An improved method to measure transfer functions using MRI.

Purpose: A previously published method for MRI-based transfer function assessment makes use of the so-called transceive phase assumption (TPA). This limits its applicability to shorter leads and/or lower field strengths. A new method is presented where the background electric field is determined from both - and -field distributions, avoiding the TPA and making it more generally applicable.

Simulation-based evaluation of SAR and flip angle homogeneity for five transmit head arrays at 14 T.

Introduction: Various research sites are pursuing 14 T MRI systems. However, both local SAR and RF transmit field inhomogeneity will increase. The aim of this simulation study is to investigate the trade-offs between peak local SAR and flip angle uniformity for five transmit coil array designs at 14 T in comparison to 7 T.

Multi-echo MR thermometry in the upper leg at 7 T using near-harmonic 2D reconstruction for initialization.

Purpose: The aim of this work is the development of a thermometry method to measure temperature increases in vivo, with a precision and accuracy sufficient for validation against thermal simulations. Such an MR thermometry model would be a valuable tool to get an indication on one of the major safety concerns in MR imaging: the tissue heating occurring due to radiofrequency (RF) exposure. To prevent excessive temperature rise, RF power deposition, expressed as specific absorption rate, cannot exceed predefined thresholds.

Tier-based formalism for safety assessment of custom-built radio-frequency transmit coils.

The purpose of this work is to propose a tier-based formalism for safety assessment of custom-built radio-frequency (RF) coils that balances validation effort with the effort put in determinating the safety factor. The formalism has three tier levels. Higher tiers require increased effort when validating electromagnetic simulation results but allow for less conservative safety factors.

A perturbation approach for ultrafast calculation of RF field enhancements near medical implants in MRI.

Patients with medical implants often are deprived of magnetic resonance imaging examination because of safety risks. One specific risk is the enhancement of the radiofrequency fields around the medical implant potentially resulting in significant tissue heating and damage. The assessment of this enhancement is a computationally demanding task, with simulations taking hours or days to converge.

The Coax Dipole: A fully flexible coaxial cable dipole antenna with flattened current distribution for body imaging at 7 Tesla.

Purpose: The coax dipole antenna, a flexible antenna for body imaging at 7T is presented. Similar to the high impedance coil, this coaxial cable antenna is fed on the central conductor and through gaps in the shield, the current passes to the outside of the antenna to generate B field. This could achieve more favorable current distributions and better adaptation to the body curvature.

SAR and temperature distributions in a database of realistic human models for 7 T cardiac imaging.

Purpose: To investigate inter-subject variability of B , SAR and temperature rise in a database of human models using a local transmit array for 7 T cardiac imaging.

Methods: Dixon images were acquired of 14 subjects and segmented in dielectric models with an eight-channel local transmit array positioned around the torso for cardiac imaging. EM simulations were done to calculate SAR distributions.

Evaluation of the radiofrequency performance of a wide-bore 1.5 T positron emission tomography/magnetic resonance imaging body coil for radiotherapy planning.

Background And Purpose: The restricted bore diameter of current simultaneous positron emission tomography/magnetic resonance imaging (PET/MRI) systems can be an impediment to achieving similar patient positioning during PET/MRI planning and radiotherapy. Our goal was to evaluate the B transmit (B ) uniformity, B efficiency, and specific absorption rate (SAR) of a novel radiofrequency (RF) body coil design, in which RF shielded PET detectors were integrated with the specific aim of enabling a wide-bore PET/MRI system.

Materials And Methods: We designed and constructed a wide-bore PET/MRI RF body coil to be integrated with a clinical MRI system.

Conditional safety margins for less conservative peak local SAR assessment: A probabilistic approach.

Purpose: The introduction of a linear safety factor to address peak local specific absorption rate (pSAR ) uncertainties (eg, intersubject variation, modeling inaccuracies) bears one considerable drawback: It often results in over-conservative scanning constraints. We present a more efficient approach to define a variable safety margin based on the conditional probability density function of the effectively obtained pSAR value, given the estimated pSAR value.

Methods: The conditional probability density function can be estimated from previously simulated data.

Introduction of the snake antenna array: Geometry optimization of a sinusoidal dipole antenna for 10.5T body imaging with lower peak SAR.

Purpose: To improve imaging performance for body MRI with a local transmit array at 10.5T, the geometry of a dipole antenna was optimized to achieve lower peak specific absorption rate (SAR) levels and a more uniform transmit profile.

Methods: Electromagnetic simulations on a phantom were used to evaluate the SAR and -performance of different dipole antenna geometries.

Homogeneous B for bilateral breast imaging at 7 T using a five dipole transmit array merged with a high density receive loop array.

To explore the use of five meandering dipole antennas in a multi-transmit setup, combined with a high density receive array for breast imaging at 7 T for improved penetration depth and more homogeneous B field. Five meandering dipole antennas and 30 receiver loops were positioned on two cups around the breasts. Finite difference time domain simulations were performed to evaluate RF safety limits of the transmit setup.

Comparing signal-to-noise ratio for prostate imaging at 7T and 3T.

Background: In MRI, the signal-to-noise ratio (SNR) theoretically increases with B field strength. However, because of attenuation of the radiofrequency (RF) fields at 7T, it is not certain if this SNR gain can be realized for prostate imaging.

Purpose/hypothesis: To investigate the SNR gain in prostate imaging at 7T as compared with 3T.

Design of a forward view antenna for prostate imaging at 7 T.

Purpose: To design a forward view antenna for prostate imaging at 7 T, which is placed between the legs of the subject in addition to a dipole array.

Materials And Methods: The forward view antenna is realized by placing a cross-dipole antenna at the end of a small rectangular waveguide. Quadrature drive of the cross-dipole can excite a circularly polarized wave propagating along the axial direction to and from the prostate region.

An 8-channel Tx/Rx dipole array combined with 16 Rx loops for high-resolution functional cardiac imaging at 7 T.

Objective: To demonstrate imaging performance for cardiac MR imaging at 7 T using a coil array of 8 transmit/receive dipole antennas and 16 receive loops.

Materials And Methods: An 8-channel dipole array was extended by adding 16 receive-only loops. Average power constraints were determined by electromagnetic simulations.

Selective proton-observed, carbon-edited (selPOCE) MRS method for measurement of glutamate and glutamine C-labeling in the human frontal cortex.

Purpose: C magnetic resonance spectroscopy (MRS) in combination with infusion of C-labeled substrates has led to unique insights into human brain metabolism and neurotransmitter cycling. However, the low sensitivity of direct C MRS and high radiofrequency power requirements has limited C MRS studies to predominantly data acquisition in large volumes of the occipital cortex. The purpose of this study is to develop an MRS technique for localized detection of C-labeling of glutamate and glutamine in the human frontal lobe.