1,721,000 research outputs found
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)
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
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
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
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
A fast, analytically based method to optimize local transmit efficiency for a transmit array
Full text linkPURPOSE: To develop an analytically based algorithm for rapid optimization of the local radiofrequency magnetic (B1+) field intensity for a given radiofrequency power through a transmit array. The analytical nature of the method will yield insight to optimization requirements and provides a valuable reference for numerically based searches. METHODS: With the knowledge of the B1+ field distribution generated by each single coil of the array, both the phases and the amplitudes of each coil current are optimized to maximize the magnitude of the B1+ field in a specific location of the body per unit of power transmitted through the array and, consequently, minimizing the whole body specific absorption rate for a given pulse sequence. RESULTS: Simulations considering the human body show that the proposed method can reduce the whole-body specific absorption rate for a given B1+ magnitude at the location of interest by a factor of about 6.3 compared to the classic birdcage current configuration, and by a factor of 3.2 compared to phase-only shimming in a case with significant coupling between the elements of the array. CONCLUSION: The proposed method can rapidly provide valuable information pertinent to the optimization of field distributions from transmit arrays. Magn Reson Med 71:432-439, 2014. (c) 2013 Wiley Periodicals, Inc
Predicting long-term temperature increase for time-dependent SAR levels with a single short-term temperature response
Full text linkPurpose Present a novel method for rapid prediction of temperature in vivo for a series of pulse sequences with differing levels and distributions of specific energy absorption rate (SAR). Theory and Methods After the temperature response to a brief period of heating is characterized, a rapid estimate of temperature during a series of periods at different heating levels is made using a linear heat equation and impulse-response (IR) concepts. Here the initial characterization and long-term prediction for a complete spine exam are made with the Pennes' bioheat equation where, at first, core body temperature is allowed to increase and local perfusion is not. Then corrections through time allowing variation in local perfusion are introduced. Results The fast IR-based method predicted maximum temperature increase within 1% of that with a full finite difference simulation, but required less than 3.5% of the computation time. Even higher accelerations are possible depending on the time step size chosen, with loss in temporal resolution. Correction for temperature-dependent perfusion requires negligible additional time and can be adjusted to be more or less conservative than the corresponding finite difference simulation. Conclusion With appropriate methods, it is possible to rapidly predict temperature increase throughout the body for actual MR examinations
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Comparison between Dielectric Resonators and a High-Permittivity Helmet to Improve the Transmit Efficiency in the Brain
No full textIn Magnetic Resonance Imaging, it is becoming common to use High-Permittivity Materials to enhance SNR and transmit efficiency. In this work we are going to compare the efficacy of two different solutions to enhance transmit efficiency, one based on an HPM helmet, and another based on dielectric resonators. These solutions have been tested when 4 dipoles are used to generate the fields. Our simulations show an increase of the transmit efficiency in the center of the brain of 23.8 % when the HPM helmet is used, while no improvement was observed for the dielectric resonators
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