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Purpose Specific absorption rate is a serious problem at high field strengths, especially for sequences involving many high power radiofrequency pulses, such as turbo spin echo (TSE). GRASE (gradient and spin echo) may overcome this problem by omitting a certain number of refocusing pulses of a TSE sequence, and replacing them with segmented echo-planar imaging readouts. Methods GRASE and TSE were compared using similar sequence parameters at a field strength of 7T. The signal-to-noise ratio (SNR) per unit time, contrast, and point spread function (PSF) were determined. High-resolution human brain images were acquired and the implementation of an inversion recovery preparation for T1 weighting was evaluated. Results TSE and GRASE images at 7T showed very similar SNR and contrast. The slightly worse PSF for GRASE is balanced by a significant reduction in scan time or increase in spatial coverage compared with TSE. Furthermore, implementing an additional inversion recovery preparation enables the acquisition of T1-weighted images with high SNR per unit time. Conclusion GRASE is highly suitable for structural scanning at ultra-high field strengths and is a valid alternative to the commonly used TSE sequence. Magn Reson Med 72:1291–1301, 2014. © 2013 Wiley Periodicals, Inc.

Purpose MRI methods sensitive to functional changes in cerebral blood volume (CBV) may map neural activity with better spatial specificity than standard functional MRI (fMRI) methods based on blood oxygen level dependent (BOLD) effect. The purpose of this study was to develop and investigate a vascular space occupancy (VASO) method with high sensitivity to CBV changes for use in human brain at 7 Tesla (T). Methods To apply 7T VASO, several high-field-specific obstacles must be overcome, e.g., low contrast-to-noise ratio (CNR) due to convergence of blood and tissue T1, increased functional BOLD signal change contamination, and radiofrequency field inhomogeneities. In the present method, CNR was increased by keeping stationary tissue magnetization in a steady-state different from flowing blood, using slice-selective saturation pulses. Interleaved acquisition of BOLD and VASO signals allowed correction for BOLD contamination. Results During visual stimulation, a relative CBV change of 28% ± 5% was measured, confined to gray matter in the occipital lobe with high sensitivity. Conclusion By carefully considering all the challenges of high-field VASO and filling behavior of the relevant vasculature, the proposed method can detect and quantify CBV changes with high CNR in human brain at 7T. Magn Reson Med 72:137–148, 2014. © 2013 Wiley Periodicals, Inc.

Purpose A novel highly accurate method for MR thermometry, effective at high field, is introduced and validated, which corrects for slow and fast field fluctuations by means of reference images. Methods An asymmetric spin-echo echo planar imaging sequence was made frequency-selective to water or a reference substance by controlling the slice-select gradient polarity and the duration of the excitation and refocusing radiofrequency pulses. Images were acquired pairwise, and the temperature-sensitive water images were corrected for field fluctuations using the reference images. In a phantom radiofrequency heating experiment, dissolved dimethyl sulfoxide was used as a reference substance. Temperature stability was tested in vivo on the human brain, referenced using subcutaneous scalp fat. Water and fat phase images were acquired only 50 ms apart. Bloch simulations validated the frequency selection accuracy. Results Asymmetric spin-echo imaging using a simple frequency selection method provides highly accurate referenced MR thermometry in phantoms and in vivo at 7 T. Effects of field fluctuations caused by field drift, breathing, and heart beat were corrected. The technique is highly robust against B1 inhomogeneities. Conclusion Frequency selection using gradient-reversal can enable fast accurate referenced in vivo MR thermometry, assisting thermal characterization of radiofrequency coils and possibly in vivo SAR monitoring. Magn Reson Med 71:524–533, 2014. © 2013 Wiley Periodicals, Inc.

Echo-planar imaging is the most widely used imaging sequence for functional magnetic resonance imaging (fMRI) due to its fast acquisition. However, it is prone to local distortions, image blurring, and signal voids. As these effects scale with echo train length and field strength, it is essential for high-resolution echo-planar imaging at ultrahigh field to address these problems. Partially parallel acquisition methods can be used to improve the image quality of echo-planar imaging. However, partially parallel acquisition can be affected by aliasing artifacts and noise enhancement. Another way to shorten the echo train length is to reduce the field-of-view (FOV) while maintaining the same spatial resolution. However, to achieve significant acceleration, the resulting FOV becomes very small. Another problem occurs when FOV selection is incomplete such that there is remaining signal aliased from the region outside the reduced FOV. In this article, a novel approach, a combination of reduced FOV imaging with partially parallel acquisition, is presented. This approach can address the problems described above of each individual method, enabling high-quality single-shot echo-planar imaging acquisition, with submillimeter isotropic resolution and good signal-to-noise ratio, for fMRI at ultrahigh field strength. This is demonstrated in fMRI of human brain at 7T with an isotropic resolution of 650 mu m. Magn Reson Med, 2012. (c) 2012 Wiley Periodicals, Inc.

We have discovered a simple and highly robust method for removal of chemical shift artifact in spin-echo MR images, which simultaneously decreases the radiofrequency power deposition (specific absorption rate). The method is demonstrated in spin-echo echo-planar imaging brain images acquired at 7 T, with complete suppression of scalp fat signal. When excitation and refocusing pulses are sufficiently different in duration, and thus also different in the amplitude of their slice-select gradients, a spatial mismatch is produced between the fat slices excited and refocused, with no overlap. Because no additional radiofrequency pulse is used to suppress fat, the specific absorption rate is significantly reduced compared with conventional approaches. This enables greater volume coverage per unit time, well suited for functional and diffusion studies using spin-echo echo-planar imaging. Moreover, the method can be generally applied to any sequence involving slice-selective excitation and at least one slice-selective refocusing pulse at high magnetic field strengths. The method is more efficient than gradient reversal methods and more robust against inhomogeneities of the static (polarizing) field (B0). Magn Reson Med, 2010. © 2010 Wiley-Liss, Inc.

PURPOSE: To overcome limitations of previous ultra-high-field arterial spin labeling (ASL) techniques concerning temporal resolution and brain coverage by utilizing the simultaneous multi-slice (SMS) approach. METHODS: An optimized, flow-alternating inversion recovery quantitative imaging of perfusion using a single subtraction II scheme was developed that tackles the challenges of 7 tesla (T) ASL. The implementation of tailored labeling radiofrequency pulses reduced the effect of transmit field ( B1+) inhomogeneities. The proposed approach utilizes an SMS echo-planar imaging (EPI) readout to efficiently achieve large brain coverage. RESULTS: A pulsed ASL (PASL) technique with large brain coverage is described and optimized that can be applied at temporal resolutions below 2.5 s, similar to those achievable at 1.5 and 3T magnetic field strength. The influences of within- and through-slice acceleration factors and reconstruction parameters on perfusion and blood-oxygenation-level-dependent (BOLD)-signal image and temporal signal-to-noise ratio (SNR) are presented. The proposed approach yielded twice the brain coverage as compared to conventional PASL at 7T, without notable loss in image quality. CONCLUSION: The presented SMS EPI PASL at 7T overcomes current limitations in SNR, temporal resolution, and spatial coverage for functional perfusion and BOLD signal as well as baseline perfusion measurements. Magn Reson Med, 2016. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

A novel highly accurate method for MR thermometry, effective at high field, is introduced and validated, which corrects for slow and fast field fluctuations by means of reference images.An asymmetric spin-echo echo planar imaging sequence was made frequency-selective to water or a reference substance by controlling the slice-select gradient polarity and the duration of the excitation and refocusing radiofrequency pulses. Images were acquired pairwise, and the temperature-sensitive water images were corrected for field fluctuations using the reference images. In a phantom radiofrequency heating experiment, dissolved dimethyl sulfoxide was used as a reference substance. Temperature stability was tested in vivo on the human brain, referenced using subcutaneous scalp fat. Water and fat phase images were acquired only 50 ms apart. Bloch simulations validated the frequency selection accuracy.Asymmetric spin-echo imaging using a simple frequency selection method provides highly accurate referenced MR thermometry in phantoms and in vivo at 7 T. Effects of field fluctuations caused by field drift, breathing, and heart beat were corrected. The technique is highly robust against B1 inhomogeneities.Frequency selection using gradient-reversal can enable fast accurate referenced in vivo MR thermometry, assisting thermal characterization of radiofrequency coils and possibly in vivo SAR monitoring.

Echo-planar imaging is the most widely used imaging sequence for functional magnetic resonance imaging (fMRI) due to its fast acquisition. However, it is prone to local distortions, image blurring, and signal voids. As these effects scale with echo train length and field strength, it is essential for high-resolution echo-planar imaging at ultrahigh field to address these problems. Partially parallel acquisition methods can be used to improve the image quality of echo-planar imaging. However, partially parallel acquisition can be affected by aliasing artifacts and noise enhancement. Another way to shorten the echo train length is to reduce the field-of-view (FOV) while maintaining the same spatial resolution. However, to achieve significant acceleration, the resulting FOV becomes very small. Another problem occurs when FOV selection is incomplete such that there is remaining signal aliased from the region outside the reduced FOV. In this article, a novel approach, a combination of reduced FOV imaging with partially parallel acquisition, is presented. This approach can address the problems described above of each individual method, enabling high-quality single-shot echo-planar imaging acquisition, with submillimeter isotropic resolution and good signal-to-noise ratio, for fMRI at ultrahigh field strength. This is demonstrated in fMRI of human brain at 7T with an isotropic resolution of 650 μm.