RESUMO
PURPOSE: In order to fully benefit from the improved signal-to-noise and contrast-to-noise ratios at 9.4T, the challenges of B1+ inhomogeneity and the long acquisition time of high-resolution 2D gradient-recalled echo (GRE) imaging were addressed. THEORY AND METHODS: Flip angle homogenized excitations were achieved by parallel transmission (pTx) of 3-spoke pulses, designed by magnitude least-squares optimization in a slice-by-slice fashion; the acquisition time reduction was achieved by simultaneous multislice (SMS) pulses. The slice-specific spokes complex radiofrequency scaling factors were applied to sinc waveforms on a per-channel basis and combined with the other pulses in an SMS slice group to form the final SMS-pTX pulse. Optimal spokes locations were derived from simulations. RESULTS: Flip angle maps from presaturation TurboFLASH showed improvement of flip angle homogenization with 3-spoke pulses over CP-mode excitation (normalized root-mean-square error [NRMSE] 0.357) as well as comparable excitation homogeneity across the single-band (NRMSE 0.119), SMS-2 (NRMSE 0.137), and SMS-3 (NRMSE 0.132) 3-spoke pulses. The application of the 3-spoke SMS-3 pulses in a 48-slice GRE protocol, which has an in-plane resolution of 0.28 × 0.28 mm, resulted in a 50% reduction of scan duration (total acquisition time 6:52 min including reference scans). CONCLUSION: Time-efficient flip angle homogenized high-resolution GRE imaging at 9.4T was accomplished by using slice-specific SMS-pTx spokes excitations. Magn Reson Med 78:1050-1058, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine.
Assuntos
Processamento de Imagem Assistida por Computador/métodos , Imageamento por Ressonância Magnética/métodos , Adulto , Algoritmos , Encéfalo/diagnóstico por imagem , Humanos , Análise dos Mínimos Quadrados , MasculinoRESUMO
PURPOSE: To eliminate a slice-position-dependent excitation error commonly observed in bipolar-gradient composite excitations such as spokes pulses in parallel transmission. THEORY AND METHODS: An undesired timing delay between subpulses in the composite pulse and their bipolar slice-selective gradient is hypothesized to cause the error. A mathematical model is presented here to relate this mismatch to an induced slice-position-dependent phase difference between the subpulses. A new navigator method is proposed to measure the timing mismatch and eliminate the error. This is demonstrated at 7 Tesla with flip-angle maps measured by a presaturation turbo-flash sequence and in vivo images acquired by a simultaneous multislice/echo-planar imaging (SMS-EPI) sequence. RESULTS: Error-free flip-angle maps were obtained in two ways: 1) by correcting the time delay directly and 2) by applying the corresponding slice-position-dependent phase differences to the subpulses. This confirms the validity of the mathematical description. The radiofrequency (RF)-gradient delay measured by the navigator method was of 6.3 µs, which agreed well with the estimate from flip-angle maps at different delay times. By applying the timing correction, accurately excited EPI images were acquired with bipolar dual-spokes SMS-2 excitations. CONCLUSION: An effective correction is proposed to mitigate slice-position-dependent errors in bipolar composite excitations caused by undesired RF-gradient timing delays. Magn Reson Med 78:1883-1890, 2017. © 2016 International Society for Magnetic Resonance in Medicine.
Assuntos
Processamento de Imagem Assistida por Computador/métodos , Imageamento por Ressonância Magnética/métodos , Encéfalo/diagnóstico por imagem , Imagem Ecoplanar , Humanos , Imagens de FantasmasRESUMO
PURPOSE: A technique is described for simultaneous multislice (SMS) excitation using radiofrequency (RF) parallel transmission (pTX). METHODS: Spatially distinct slices are simultaneously excited by applying different RF frequencies on groups of elements of a multichannel transmit array. The localized transmit sensitivities of the coil geometry are thereby exploited to reduce RF power. The method is capable of achieving SMS-excitation using single-slice RF pulses, or multiband pulses. SMS-pTX is demonstrated using eight-channel parallel RF transmission on a dual-ring pTX coil at 3 T. The effect on B(1)(+) homogeneity and specific absorption rate (SAR) is evaluated experimentally and by simulations. Slice-GRAPPA reconstruction was used for separation of the collapsed slice signals. RESULTS: Phantom and in vivo brain data acquired with fast low-angle shot (FLASH) and blipped-controlled aliasing results in higher acceleration (CAIPIRINHA) echo-planar imaging are presented at SMS excitation factors of two, four, and six. We also show that with our pTX coil design, slice placement, and binary division of transmitters, SMS-pTX excitations can achieve the same mean flip angles excitations at â¼30% lower RF power than a conventional SMS approach with multiband RF pulses. CONCLUSION: The proposed SMS-pTX allows SMS excitations at reduced RF power by exploiting the local B(1)(+) sensitivities of suitable multielement pTX arrays.