Three‐dimensional static and dynamic parallel transmission of the human heart at 7 T

Abstract

Three‐dimensional (3D) human heart imaging at ultra‐high fields is highly challenging due to respiratory and cardiac motion‐induced artifacts as well as spatially heterogeneous B1+0.25em profiles. In this study, we investigate the feasibility of applying 3D flip angle (FA) homogenization targeting the whole heart via static phase‐only and dynamic kT‐point in vivo parallel transmission at 7 T. 3D B1+ maps of the thorax were acquired under free breathing in eight subjects to compute parallel transmission pulses that improve excitation homogeneity in the human heart. To analyze the number of kT‐points required, excitation homogeneity and radiofrequency (RF) power were compared using different regions of interest in six subjects with different body mass index (BMI) values of 20‐34 kg/m2 for a wide range of regularization parameters. One subset of the optimized subject‐specific pulses was applied in vivo on a 7 T scanner for six subjects in Cartesian 3D breath‐hold scans as well as in two subjects in a radial phase‐encoded 3D free‐breathing scan. Across all subjects, 3‐4 kT‐points achieved a good tradeoff between RF power and nominal FA homogeneity. For subjects with a BMI in the normal range, the 4 kT‐point pulses reliably improved the coefficient of variation by less than 10% compared with less than 25% achieved by static phase‐only parallel transmission. in vivo measurements on a 7 T scanner validated the B1+0.25em estimations and the pulse design, despite neglecting ΔB0 in the optimizations and Bloch simulations. This study demonstrates in vivo that kT‐point pTx pulses are highly suitable for mitigating nominal FA heterogeneities across the entire 3D heart volume at 7 T. Furthermore, 3‐4 kT‐points demonstrate a practical tradeoff between nominal FA heterogeneity mitigation and RF power.

Calibration‐free pTx of the human heart at 7T via 3D universal pulses

“…All UPs were evaluated using the CV that is defined as the SD of the FA divided by the mean FA in the heart ROI. 29 (1) min…”

Motion‐compensated fat‐water imaging for 3D cardiac MRI at ultra‐high fields

“…Applications targeting the human body at 7T using either fixed volunteer-unspecific B + 1 shimming, 21,23 tailored B + 1 shimming, 58 or dynamic pTx techniques. 24,30 Fixed volunteer-unspecific approaches do not require additional RF management, but may lead to large B + 1 variations within the target region 23 or can lead to an extended acquisition time if the TIAMO method is used. 32 In this work, tailored B + 1 shimming was performed for three transversal slices covering the heart using channel-wise relative 3D B + 1 maps obtained within the entire thorax.…”

“…The detailed setup of the coil, as well as the applied power limits, were identical to previous publications. 24,41 The study was performed after obtaining ethical approval by the local institutional review board and after providing written informed consent from 11 healthy volunteers (age: 30 ± 5 years; max\min: 35\21 years; body mass index: 24 ± 5 kg/m 2 ; max\min: 34\19 kg/m 2 ; male\ female: 7\4). A three-lead wireless ECG was used for retrospective cardiac phase binning, following Ref.…”

Respiration induced B1+ changes and their impact on universal and tailored 3D kT‐point parallel transmission pulses for 7T cardiac imaging

“…Respiration robustness was achieved extending the tailored‐NRR approach 9 to using multiple RR ‐maps of different respiration states. Code for the design of tailored‐RSpec and tailored‐RRob pulses and the used RR‐DB ‐maps can be downloaded from https://github.com/chaigner/tailored-RRob.…”

“…Tailored-RSpec, tailored-RRob, and UP-DB pulses were designed from RR-SB/DB B + 1 -maps and the respirationrobust UP-SBDB was designed from both, NRR-SB and RR-DB B + 1 -maps. Respiration robustness was achieved extending the tailored-NRR approach 9 to using multiple RR B + 1 -maps of different respiration states. Code for the design of tailored-RSpec and tailored-RRob pulses and the used RR-DB B + 1 -maps can be downloaded from https://github.com/chaig ner/tailo red-RRob.…”