Numerical comparison of local transceiver arrays of fractionated dipoles and microstrip antennas for body imaging at 7 T

Stelter, Jonathan; Ladd, Mark E.; Fiedler, Thomas M.

doi:10.1002/nbm.4722

Cited by 3 publications

(1 citation statement)

References 43 publications

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“…This excitation uniformity performance improvement was at the cost of greater local SAR, however. Similarly, at 7T, Stelter et al 15 compared the performance of dipole and microstrip arrays and their combinations with loop elements (up to 16 elements) for body imaging. They found that dipole‐based and microstrip‐based arrays have similar transmit performance, but that hybrid arrangements (loops + dipoles or microstrips) have a slight performance advantage.…”

Section: Introductionmentioning

confidence: 99%

Comparison of tight‐fitting 7T parallel‐transmit head array designs using excitation uniformity and local specific absorption rate metrics

Kazemivalipour,

Wald,

Guerin

2023

Magnetic Resonance in Med

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PurposeWe model the performance of parallel transmission (pTx) arrays with 8, 16, 24, and 32 channels and varying loop sizes built on a close‐fitting helmet for brain imaging at 7 T and compare their local specific absorption rate (SAR) and flip‐angle performances to that of birdcage coil (used as a baseline) and cylindrical 8‐channel and 16‐channel pTx coils (single‐row and dual‐row).MethodsWe use the co‐simulation approach along with MATLAB scripting for batch‐mode simulation of the coils. For each coil, we extracted B1+ maps and SAR matrices, which we compressed using the virtual observation points algorithm, and designed slice‐selective RF shimming pTx pulses with multiple local SAR and peak power constraints to generate L‐curves in the transverse, coronal, and sagittal orientations.ResultsHelmet designs outperformed cylindrical pTx arrays at a constant number of channels in the flip‐angle uniformity at a constant local SAR metric: up to 29% for 8‐channel arrays, and up to 34% for 16‐channel arrays, depending on the slice orientation. For all helmet arrays, increasing the loop diameter led to better local SAR versus flip‐angle uniformity tradeoffs, although this effect was more pronounced for the 8‐channel and 16‐channel systems than the 24‐channel and 32‐channel systems, as the former have more limited degrees of freedom and therefore benefit more from loop‐size optimization.ConclusionHelmet pTx arrays significantly outperformed cylindrical arrays with the same number of channels in local SAR and flip‐angle uniformity metrics. This improvement was especially pronounced for non‐transverse slice excitations. Loop diameter optimization for helmets appears to favor large loops, compatible with nearest‐neighbor decoupling by overlap.

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Section: Introductionmentioning

confidence: 99%

Comparison of tight‐fitting 7T parallel‐transmit head array designs using excitation uniformity and local specific absorption rate metrics

Kazemivalipour,

Wald,

Guerin

2023

Magnetic Resonance in Med

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Local and whole‐body SAR in UHF body imaging: Implications for SAR matrix compression

Fiedler,

Ladd,

Orzada

2024

Magnetic Resonance in Med

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PurposeTransmit arrays for body imaging have characteristics of both volume and local transmit coils. This study evaluates two specific absorption rate (SAR) aspects, local and whole‐body SAR, of arrays for body imaging at 7 T and also for a 3 T birdcage.MethodsSimulations were performed for six antenna arrays at 7 T and one 3 T birdcage. Local SAR matrices and the whole‐body SAR matrix were computed and evaluated with random shims. A set of reduced local SAR matrices was determined by removing all matrices dominated by the whole‐body SAR matrix.ResultsThe results indicate that all RF transmit coils for body imaging in this study are constrained by the local SAR limit. The ratio between local and whole‐body SAR is nevertheless smaller for arrays with large FOV, as these arrays also expose a larger part of the human body.By using the whole‐body SAR matrix, the number of local SAR matrices can be reduced (e.g., 33.3% matrices remained for an 8‐channel local array and 89.7% for a 30‐channel remote array; 12.1% for the 3 T birdcage).ConclusionFor transmit antenna arrays used for body imaging at 7 T as well as for the 3 T birdcage, all evaluated cases show that the local SAR limit was reached before reaching the whole‐body SAR limit. Nevertheless, the whole‐body SAR matrix can be used to reduce the number of local SAR matrices, which is important to reduce memory and computing time for a virtual observation points (VOP) compression. This step can be included as a pre‐compression prior to a VOP compression.

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A Review of Parallel Transmit Arrays for Ultra-High Field MR Imaging

Choi

Webb

Orzada

et al. 2024

IEEE Rev. Biomed. Eng.

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Numerical comparison of local transceiver arrays of fractionated dipoles and microstrip antennas for body imaging at 7 T

Cited by 3 publications

References 43 publications

Comparison of tight‐fitting 7T parallel‐transmit head array designs using excitation uniformity and local specific absorption rate metrics

Comparison of tight‐fitting 7T parallel‐transmit head array designs using excitation uniformity and local specific absorption rate metrics

Local and whole‐body SAR in UHF body imaging: Implications for SAR matrix compression

A Review of Parallel Transmit Arrays for Ultra-High Field MR Imaging

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