1,721,072 research outputs found
A Tool for Coil Sensitivity Analysis for an Arbitrary Surface Coil near Arbitrary Spherical Sample
No full textIn this paper we provide a tool to quickly estimate the coil sensitivity of a surface coil near an arbitrary spherical sample. The tool is very fast because it relies on analytical solutions to estimate the B-1 field distribution in the sample, and the power P-c dissipated by the coil resistance and the power P-s dissipated by the induced electric fields in the sample. We have provided plots of the coil sensitivity and of the ratio P-s/P-C for several parameters such as operating frequency, coil radius, distance of the coil from the sample
Algorithm 934: Fortran 90 subroutines to compute Mathieu functions for complex values of the parameter
Full text linkSoftware to compute angular and radial Mathieu functions is provided in the case that the parameter q is a complex variable and the independent variable x is real. After an introduction on the notation and the definitions of Mathieu functions and their related properties, Fortran 90 subroutines to compute them are described and validated with some comparisons. A sample application is also provided. © 2013 ACM
Transmit Efficiency Across a Range of Field Strengths, Relative Permittivities and Transmit Coils
No full textTransmit efficiency is an important parameter in MRI to determine the quality of a coil, and it is affected by many parameters such as the frequency and the shape of the patient. The presence of a High-Permittivity Helmet significantly impacts the radiofrequency fields distribution. Through numerical simulations we have evaluated the dependence of the transmit efficiency with respect to the variation of many parameters, such as the frequency, the permittivity of the helmet, and the kind of used coil: the performances of a birdcage and of an 8 dipoles transmit array are compared
Determination of the Larmor Frequency for Highest Transmit Efficiency in the Head
No full textMRI scanners with higher B0 fields typically provide a stronger Signal to Noise Ratio (SNR) which results in MRI images with their resolution or shorter acquisition time [1]. Stronger B0 fields require higher operating frequencies of the B1 field, and the shorter wavelengths cause in homogeneities in the B1 field distribution due to scattering and interference in the tissues. At higher B0 RF fields are limited to not cause excessive absorption of power in the tissues. Specifically, electric fields induce currents in conductive tissues causing heat dissipation which raise local temperature potentially leading to patient discomfort and even tissue damage. Hence, local Specific Absorprtion Rate (SAR) and temperature are limited
High-permittivity pads to enhance SNR and transmit efficiency in MRI of the heart at 7T: a simulation study
Full text linkObjective: High-permittivity pads have shown promising results in enhancing SNR and transmit efficiency when used for MRI of the brain, but fewer studies have been conducted to examine the performance of high-permittivity pads in other parts of the patient. In this work, we evaluate the impact on SNR and transmit efficiency distributions when high-permittivity pads with different thickness are positioned near the chest of the patient in combination with a transmit/receive array coil. Methods: The performance of the pads is evaluated through numerical simulations, and both the SNR distribution and the transmit efficiency maps are compared with those obtained when the pads are not present and the distance between the coils and the patient is minimal. The average improvement of SNR and transmit efficiency in the heart is also evaluated for different values of the permittivity of the pads. Results: In the scenario examined, high-permittivity pads can increase SNR and transmit efficiency in the heart volume by as much as 16% and 65%, respectively
Optimization of the order and spacing of sequences in an MRI exam to reduce the maximum temperature and thermal dose
Full text linkPurpose: Evaluate the possibility to reduce specific energy absorption rate (SAR)-induced maximum temperature and thermal dose by rearranging the order and spacing of sequences without increasing duration of the MRI examination. Methods: Using numerical simulations based on an actual SAR-intensive MRI examination, optimizations to reduce either maximum temperature or thermal dose were performed. For each permutation of groups of sequences having the same patient table position, temperature and thermal dose were computed very rapidly using recently published methods. Disposition of sequences was further adjusted by optimizing the spacing between each sequence without exceeding the original exam duration. Results: The maximum simulated temperature in the original exam was 42.38°C, and the maximum thermal dose was 3.23 cumulative effective minutes at 43°C (CEM43). After optimization to reduce maximum temperature, it was 41.77°C, and after optimization to minimize the thermal dose, it was 1.42 CEM43. Conclusion: It is possible to reduce maximum temperature and thermal dose in the exam by changing the arrangement and spacing of the sequences without increasing the duration of the exam (by increasing TR or adding delays) or compromising image quality (by reducing flip angles)
Propagation of a the Fields from a Single Coil in a High-Permittivity Helmet for MRI Applications
No full textUse of High-Permittivity Materials can improve both the Signal to Noise Ratio and the Transmit Efficiency in Magnetic Resonance Imaging systems. The physical mechanisms that contribute to such enhancements are still not fully explored. For a better understanding of these mechanisms, we compare the propagation of the electromagnetic fields when no High-Permittivity helmet is used, and when two different High-Permittivity helmets are used. The results show the helmet contributing to extend the fields far from the coil, therefore providing a more uniform distribution of the fields all around the head
Safety evaluation of algorithms for local excitation with a transmit array considering thermoregulatory responses
No full textIn this paper we evaluate the safety (maximum local SAR and temperature) of three different algorithms to drive the currents of a transmit array: 1) the birdcage coil algorithm; 2) the phase-only optimization algorithm; 3) the phase and amplitude optimization algorithm. The last two algorithms are evaluated when optimizing the transmit efficiency of RF fields in two different locations, one close to the center, and one close to the extremities of a human body subject. All the fields were normalized so that the whole-body average SAR is equal to 2W/kg, and temperature simulations were computed for a continuous exposure 1 hour long
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