Comparison of linear and circular polarization for magnetic resonance imaging

Glover, Gary H.; Hayes, Cecil E.; Pelc, Norbert J.; Edelstein, William A.; Mueller, O.; Hart, H. R.; Hardy, Christopher J.; O’Donnell, Matthew; Barber, William J.

doi:10.1016/0022-2364(85)90349-x

Cited by 203 publications

(200 citation statements)

References 3 publications

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“…This cautionary note is not restricted to high field only. In fact, in low field 13 C MR spectroscopy of proton decoupling, a similar condition for exceeding the safe SAR limits exists (25). As such, monitoring of the RF power delivered to living subjects must be refined to make accurate predictions for safety implications of RF deposition.…”

Section: Discussionmentioning

confidence: 99%

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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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Section: Discussionmentioning

confidence: 99%

“…These are possible consequences of inhomogeneous distribution of electromagnetic fields over a human subject (13). The rotating magnetic field (B1) inhomogeneities, which become more significant above 100 MHz, will give rise to inhomogeneous distribution of signal and SAR intensities (9).…”

mentioning

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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“…For a spin echo (SE) sequence with excitation flip angle ␣(x) and refocusing flip angle ␤(x), the signal intensity for noninteracting spins without transverse coherence, and assuming that the repetition time TR Ͼ Ͼ longitudinal relaxation time T 1 , can be written as follows (27,28),…”

Section: Transmission Fieldmentioning

confidence: 99%

In vivo method for correcting transmit/receive nonuniformities with phased array coils

Wang

Qiu

Constable

2005

Magnetic Resonance in Med

102

111

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Phased array coils are finding widespread applications in both the research and the clinical setting. However, intensity nonuniformities with such coils can reduce the potential benefits of these coils, particularly for applications such as tissue segmentation. In this work, a method is described for correcting the nonuniform signal response based on in vivo measures of both the transmission field of body coil and the reception sensitivity of phased array coils, separately. For a uniform phantom, the reception sensitivity can be calculated using both Bloch equations and transmission field maps. For a heterogeneous object such as a brain, a minimal contrast acquisition must be obtained to map the receiver nonuniformities. This transmit field/ receiver sensitivity (TFRS) approach is compared with the standard methods of using the body coil to obtain a reference scan and low-pass filtering. The quantitative comparison results shows that the TFRS approach provides superior results in correcting intensity nonuniformities for a uniform phantom. This approach reduces the ratio between signal intensity SD of an image and its mean intensity from approximately 21% before correction to 13% after correction. Results are also shown demonstrating the utility of this approach in vivo with human brain images. The method is general and can be applied with most pulse sequences, any coil combination for transmission and reception, and in any anatomic region. Magn Reson Med 53:666 -674, 2005.

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“…Hoult and Lauterbur [3] and Chen et al [4] developed methods to calculate the loss and SNR in the human subject. Glover et al [5] compared the power deposition and SNR in a lossy body-size phantom and in human bodies for linear and quadrature excitation at 63 MHz. Edelstein et al [6] introduced the concept of intrinsic SNR (ISNR) to MR human imaging.…”

Section: Introductionmentioning

confidence: 99%

Calculations ofB 1 distribution, specific energy absorption rate, and intrinsic signal-to-noise ratio for a body-size birdcage coil loaded with different human subjects at 64 and 128 MHz

2005

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A numerical model of a female body is developed to study the effects of different body types with different coil drive methods on radio-frequency magnetic (B 1 ) field distribution, specific energy absorption rate (SAR), and intrinsic signal-to-noise ratio (ISNR) for a body-size birdcage coil at 64 and 128 MHz. The coil is loaded with either a larger, more muscular male body model (subject 1) or a newly developed female body model (subject 2), and driven with two-port (quadrature), four-port, or many (ideal) sources. Loading the coil with subject 1 results in significantly less homogeneous B 1 field, higher SAR, and lower ISNR than those for subject 2 at both frequencies. This dependence of MR performance and safety measures on body type indicates a need for a variety of numerical models representative of a diverse population for future calculations. The different drive methods result in similar B 1 field patterns, SAR, and ISNR in all cases.

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Comparison of linear and circular polarization for magnetic resonance imaging

Cited by 203 publications

References 3 publications

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

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

In vivo method for correcting transmit/receive nonuniformities with phased array coils

Calculations ofB 1 distribution, specific energy absorption rate, and intrinsic signal-to-noise ratio for a body-size birdcage coil loaded with different human subjects at 64 and 128 MHz

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