Human magnetic resonance imaging at 8 T

Robitaille, Pierre‐Marie; Abduljalil, Amir M.; Kangarlu, Alayar; Zhang, X.; Yu, Ying; Burgess, Richard; Bair, S.; Noa, P.J.; Yang, Lining; Zhu, Honglin; Palmer, B. S.; Jiang, Zongchen; Chakeres, Donald W.; Spigos, Dimitrios G.

doi:10.1002/(sici)1099-1492(199810)11:6<263::aid-nbm549>3.0.co;2-0

Cited by 134 publications

(90 citation statements)

References 9 publications

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“…The RF power absorbed by the biological tissue increases with static magnetic field. Theory predicts a square law dependence of SAR on field strength at lower strength values and a slightly weakened dependence at higher values, which has also been verified experimentally [10][11][12]. Among many methods to reduce SAR, some particularly relevant to echo-train imaging sequences such as the turbo spin-echo (TSE) include: lowering the flip angle of the refocusing pulses and hence reducing the power deposited by them [13,14], stretching the rf pulses, reshaping [15] and modulating the refocusing flip angle throughout the echo train [16].…”

Section: Specific Absorption Ratio (Sar)supporting

confidence: 62%

High-Resolution Black-Blood MRI of the Carotid Vessel Wall Using Phased-Array Coils at 1.5 and 3 Tesla1

et al. 2005

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Purpose-To investigate the magnetic field dependence of the signal-to-noise ratio (SNR) for carotid vessel wall magnetic resonance imaging (MRI) using phased-array (PA) surface coils by comparing images obtained at 1.5 T and 3 T, and to determine to what extent the improved SNR at the higher field can be traded for improved spatial resolution.Materials and methods-Two pairs of dual-element PA coils were constructed for operation at the two field strengths. The individual elements of each PA were matched to 50 Ω impedance on the neck and tuned at the respective frequencies. The coils were evaluated on a cylindrical phantom positioned with its axis parallel to the main field, and with the coils placed on either side of the phantom parallel to the sagittal plane. In-vivo MR images of the carotid arteries were obtained in five subjects at both field strengths with a fast spin-echo double-inversion black-blood pulse sequence with fat saturation. SNR was measured at both field strengths using standard techniques.Results-At a depth corresponding to the average location of the carotid arteries in the study subjects, mean phantom SNR for the two coils was higher at 3 T by a factor of 2.5. The greater than linear increase is due to only partial coil loading of these relatively small coils. The practically achievable average SNR gain in vivo was 2.1. The lower in vivo SNR gain is attributed to a reduction in T 2 and prolongation of T 1 at the higher field strength and, to a lesser extent, the requirement for reduced refocusing pulse flip angle to operate within specific absorption ratio limits. The superior SNR at 3 T appears to provide considerably improved vessel wall delineation.Conclusions-Carotid artery vessel wall MRI using phased-array surface coils provides a considerable increase in SNR when field strength is raised from 1.5 T to 3 T. This increase can be traded for enhanced in-plane resolution.

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Section: Specific Absorption Ratio (Sar)supporting

confidence: 62%

High-Resolution Black-Blood MRI of the Carotid Vessel Wall Using Phased-Array Coils at 1.5 and 3 Tesla1

et al. 2005

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“…This MR system uses Paravison acquisition and routine imaging processing software (1)(2)(3)(4)(5)(6). A 16-strut transverse electromagnetic (TEM) RF coil 21 cm long was used in four-port excitation configuration for imaging and temperature experiments.…”

Section: -T Human Mr Systemmentioning

confidence: 99%

8.0‐Tesla human MR system: Temperature changes associated with radiofrequency‐induced heating of a head phantom

Kangarlu

Shellock

Chakeres

2003

Magnetic Resonance Imaging

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Purpose: To investigate if the heat induced in biological tissues by typical radio frequency (RF) energy associated with an 8.0-Tesla magnetic resonance (MR) system causes excessive temperature changes. Materials and Methods:Fluoroptic thermometry was used to measure temperatures in multiple positions in a head phantom made of ground turkey breast. A series of experiments were conducted with measurements obtained at RF power levels ranging from a specific absorption rate (SAR) of up to 4.0 W/kg for 10 minutes. Results:The highest temperature increases were up to 0.7°C. An inhomogeneous heating pattern was observed. In general, the deep regions within the phantom registered higher temperature increases compared to the peripheral sites. Conclusion:The expectation of an inhomogeneous RF distribution in ultra high field systems (Ͼ 4 T) was confirmed. At a frequency of 340 MHz and in-tissue RF wave length of about 10 cm, the RF inhomogeneity was measured to create higher temperatures in deeper regions of a human head phantom compared to peripheral tissues. Our results agree with the computational electromagnetic calculations for such frequencies. Importantly, these experiments indicated that there were no regions of heating that exceeded the current FDA guidelines.

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“…Recently, two human systems operating at fields higher than 4 T became available. The 8 T system has generated a great deal of interesting data (22), mostly in anatomic imaging. With the 7 T whole-body system in our laboratory, we have begun experimental fMRI studies in humans for the first time at magnetic field strengths that significantly exceed 4 T. The experiments were conducted using a surface coil with a visual stimulation paradigm.…”

mentioning

confidence: 99%

Imaging brain function in humans at 7 Tesla

Yacoub

Shmuel

Pfeuffer

et al. 2001

Magnetic Resonance in Med

411

333

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This article describes experimental studies performed to demonstrate the feasibility of BOLD fMRI using echo-planar imaging (EPI) at 7 T and to characterize the BOLD response in humans at this ultrahigh magnetic field. Visual stimulation studies were performed in normal subjects using high-resolution multishot EPI sequences. Changes in R* 2 arising from visual stimulation were experimentally determined using fMRI measurements obtained at multiple echo times. The results obtained at 7 T were compared to those at 4 T. Experimental data indicate that fMRI can be reliably performed at 7 T and that at this field strength both the sensitivity and spatial specificity of the BOLD response are increased. This study suggests that ultrahigh field MR systems are advantageous for functional mapping in humans. Magn Reson Med 45:588 -594, 2001.

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Human magnetic resonance imaging at 8 T

Cited by 134 publications

References 9 publications

High-Resolution Black-Blood MRI of the Carotid Vessel Wall Using Phased-Array Coils at 1.5 and 3 Tesla1

High-Resolution Black-Blood MRI of the Carotid Vessel Wall Using Phased-Array Coils at 1.5 and 3 Tesla1

8.0‐Tesla human MR system: Temperature changes associated with radiofrequency‐induced heating of a head phantom

Imaging brain function in humans at 7 Tesla

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