Reducing SAR in 7T brain fMRI by circumventing fat suppression while removing the lipid signal through a parallel acquisition approach

Seginer, Amir; Furman‐Haran, Edna; Goldberg, Ilan; Schmidt, Rita

doi:10.1038/s41598-021-94692-6

Cited by 4 publications

(6 citation statements)

References 49 publications

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“…An alternative low-SAR fat saturation approach that leverages parallel acceleration techniques has been suggested and verified in echo planar head imaging. 43 …”

Section: Technological Challenges and Image Optimizationmentioning

confidence: 99%

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7 T Musculoskeletal MRI

2022

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This review summarizes the current state-of-the-art of musculoskeletal 7 T magnetic resonance imaging (MRI), the associated technological challenges, and gives an overview of current and future clinical applications of 1H-based 7 T MRI. The higher signal-to-noise ratio at 7 T is predominantly used for increased spatial resolution and thus the visualization of anatomical details or subtle lesions rather than to accelerate the sequences. For musculoskeletal MRI, turbo spin echo pulse sequences are particularly useful, but with altered relaxation times, B1 inhomogeneity, and increased artifacts at 7 T; specific absorption rate limitation issues quickly arise for turbo spin echo pulse sequences. The development of dedicated pulse sequence techniques in the last 2 decades and the increasing availability of specialized coils now facilitate several clinical musculoskeletal applications. 7 T MRI is performed in vivo in a wide range of applications for the knee joint and other anatomical areas, such as ultra-high-resolution nerve imaging or bone trabecular microarchitecture imaging. So far, however, it has not been shown systematically whether the higher field strength compared with the established 3 T MRI systems translates into clinical advantages, such as an early-stage identification of tissue damage allowing for preventive therapy or an influence on treatment decisions and patient outcome. At the moment, results tend to suggest that 7 T MRI will be reserved for answering specific, targeted musculoskeletal questions rather than for a broad application, as is the case for 3 T MRI. Future data regarding the implementation of clinical use cases are expected to clarify if 7 T musculoskeletal MRI applications with higher diagnostic accuracy result in patient benefits compared with MRI at lower field strengths.

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“…An alternative low-SAR fat saturation approach that leverages parallel acceleration techniques has been suggested and verified in echo planar head imaging. 43 …”

Section: Technological Challenges and Image Optimizationmentioning

confidence: 99%

“…3), at the cost of some reduction of the water signal, in particular in areas of B0 inhomogeneity. An alternative low-SAR fat saturation approach that leverages parallel acceleration techniques has been suggested and verified in echo planar head imaging 43 …”

Section: Technological Challenges and Image Optimizationmentioning

confidence: 99%

7 T Musculoskeletal MRI

2022

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“…This MUSE‐like water/fat‐resolved algorithm will be referred to as “water‐fat MUSE”. Water/fat components can be disentangled by solving the SENSE‐based water/fat separation 43–45 using a similar system as in Equation 3), dropping B 0 (

{\overset{truê}{Ψ}}_{B}

) and diffusion phase (

\overset{Φ}{true}

) operators and calculating water/fat images for each chemical shift‐encoding point

n

and each shot

l

. Solving for

N

chemical shift‐encoding points and

L

shots data simultaneously, the joint system can be constructed as:

\begin{matrix} true \\ S = [\overset{truê}{normalI} \overset{truê}{normalI}] [\begin{matrix} \overset{F}{true} \\ 0 \end{matrix} \begin{matrix} 0 \\ {\overset{truê}{normalΨ}}_{f} \overset{F}{true} \end{matrix}] [\begin{matrix} {\overset{truê}{normalC}}_{l} \\ 0 \end{matrix} \begin{matrix} 0 \\ {\overset{truê}{normalC}}_{l} \end{matrix}] [\begin{matrix} {\overset{true\sim}{normalΡ}}_{w} \\ {\overset{true\sim}{normalΡ}}_{f} \end{matrix}] = {\overset{A}{true}}_{normall} \overset{true\sim}{normalX}, \end{matrix}

where the coil sensitivity operator

{\overset{truê}{C}}_{l}

is slightly modified to disable the shot combination step (i.e., every shot data will be treated as an independent undersampled case),

\overset{X}{true} = [{\overset{true\sim}{Ρ}}_{w}, {\overset{true\sim}{Ρ}}_{f}]

…”

Section: Theorymentioning

confidence: 99%

“…However, in EPI images without fat suppression, the spatial displacement of fat signals should also be addressed for proper phase extraction. One solution is to use a sensitivity encoding for fast MRI (SENSE)‐based water/fat separation 43 , 44 , 45 instead of conventional SENSE in the MUSE implementation. This MUSE‐like water/fat‐resolved algorithm will be referred to as “water‐fat MUSE”.…”

Section: Theorymentioning

confidence: 99%

Water/fat separation for self‐navigated diffusion‐weighted multishot echo‐planar imaging

Dong

Riedel

Koolstra³

et al. 2022

NMR in Biomedicine

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The purpose of this study was to develop a self-navigation strategy to improve scan efficiency and image quality of water/fat-separated, diffusion-weighted multishot echo-planar imaging (ms-EPI). This is accomplished by acquiring chemical shiftencoded diffusion-weighted data and using an appropriate water-fat and diffusionencoded signal model to enable reconstruction directly from k-space data. Multishot EPI provides reduced geometric distortion and improved signal-to-noise ratio in diffusion-weighted imaging compared with single-shot approaches. Multishot acquisitions require corrections for physiological motion-induced shot-to-shot phase errors using either extra navigators or self-navigation principles. In addition, proper fat suppression is important, especially in regions with large B 0 inhomogeneity. This makes the use of chemical shift encoding attractive. However, when combined with ms-EPI, shot-to-shot phase navigation can be challenging because of the spatial displacement of fat signals along the phase-encoding direction. In this work, a new model-based, self-navigated water/fat separation reconstruction algorithm is proposed. Experiments in legs and in the head-neck region of 10 subjects were performed to validate the algorithm. The results are compared with an image-based, two-dimensional (2D) navigated water/fat separation approach for ms-EPI and with a conventional fat saturation approach. Compared with the 2D navigated method, the use of self-navigation reduced the shot duration time by 30%-35%. The proposed algorithm provided improved diffusion-weighted water images in both leg and head-neck regions compared with the 2D navigator-based approach. The proposed algorithm also produced better fat suppression compared with the conventional fat saturation technique in the B 0 inhomogeneous regions. In conclusion, the proposed self-navigated reconstruction algorithm can produce superior water-only diffusionweighted EPI images with less artefacts compared with the existing methods.

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“…However, increasing the frequency of operation signifies that the wavelength is reduced, which, in combination with the frequency dependency of the electrical properties of materials [ 4 , 5 , 6 ], reduces the effectiveness of the RF transmission (|B 1 + |) field inside the imaging target [ 7 , 8 , 9 , 10 , 11 ]. This results in field non-uniformity and the production of local hot spots of absorbed energy, as measured by the specific absorption rate (SAR) [ 12 , 13 , 14 ].…”

Section: Introductionmentioning

confidence: 99%

Study on the Effect of Non-Symmetrical Current Distribution Controlled by Capacitor Placement in Radio-Frequency Coils for 7T MRI

et al. 2022

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In this paper, we present a study on the effects of varying the position of a single tuning capacitor in a circular loop coil as a mechanism to control and produce non-symmetric current distribution, such that could be used for magnetic resonance imaging (MRI) operating at ultra-high frequency (UHF). This study aims to demonstrate that the position of the tuning capacitor of a circular loop could improve the coupling between adjacent coils, used to optimize transmission field uniformity or intensity, improve signal-to-noise ratio (SNR) or specific absorption rate (SAR). A typical loop coil used in MRI consists of symmetrically distributed capacitors along the coil; this design is able to produce uniform current distributions inside the coil. However, in UHF conditions, the magnetic flux density (|B1+|) field produced by this setup may exhibit field distortion, requiring a method of controlling the field distribution and improving the field intensity of the circular loop coil. The control mechanism investigated in this study is based on the position of the tuning capacitor in the circular coil, the capacitor position was varied from 15° to 345°, in steps of 15°. We performed electromagnetic (EM) simulations, fabricated the coils, and performed MRI experiments at 7T, with each of the coils with capacitor position from 15° to 345° to determine the effects on field intensity, coupling between adjacent coils, SAR, and applications for field uniformity optimization. For the case of free space, a coil with capacitor position at 15° showed higher field intensity compared to the reference coil; while an improved decoupling was achieved when a coil had the capacitor placed at 180° and the other coil at 90°; in a similar matter, we discuss the results for SAR, field uniformity and an application with an array coil for the spinal cord.

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Reducing SAR in 7T brain fMRI by circumventing fat suppression while removing the lipid signal through a parallel acquisition approach

Cited by 4 publications

References 49 publications

7 T Musculoskeletal MRI

7 T Musculoskeletal MRI

Water/fat separation for self‐navigated diffusion‐weighted multishot echo‐planar imaging

Study on the Effect of Non-Symmetrical Current Distribution Controlled by Capacitor Placement in Radio-Frequency Coils for 7T MRI

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