Understanding and manipulating the RF fields at high field MRI

Ibrahim, Tamer S.; Hue, Yik-Kiong; Tang, Lin

doi:10.1002/nbm.1406

Cited by 48 publications

(56 citation statements)

References 36 publications

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“…Sample-induced inhomogeneities in the distribution of electromagnetic (EM) energy for human MRI at high magnetic fields are well documented [1][2][3][4][5][6][7]. These inhomogeneities arise primarily from the dielectric properties of tissue, which result in partial constructive and destructive RF interactions from complex wave behavior.…”

Section: Introductionmentioning

confidence: 98%

New high dielectric constant materials for tailoring theB1+distribution at high magnetic fields

Haines

Smith

Webb

2010

Journal of Magnetic Resonance

121

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Section: Introductionmentioning

confidence: 98%

New high dielectric constant materials for tailoring theB1+distribution at high magnetic fields

Haines

Smith

Webb

2010

Journal of Magnetic Resonance

121

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“…1d). The FDTD package incorporates true transmission line modeling for capturing the mechanism of excitation and reception, and, therefore, accurately predicting coil input impedance and coupling between coil elements [27,28,36,37]. …”

Section: Methodsmentioning

confidence: 99%

A new RF transmit coil for foot and ankle imaging at 7T MRI

Santini

Kim

Wood

et al. 2018

Magnetic Resonance Imaging

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A four-channel Tic-Tac-Toe (TTT) transmit RF coil was designed and constructed for foot and ankle imaging at 7T MRI. Numerical simulations using an in-house developed FDTD package and experimental analyses using a homogenous phantom show an excellent agreement in terms of B1 + field distribution and s-parameters. Simulations performed on an anatomically detailed human lower leg model demonstrated an B1+ field distribution with a coefficient of variation (CV) of 23.9%/15.6%/28.8% and average B1 + of 0.33 μT/0.56 μT/0.43 μT for 1 W input power (i.e., 0.25 W per channel) in the ankle/calcaneus/mid foot respectively. In-vivo B1 + mapping shows an average B1 + of 0.29 μT over the entire foot/ankle. This newly developed RF coil also presents acceptable levels of average SAR (0.07 W/kg for 10 g per 1 W of input power) and peak SAR (0.34 W/kg for 10 g per 1 W of input power) over the whole lower leg. Preliminary in-vivo images in the foot/ankle were acquired using the T2-DESS MRI sequence without the use of a dedicated receive-only array.

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“…An in-house finite difference time domain [FDTD] package was utilized [6, 33] in order to calculate the B 1 + field [transmit field responsible for excitation] and specific absorption rate [34] parameters. An eight channel receive-only insert array was designed and utilized with the TEM coil [26].…”

Section: Methodsmentioning

confidence: 99%

Ultra-high field upper extremity peripheral nerve and non-contrast enhanced vascular imaging

et al. 2017

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ObjectiveThe purpose of this study was to explore the efficacy of Ultra-high field [UHF] 7 Tesla [T] MRI as compared to 3T MRI in non-contrast enhanced [nCE] imaging of structural anatomy in the elbow, forearm, and hand [upper extremity].Materials and methodA wide range of sequences including T1 weighted [T1] volumetric interpolate breath-hold exam [VIBE], T2 weighted [T2] double-echo steady state [DESS], susceptibility weighted imaging [SWI], time-of-flight [TOF], diffusion tensor imaging [DTI], and diffusion spectrum imaging [DSI] were optimized and incorporated with a radiofrequency [RF] coil system composed of a transverse electromagnetic [TEM] transmit coil combined with an 8-channel receive-only array for 7T upper extremity [UE] imaging. In addition, Siemens optimized protocol/sequences were used on a 3T scanner and the resulting images from T1 VIBE and T2 DESS were compared to that obtained at 7T qualitatively and quantitatively [SWI was only qualitatively compared]. DSI studio was utilized to identify nerves based on analysis of diffusion weighted derived fractional anisotropy images. Images of forearm vasculature were extracted using a paint grow manual segmentation method based on MIPAV [Medical Image Processing, Analysis, and Visualization].ResultsHigh resolution and high quality signal-to-noise ratio [SNR] and contrast-to-noise ratio [CNR]—images of the hand, forearm, and elbow were acquired with nearly homogeneous 7T excitation. Measured [performed on the T1 VIBE and T2 DESS sequences] SNR and CNR values were almost doubled at 7T vs. 3T. Cartilage, synovial fluid and tendon structures could be seen with higher clarity in the 7T T1 and T2 weighted images. SWI allowed high resolution and better quality imaging of large and medium sized arteries and veins, capillary networks and arteriovenous anastomoses at 7T when compared to 3T. 7T diffusion weighted sequence [not performed at 3T] demonstrates that the forearm nerves are clearly delineated by fiber tractography. The proper digital palmar arteries and superficial palmar arch could also be clearly visualized using TOF nCE 7T MRI.ConclusionUltra-high resolution neurovascular imaging in upper extremities is possible at 7T without use of renal toxic intravenous contrast. 7T MRI can provide superior peripheral nerve [based on fiber anisotropy and diffusion coefficient parameters derived from diffusion tensor/spectrum imaging] and vascular [nCE MRA and vessel segmentation] imaging.

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Understanding and manipulating the RF fields at high field MRI

Cited by 48 publications

References 36 publications

New high dielectric constant materials for tailoring theB1+distribution at high magnetic fields

New high dielectric constant materials for tailoring theB1+distribution at high magnetic fields

A new RF transmit coil for foot and ankle imaging at 7T MRI

Ultra-high field upper extremity peripheral nerve and non-contrast enhanced vascular imaging

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