Abstract
To introduce and investigate a method for free-breathing three-dimensional (3D) mapping of the human body at ultrahigh field (UHF), which can be used to generate homogenous flip angle (FA) distributions in the human body at UHF. A 3D relative mapping sequence with a radial phase-encoding (RPE) k-space trajectory was developed and applied in 11 healthy subjects at 7T. An RPE-based actual flip angle mapping method was applied with a dedicated shim setting to calibrate the relative maps yielding absolute maps of the individual transmit channels. The method was evaluated in a motion phantom and by multidimensional in vivo measurements. Additionally, 3D gradient echo scans with and without static phase-only shims were used to qualitatively validate shim predictions. The phantom validation revealed good agreement for maps between dynamic measurement and static reference acquisition. The proposed 3D method was successfully validated in vivo by comparing magnitude and phase distributions with a 2D Cartesian reference. 3D maps free from visible motion artifacts were successfully acquired for 11 subjects with body mass indexes ranging from 19 kg/m2 to 34 kg/m2 . 3D respiration-resolved absolute maps indicated FA differences between inhalation and exhalation up to 15% for one channel and up to 24% for combined channels for shallow breathing. The proposed method provides respiration-resolved absolute 3D maps of the human body at UHF, which enables the investigation and development of 3D shimming and parallel transmission methods to further enhance body imaging at UHF.
Highlights
One of the major challenges at ultrahigh magnetic field (UHF) is the heterogeneous amplitude of the transmit (Tx) magnetic field (B1+)
A, Comparison of magnitude and phase of estimated, absolute maps with dynamic data with exhale and inhale motion states, and data from static acquisition for exhale and inhale position, with inhale position indicated by a solid line and exhale position by a dashed line, with a peak-to-peak difference of 30 mm
The presented respiration-resolved, absolute multichannel B1+ mapping method is based on a relative B1+ mapping method,[14] providing proton-density relative 2D B1+ maps in the human head
Summary
One of the major challenges at ultrahigh magnetic field (UHF) is the heterogeneous amplitude of the transmit (Tx) magnetic field (B1+). It yields a spatially heterogeneous flip angle (FA) and, spatially varying contrast-limiting UHF applications. This issue has been addressed, among other solutions, using multitransmit coils in combination with several different parallel transmission (pTx) strategies.[1,2]. Static pTx methods are often sufficient to achieve acceptable FA homogeneity for small two-dimensional (2D) regions of interest (ROIs) such as used for the prostate,[4] whereas the complexity of the pTx strategies needs to increase when large organs such as the liver[5] are targeted. It is expected that an extension of the ROI to cover a larger 3D volume in the human body requires more complex FA optimization strategies
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