<i>In vivo</i> magnetic resonance imaging and spectroscopy of humans with a 4 t whole‐body magnet

Barfuss, H.; Fischer, H.; Hentschel, D.; Ladebeck, Ralf; Oppelt, A; Wittig, Roland; Duerr, W.; Oppelt, Ralph

doi:10.1002/nbm.1940030106

Cited by 110 publications

(80 citation statements)

References 26 publications

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“…Therefore, wave superposition effects are expected to contribute to the B 1 field distribution, as has been observed previously at high field (18,19). The distribution of the B 1 field in the human head at 4.7 T is shown in Fig.…”

Section: B 1 Field Mapssupporting

confidence: 65%

High‐resolution fast spin echo imaging of the human brain at 4.7 T: Implementation and sequence characteristics

Thomas

Vita

Roberts³

et al. 2004

Magnetic Resonance in Med

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In this work, a number of important issues associated with fast spin echo (FSE) imaging of the human brain at 4.7 T are addressed. It is shown that FSE enables the acquisition of images with high resolution and good tissue contrast throughout the brain at high field strength. By employing an echo spacing (ES) of 22 ms, one can use large flip angle refocusing pulses (162°) and a low acquisition bandwidth (50 kHz) to maximize the signal-to-noise ratio (SNR). A new method of phase encode (PE) ordering (called "feathering") designed to reduce image artifacts is described, and the contributions of RF (B 1 ) inhomogeneity, different echo coherence pathways, and magnetization transfer (MT) to FSE signal intensity and contrast are investigated. B 1 inhomogeneity is measured and its effect is shown to be relatively minor for high-field FSE, due to the self-compensating characteristics of the sequence. Thirty-four slice data sets (slice thickness ‫؍‬ 2 Over recent years, the use of high-field MR scanners for imaging the human body has become increasingly widespread, due to the improvements they offer in signal-tonoise ratio (SNR) and susceptibility contrast (1,2). These benefits can be exploited to allow 1) the acquisition of MR images with higher spatial resolution and thus greater anatomical detail, 2) more robust detection of susceptibility-related changes in signal intensity (e.g., BOLD contrast in functional MRI (fMRI)), and 3) easier discrimination of metabolite signals in MR spectra, due to increased dispersion as well as higher SNR.Along with the advantages of high-field MR systems, there are also associated problems, ranging from technical challenges to safety concerns. At higher fields, the wavelength of the RF excitation pulses approaches the dimensions of the object being imaged, causing inhomogeneities in the RF (B 1 ) field via RF focusing effects (3). As a result, the RF flip angle varies over the imaging field of view (FOV), potentially leading to variations in both signal intensity and contrast across images. In addition, the RF power required for spin excitation increases with the magnetic field strength. This means, for some RF-intensive pulse sequences, that the power deposition may exceed permissible specific absorption rate (SAR) safety limits.In this work, we describe the implementation and application of a standard clinical MRI pulse sequence on a high-field 4.7-T whole-body system. The pulse sequence we implemented is fast spin echo (FSE) (4,5). FSE is used clinically as an efficient method for obtaining high-quality T 2 -weighted images. However, the extension to high field is not necessarily straightforward, due to the aforementioned issues of B 1 inhomogeneity and SAR power restrictions. In a recent study, we presented our initial results concerning FSE imaging at 4.7 T (6). Here we investigate in more detail the factors involved in using FSE as a technique for imaging the human brain at 4.7 T. To do this, we assess the extent of B 1 variation within the brain and its effect on image signal in...

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Section: B 1 Field Mapssupporting

confidence: 65%

High‐resolution fast spin echo imaging of the human brain at 4.7 T: Implementation and sequence characteristics

Thomas

Vita

Roberts³

et al. 2004

Magnetic Resonance in Med

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“…In addition, it has been shown that the homogeneity of the RF magnetic field (B 1 ) will progressively decrease in large samples with increased RF frequency. This is mainly caused by the dielectric resonance phenomenon (1). Birdcage (2), transverse electromagnetic (TEM) (3), and microstrip (4) coils are three well-known kinds of volume coils for MRI.…”

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confidence: 99%

B₁ field, SAR, and SNR comparisons for birdcage, TEM, and microstrip coils at 7T

Wang

Shen

2006

Magnetic Resonance Imaging

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Purpose:To compare the performance of birdcage, transverse electromagnetic (TEM) and microstrip volume coils at 7T under the same geometric conditions. Materials and Methods: Birdcage, TEM, and microstrip coils are modeled with the same dimensions. The finite difference time domain (FDTD) method is adopted to calculate the electromagnetic fields of the coils. Further, B 1 field, specific absorption rate (SAR) and signal-to-noise ratio (SNR) are calculated for these coils. Results:In the unloaded case, within the central axial plane, the variation of B 1 field magnitude over 18-cm distance is about 15% for the birdcage coil, 23% for the TEM coil, and 38% for the microstrip coil. In the loaded case, the percentages of the samples on the central axial plane, which have B 1 field magnitude within Ϯ20% of the average B 1 field magnitude, are about 57% for the birdcage, 72% for the TEM, and 59% for the microstrip coil. Average SAR levels are 11.4% and 42.9% higher in the birdcage than those in the TEM and microstrip coils, respectively. The average relative SNR on the central axial plane for the shielded birdcage, TEM, and microstrip coils are 1, 1.07, and 1.48, respectively. Conclusion:The birdcage coil has the best unloaded B 1 field homogeneity, and the TEM coil has the best loaded B 1 field homogeneity and the lowest radiation loss; while the microstrip coil is better in SAR and SNR at 7T than the birdcage and TEM coils.

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“…Human tissue is not very magnetic (i.e., it has a susceptibility of | |Ͻ Ͻ1), so from Eq. [5] we can assume that r Ϸ 1. Equation [8] …”

Section: Rf Wavelengthmentioning

confidence: 99%

Imaging artifacts at 3.0T

Bernstein

Huston

Ward

2006

Magnetic Resonance Imaging

230

214

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Clinical MRI at a field strength of 3.0T is finding increasing use. However, along with the advantages of 3.0T, such as increased SNR, there can be drawbacks, including increased levels of imaging artifacts. Although every imaging artifact observed at 3.0T can also be present at 1.5T, the intensity level is often higher at 3.0T and thus the artifact is more objectionable. This review describes some of the imaging artifacts that are commonly observed with 3.0T imaging, and their root causes. When possible, countermeasures that reduce the artifact level are described. J. Magn. Reson. Imaging 2006. © 2006 Wiley‐Liss, Inc.

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In vivo magnetic resonance imaging and spectroscopy of humans with a 4 t whole‐body magnet

Cited by 110 publications

References 26 publications

High‐resolution fast spin echo imaging of the human brain at 4.7 T: Implementation and sequence characteristics

High‐resolution fast spin echo imaging of the human brain at 4.7 T: Implementation and sequence characteristics

B₁ field, SAR, and SNR comparisons for birdcage, TEM, and microstrip coils at 7T

Imaging artifacts at 3.0T

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In vivo magnetic resonance imaging and spectroscopy of humans with a 4 t whole‐body magnet

Cited by 110 publications

References 26 publications

High‐resolution fast spin echo imaging of the human brain at 4.7 T: Implementation and sequence characteristics

High‐resolution fast spin echo imaging of the human brain at 4.7 T: Implementation and sequence characteristics

B1 field, SAR, and SNR comparisons for birdcage, TEM, and microstrip coils at 7T

Imaging artifacts at 3.0T

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B₁ field, SAR, and SNR comparisons for birdcage, TEM, and microstrip coils at 7T