Combination of optimized transmit arrays and some receive array reconstruction methods can yield homogeneous images at very high frequencies

Collins, Christopher M.; Liu, Wanzhan; Swift, Bryan J.; Smith, Michael B.

doi:10.1002/mrm.20729

Cited by 110 publications

(111 citation statements)

References 20 publications

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“…Two elliptical array coils containing 16 stripline TEM elements (8) were modeled and driven at 300 MHz. The coil geometries were identical to those modeled previously (9,10). The array geometry is shown in Fig.…”

Section: Methodsmentioning

confidence: 99%

“…The desired current distributions were found using an optimization routine that capitalized on the superposition of the different magnetic fields (9,10). During optimization, the magnitude and phase of the separate coil currents were varied to produce a homogeneous distribution of the magnitude of the pertinent circularly polarized component of the B 1 field ( ) over three different regions: (1) the entire head portion of the model, excluding neck and shoulders (optimizing whole head), (2) the entire brain including white matter, gray matter, and CSF (optimizing whole brain), and (3) a single axial slice of the brain (optimizing brain slice).…”

Section: Methodsmentioning

confidence: 99%

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Calculation of SAR for transmit coil arrays

Mao

Wang

Smith

et al. 2007

Concepts Magn Reson

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Transmit coil arrays allowing independent control of individual coil drives facilitate adjustment of the B 1 field distribution, but when the B 1 field distribution is changed the electric field and SAR distributions are also altered. This makes safety evaluation of the transmit array a challenging problem because there are potentially an infinite number of possible field distributions in the sample. Local SAR levels can be estimated with numerical calculations, but it is not practical to perform separate full numerical calculations for every current distribution of interest. Here we evaluate superposition of separate electric field calculations-one for each coil-for predicting SAR in a full numerical calculation where all coils are driven simultaneously. It is important to perform such an evaluation because the effects of coil coupling may alter the result. It is shown that while there is good agreement between the superimposed and simultaneous drive results when using current sources in the simulations, the agreement is not as good when voltage sources are used. Finally, we compare maximum local SAR levels for B 1 field distributions that are either unshimmed or shimmed over one of three regions of interest. When B 1 field homogeneity is improved in a small region of interest without regard for SAR, the maximum local SAR can become very high.

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Calculation of SAR for transmit coil arrays

Mao

Wang

Smith

et al. 2007

Concepts Magn Reson

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“…RF shimming offers control over these three parameters by manipulating the combined magnetic and electric field in the patient by changing the phases and/or amplitudes of individual RF coil elements (12)(13)(14)(15). The challenge lies in finding the phase and amplitude settings that result in the most favorable trade-off between B 1 ϩ , SAR, and power.…”

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confidence: 99%

SAR and power implications of different RF shimming strategies in the pelvis for 7T MRI

Bergen

Berg

Klomp

et al. 2009

Magnetic Resonance Imaging

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Purpose: To determine the best radiofrequency (RF) shimming method for 7 T body imaging that provides sufficient B 1 ϩ excitation inside the target region while energy deposition (SAR) and power demands are as low as possible and that does not incorporate anatomy specific electric field information inside the patient models, as this information is not available in practice. Materials and Methods:Finite difference time domain (FDTD) simulations were used to evaluate five RF shimming strategies for the pelvis inside a body coil. The results were compared to the theoretical best solution that could be achieved if the electric field inside the patient was known.Results: Most of the RF shimming strategies were successful. However, between the different strategies a factor of two difference in average SAR reduction, a factor of three difference in local maximum SAR reduction, and a factor of 20 difference in power efficiency was observed. Phase matching was found to be the most promising RF shimming method for the body coil used and patient models. Conclusion:RF shimming can reduce the SAR and improve power efficiency in an accurate patient model without knowing the electric field. However, choosing the right method is critical to prevent unexpected behavior in local SAR deposition. THE INCREASE in main magnetic field strength for human magnetic resonance imaging (MRI) toward 7 T has shown large benefits for head imaging, where the higher signal-to-noise ratio can be used to increase scan speed and/or to obtain a higher resolution (1-4) and where the higher spectral resolution (5,6) and the increased sensitivity for various contrast mechanisms such as blood oxygenation level-dependent (BOLD) (4,7,8) have proven useful.Recent studies have shown that 7 T MRI is not limited to the head, but that imaging of the body is also possible (9 -11). However, this ultrahigh magnetic field imaging is hampered by difficulties related to the control of the radiofrequency (RF) excitation field. In the head these difficulties are not too severe and imaging is possible with traditional imaging concepts. In the body this becomes increasingly challenging due to its larger anatomical dimensions. As the Larmor frequency increases with the main magnetic field strength, the wavelength of the RF excitation field becomes smaller, which leads to more pronounced interference effects both in the RF excitation (B 1 ϩ ) field and in the accompanying electric field. The former can result in inhomogeneous B 1 ϩ fields with excitation voids in the region of interest (ROI) (9,12,13) and the latter can cause local maxima in the electric field, which result in large local energy depositions (specific absorption rate, SAR). This poses a safety concern because of possible local overheating of the patient. A second effect of the increase in Larmor frequency is that the RF penetration depth becomes smaller. A larger amount of power is therefore required to reach the center of the body with an increasing SAR as a result. In addition, the chemical shift also in...

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“…To overcome the aforementioned challenges, several techniques have been proposed. The most widely considered are RF shimming [30,31], Transmit SENSE [32], and TIAMO [33]. These Before we discuss the parallel RF transmission technology, let us firstly discuss the conventional excitation for a classical MRI system.…”

Section: Parallel Rf Transmissionmentioning

confidence: 99%

Power amplifier for magnetic resonance imaging using unconventional Cartesian feedback loop

Abuelhaija

Solbach

Buck

2015

2015 German Microwave Conference

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Combination of optimized transmit arrays and some receive array reconstruction methods can yield homogeneous images at very high frequencies

Abstract: Image inhomogeneity related to high radiofrequencies is one of the major challenges for high field imaging. This inhomogeneity can be thought of as having 2 radiofrequency-field related contributors: the transmit field distribution and the reception field

Cited by 110 publications

References 20 publications

Calculation of SAR for transmit coil arrays

Calculation of SAR for transmit coil arrays

SAR and power implications of different RF shimming strategies in the pelvis for 7T MRI

Power amplifier for magnetic resonance imaging using unconventional Cartesian feedback loop

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