At high magnetic field strengths (!3T), the radiofrequency wavelength used in MRI is of the same order of magnitude of (or smaller than) the typical sample size, making transmit magnetic field (B þ 1 ) inhomogeneities more prominent. Methods such as radiofrequency-shimming and transmit SENSE have been proposed to mitigate these undesirable effects. A prerequisite for such approaches is an accurate and rapid characterization of the B þ 1 field in the organ of interest. In this work, a new phase-sensitive three-dimensional B þ 1 -mapping technique is introduced that allows the acquisition of a 64 3 64 3 8 B þ 1 -map in~20 s, yielding an accurate mapping of the relative B þ 1 with a 10-fold dynamic range (0.2-2 times the nominal B þ 1 ). Moreover, the predominant use of low flip angle excitations in the presented sequence minimizes specific absorption rate, which is an important asset for in vivo B þ 1 -shimming procedures at high magnetic fields. INTRODUCTIONAt high magnetic field strengths (!3T), the radiofrequency (RF) wavelength used in MRI becomes smaller than the typical sample size, making transmit magnetic field (B þ 1 ) inhomogeneities more prominent. RF-shimming (1) and transmit SENSE (2,3) or combinations of these methods (4) have been proposed to mitigate these B þ 1 variations. However, a prerequisite of such methods is to accurately map the B þ 1 field in short experimental times.Several approaches had been developed in the past 20 years to determine the B þ 1 distribution. The technique of most straightforward implementation is the double-angle method (DAM), which calculates B þ 1 from the ratio of two images acquired by using flip angles a 1 and a 2 ¼ 2a 1 and a very long repetition time (5). Another approach relies on a sequence made of three RF pulses (a, 2a, a) in order to generate both a spin echo and stimulated echo signal, which encode the transmit magnetic field information (6,7). Recently, a technique dubbed actual flip angle imaging has been introduced where two images are acquired after excitations with the same flip angle a but different repetition times TR 1 and TR 2 (8). An appropriate choice of TR 1 and TR 2 removes the T 1 -sensitivity of ratio between the two signals, which therefore only depends on B þ 1 and TR 2 /TR 1 . Another method uses the 180 signal null to build a flip angle map (9). An innovative approach consists in the measurement of B þ 1 from the phase of the MR signal (10,11). Recently, with the availability of higher magnetic fields (implying increased B þ 1 inhomogeneity) and multiple transmit channels, B þ 1 measurements have become increasingly important, and many new methodologies have been proposed (12-18).One of the main challenges for any B þ 1 -mapping strategy to be used in the context of high static magnetic fields and parallel transmission is the range in which the measured B þ 1 is accurate and precise. A B þ 1 mapping methodology should have a large range where the assumptions made to calculate B þ 1 are valid, and the B þ 1 calculation should in that ran...
Purpose: At high magnetic field strengths (B 0 ! 3 T), the shorter radiofrequency wavelength produces an inhomogeneous distribution of the transmit magnetic field. This can lead to variable contrast across the brain which is particularly pronounced in T 2 -weighted imaging that requires multiple radiofrequency pulses. To obtain T 2 -weighted images with uniform contrast throughout the whole brain at 7 T, short (2-3 ms) 3D tailored radiofrequency pulses (k T -points) were integrated into a 3D variable flip angle turbo spin echo sequence. Methods: The excitation and refocusing "hard" pulses of a variable flip angle turbo spin echo sequence were replaced with k T -point pulses. Spatially resolved extended phase graph simulations and in vivo acquisitions at 7 T, utilizing both single channel and parallel-transmit systems, were used to test different k T -point configurations. Results: Simulations indicated that an extended optimized k-space trajectory ensured a more homogeneous signal throughout images. In vivo experiments showed that high quality T 2 -weighted brain images with uniform signal and contrast were obtained at 7 T by using the proposed methodology. Conclusion: This work demonstrates that T 2 -weighted images devoid of artifacts resulting from B 1 þ inhomogeneity can be obtained at high field through the optimization of extended k Tpoint pulses.
pulses was demonstrated via simulations. Images acquired with dynamic k T -points showed systematic improvement of signal and contrast at 7T over regular TSE-especially in cerebellar and temporal lobe regions without the need of parallel transmission. Conclusion Designing dynamic k T -points for a 3D TSE sequence allows the acquisition of T 2 -weighted brain images on a single-transmit system at ultra-high field with reduced dropout and only mild residual effects due to the B 1 + inhomogeneity.
With the increasing development of transgenic mouse models of neurodegenerative diseases allowing improved understanding of the underlying mechanisms of these disorders, robust quantitative mapping techniques are also needed in rodents. MP2RAGE has shown great potential for structural imaging in humans at high fields. In the present work, MP2RAGE was successfully implemented at 9.4T and 14.1T. Following fractionated injections of MnCl, MP2RAGE images were acquired allowing simultaneous depiction and T mapping of structures in the mouse brain at both fields. In addition, T maps demonstrated significant T shortenings in different structures of the mouse brain (p < 0.0008 at 9.4T, p < 0.000001 at 14.1T). T values recovered to the levels of saline-injected animals 1 month after the last injection except in the pituitary gland. We believe that MP2RAGE represents an important prospective translational tool for further structural MRI.
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