Reducing SAR in parallel excitation using variable-density spirals: a simulation-based study

Liu, Yinan; Ke, Feng; McDougall, Mary P.; Wright, Steven M.; Ji, Jim

doi:10.1016/j.mri.2008.02.003

Cited by 17 publications

(20 citation statements)

References 23 publications

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“…As an alternative method for SAR reduction, variable‐density k ‐space trajectories have been proposed (13). These trajectories have the additional benefit of reduced coherent aliased energy.…”

Section: Discussionmentioning

confidence: 99%

“…In particular, the excitation profiles of long RF pulses are more prone to distortions caused by relaxation and off‐resonance. An alternative to this is variable‐rate selective excitation (VERSE) (1, 2, 10–17), which is a general RF and gradient reshaping principle. VERSE jointly reshapes RF and gradient pulses to meet temporal or peak RF constraints, without changing the pulses' on‐resonance excitation profiles.…”

mentioning

confidence: 99%

“…VERSE jointly reshapes RF and gradient pulses to meet temporal or peak RF constraints, without changing the pulses' on‐resonance excitation profiles. It has been used to (i) reduce RF Power (2, 10–13), (ii) overcome peak RF power limitations (1, 10, 14), and (iii) shorten pulse duration (1, 10, 15), though these three categories are not exclusive.…”

mentioning

confidence: 99%

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Time‐optimal design for multidimensional and parallel transmit variable‐rate selective excitation

Lee

Lustig

Grissom

et al. 2009

Magnetic Resonance in Med

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Variable-rate selective excitation (VERSE) is a radio frequency (RF) pulse reshaping technique. It is most commonly used to reduce the peak magnitude and specific absorption rate (SAR) of RF pulses by reshaping pulses and gradient waveforms to reduce RF magnitude while preserving excitation profiles. In this work, a general time-optimal VERSE algorithm for multidimensional and parallel transmit pulses is presented. Time optimality is achieved by translating peak RF limits to gradient upper bounds in excitation k -space. The limits are fed into a time-optimal gradient waveform design technique. Effective SAR reduction is achieved by reducing peak RF subject to a fixed pulse length. The presented method is different from other VERSE techniques in that it provides a noniterative time-optimal multidimensional solution, which drastically simplifies VERSE designs. Examples are given for 1D and 2D single channel and 2D parallel transmit pulses.Magn In magnetic resonance imaging (MRI), spatially selective excitation radio frequency (RF) pulses are often limited by hardware and pulse sequence constraints, such as RF power, peak RF magnitude, and SAR. In most imaging scenarios it is desirable to excite sharp slice profiles. However, such target profiles often result in oscillatory RF waveforms with high peak RF and unacceptable levels of SAR. High peak RF magnitude and SAR effectively set lower limits on the pulse length. In some fast imaging applications, the time required to perform excitation can reduce the available data acquisition time, resulting in SNR degradation (1). In other fast imaging applications, shorter TRs can be achieved by reducing SAR (2). Parallel transmission has recently gained the interest of researchers (3-9) because of the additional degrees of freedom afforded by excitation with multiple independent transmit coils. This can be used to shorten pulse duration, increase spatial resolution, and control RF power deposition (4). Numerical optimizationbased pulse design methods are widely used to design parallel transmit pulses (3-9). Numerical optimization is necessary to meet the unique requirements of parallel RF

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Section: Discussionmentioning

confidence: 99%

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confidence: 99%

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Time‐optimal design for multidimensional and parallel transmit variable‐rate selective excitation

Lee

Lustig

Grissom

et al. 2009

Magnetic Resonance in Med

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“…Dedicated parallel RF transmission coil design (transmit SENSE) is currently under development to further improve the image quality of abdominal examinations at high magnetic field strength. 10,11 Furthermore, magnetic field strength in whole-body scanners has recently been increased up to 7.0 T, leading to markedly pronounced RF inhomogeneity-related problems even in examinations of head and lower extremities and will also require further hardware development.…”

Section: Larmor Frequency and Corresponding Wavelengthmentioning

confidence: 99%

Current Problems and Future Opportunities of Abdominal Magnetic Resonance Imaging at Higher Field Strengths

Springer¹,

Martirosian²,

Boss³

et al. 2010

Topics in Magnetic Resonance Imaging

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Introduction of high-field-strength whole-body MR scanners to clinical routine made abdominal magnetic resonance (MR) imaging widely available. Higher field strength provides improved signal yield, but other issues such as shorter wavelength and increased power deposition of radiofrequency in tissue must also be taken into account. This review describes current problems and future opportunities of abdominal MR imaging at 3.0 T under special consideration of relevant physical properties and technical challenges: impact of higher field strength on signal-to-noise ratio, Larmor frequency, and chemical shift effects are elucidated in detail. Furthermore, changes in longitudinal and transverse relaxation times as well as increased susceptibility effects at 3.0 T are reported. General safety issues and limitations in radiofrequency power deposition are discussed. Subsequently, implications of the previously mentioned changed MR properties at 3.0 T on clinical abdominal examinations applying different sequence types are described.

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“…The speedup of excitation can also achieve reduced specified absorption rate (SAR) with shortened RF duration. Method that employs variable rate [6] spirals k-space trajectory [7] for SAR reduction in parallel excitation is proposed.…”

Section: Introductionmentioning

confidence: 99%

Coil reduction in parallel excitation with large array

Feng

2010

2010 Annual International Conference of the IEEE Engineering in Medicine and Biology

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Parallel excitation using coil array has been introduced in MRI as an effective method to design multi-dimensional spatially selective RF pulses. Generally, all transmit coil elements are used simultaneously in parallel excitation. In case of large array, RF pulse design requires significant computation time and memory size. In this paper, a method using reduced number of coils for parallel excitation is proposed to take advantage of localized coil transmit sensitivity in large array. Specifically, a subset of coils in the large array can be selected based on coil sensitivity coverage and desired excitation pattern. Simulation results with both spiral and EPI k-space trajectories show that proposed method is effective in achieving high excitation resolution at different acceleration factors while the computation time and memory requirement are greatly reduced.

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Reducing SAR in parallel excitation using variable-density spirals: a simulation-based study

Cited by 17 publications

References 23 publications

Time‐optimal design for multidimensional and parallel transmit variable‐rate selective excitation

Time‐optimal design for multidimensional and parallel transmit variable‐rate selective excitation

Current Problems and Future Opportunities of Abdominal Magnetic Resonance Imaging at Higher Field Strengths

Coil reduction in parallel excitation with large array

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