence and the echo formation is studied. The new theory is verified by experiments on a model system and compared to the other two methods: DC-CDI and RF-CDI. Finally, an application of the AC-CDI sequence to biological systems is demonstrated by an experiment on a wood twig in which an increase of approximately 30% was obtained at AC as compared to DC electric current.

Specific absorption rate study for radiofrequency current density imaging using a two-dimensional finite element model

Radiofrequency current density imaging is an MR technique that images tissue conductivity contrast. Compared to conventional MRI, RF-CDI uses two additional sources of RF power to be absorbed and that must be evaluated in terms of proper parameter optimization to prevent excessive tissue heating and effects on the nervous system. In view of possible future clinical use of RF-CDI, a simple 2D finite element model of a rat brain was built to simulate current density distribution and distribution of absorbed RF power, i.e., SAR and related tissue heating. Current density in the rat brain was also evaluated qualitatively and quantitatively in an in vivo RF-CDI experiment. The results demonstrate that a numerical model can predict SAR and tissue temperature changes. The study also shows that substantial sensitivity and resolution of RF-CDI can be achieved using imaging parameters that produce SAR and temperature changes within allowed limits.

Changes in the complex permittivity during spreading depression in rat cortex

With recent developments in current density imaging (CDI), it is feasible to utilize this new technique in brain imaging applications. Since CDI's ability to measure changes in current density depends on a concomitant activity-dependent change in the conductivity of the brain tissue, we have examined the changes in complex conductivity during spreading depression (SD) in rodent neocortex using a coaxial probe. SD was chosen because it is often referred to as an animal model of cerebral ischemia and migraine with aura. The conductivity measurements revealed a change with short latency (30-60 s) followed by a change with a longer latency (200-300 s). This change in conductivity with short latency has not been reported before, and we conjecture that it may be the priming or triggering mechanism prior to the main SD episode. A 20% change in conductivity during SD is sufficiently large to be measured by CDI. Therefore, the ability to measure changes in the conductivity, as opposed to metabolic changes, makes CDI a viable approach to the study of ischemia and migraine with aura.

Radiofrequency current density imaging of kainate-evoked depolarization

The purpose of this study was to examine whether radiofrequency current density imaging (RF-CDI) can quantitatively monitor depolarizations evoked by excitatory amino acids in a rat's brain. To evoke depolarization, a glutamate receptor agonist, kainate, was administered into the right lateral ventricle. First, electroencephalographic activity was recorded in a basal condition and after the application of kainate. Complex behavioral patterns were observed. Second, impedance measurements were performed to assess the change in conductivity of the brain due to kainate at the Larmor frequency of the imager. Calculated changes were about 17%. Third, a set of current density images was obtained with RF-CDI before and after the administration of kainate. Kainate-induced excitatory changes were observed on current density images as brighter regions, mainly in the hippocampal area compared with the same area in the basal condition.

SAR and B1 field distributions in a heterogeneous human head model within a birdcage coil. Specific energy absorption rate

Calculations of radiofrequency magnetic (B1) field and specific energy absorption rate (SAR) distributions in a sphere of tissue and a multi-tissue human head model in a 12-element birdcage coil are presented. The coil model is driven in linear and quadrature modes at 63, 175, 200, and 300 MHz. Plots of B1 field magnitude and SAR distributions, average SAR, maximum local SAR, and measures of B1 field homogeneity and signal-to-noise ratio are given. SAR levels for arbitrary pulse sequences can be estimated from the calculated data. Maximum local SAR levels are lower at lower frequencies, in quadrature rather than in linear coils, and in linear fields oriented posterior-to-anterior rather than left-to-right in the head. It should be possible to perform many experiments in the head at frequencies up to 300 MHz without exceeding standard limits for local or average SAR levels.

Contrast-modified gradient echo imaging using rotary echo preparatory pulses

The use of on-resonance 121 binomial composite pulses in two- or three-dimensional magnetization-prepared gradient-recalled echo magnetic resonance imaging experiments generates rotary echoes, leading to an increase in contrast range that is, in part, determined by the ratio of T2 to T1. In comparison with other fast gradient-recalled echo imaging techniques designed for enhanced T2 contrast, this method is more robust with respect to radiofrequency field inhomogeneity and less sensitive with respect to motion artifacts. Three-dimensional parametric images may be calculated using least-squares fitting based on a simple model for steady-state longitudinal magnetization during the imaging sequences.