Generalized Reciprocity

Insko, Erik K.; Elliott, Mark A.; Schotland, John C.; Leigh, John S.

doi:10.1006/jmre.1997.1355

Cited by 37 publications

(48 citation statements)

References 6 publications

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“…In consequence, Eq. [23] gives results that are numerically more accurate for low values of N. The practical issue now is to obtain expressions similar to Eq. [23] relating current intensity to field amplitude for each of the two modes of the shielded discrete elliptical coil.…”

Section: Elliptical Birdcage (Discrete Implementation)mentioning

confidence: 99%

“…[23] gives results that are numerically more accurate for low values of N. The practical issue now is to obtain expressions similar to Eq. [23] relating current intensity to field amplitude for each of the two modes of the shielded discrete elliptical coil. Since the direct application of the method of images (as implemented for the discrete circular birdcage) is not possible with an elliptical shield, the computation of the exact field lines, and hence the degree of inhomogeneity produced by such a coil, would involve a second confor-…”

Section: Elliptical Birdcage (Discrete Implementation)mentioning

confidence: 99%

“…In fact, if the coil elements in the shielded case are positioned in the same manner and are driven by the same excitation currents as in the unshielded case, the sensitivities will be obtainable directly from Eqs. [20] and [23]. By analogy with the circular geometry (Eq.…”

Section: Elliptical Birdcage (Discrete Implementation)mentioning

confidence: 99%

“…The same feed configuration can also be shown, by applying the principle of reciprocity (17,23), to provide correct compensation for the different port sensitivities on reception. The hybrid combination will contain equal contributions from both ports, and the SNR of the output signal will be given by the Pythagorean sum of the original SNRs (24):…”

Section: Port Imbalance and Noise Performance In Quadrature Operationmentioning

confidence: 99%

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Sensitivity calculations and comparisons for shielded elliptical and circular birdcage coils

Zanche

Allen

2002

Magnetic Resonance in Med

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An analytical framework is presented which justifies the use of a confocal elliptical shield (previously adopted on empirical or numerical grounds) to limit the spatial extent of the radiofrequency (RF) field produced by an elliptical birdcage coil, while concurrently minimizing field inhomogeneities. From this analytical capability it is further shown that the relative linear mode geometric sensitivities depend only on the shield dimensions and can be readily evaluated, thus providing an upper bound to the improvement in signal-to-noise ratio (SNR) of the damped elliptical birdcage over a circular coil of similar dimensions. The mismatch in the linear mode sensitivities can also be evaluated, and a method for the design of an appropriate feed configuration that ensures quadrature operation is given. Magn Reson Med 47:364 -371, 2002.

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Section: Elliptical Birdcage (Discrete Implementation)mentioning

confidence: 99%

Section: Elliptical Birdcage (Discrete Implementation)mentioning

confidence: 99%

Section: Elliptical Birdcage (Discrete Implementation)mentioning

confidence: 99%

Section: Port Imbalance and Noise Performance In Quadrature Operationmentioning

confidence: 99%

See 2 more Smart Citations

Sensitivity calculations and comparisons for shielded elliptical and circular birdcage coils

Zanche

Allen

2002

Magnetic Resonance in Med

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show abstract

“…Assuming the main magnetic field is oriented along the z-direction, the principle of reciprocity (24,25) permits expressing the sensitivity s g (r) of each coil element g at point r as a function of the magnetic H field that the coils generate when driven with unit input current I 0 at the Larmor frequency (13,17,26):…”

Section: Theorymentioning

confidence: 99%

Optimizing signal‐to‐noise ratio of high‐resolution parallel single‐shot diffusion‐weighted echo‐planar imaging at ultrahigh field strengths

Reischauer

Vorburger

Wilm

et al. 2011

Magnetic Resonance in Med

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The potential signal-to-noise ratio (SNR) gain at ultrahigh field strengths offers the promise of higher image resolution in single-shot diffusion-weighted echo-planar imaging the challenge being reduced T 2 and T 2 * relaxation times and increased B 0 inhomogeneity which lead to geometric distortions and image blurring. These can be addressed using parallel imaging (PI) methods for which a greater range of feasible reduction factors has been predicted at ultrahigh field strengths-the tradeoff being an associated SNR loss. Using comprehensive simulations, the SNR of high-resolution diffusion-weighted echo-planar imaging in combination with spinecho and stimulated-echo acquisition is explored at 7 T and compared to 3 T. To this end, PI performance is simulated for coil arrays with a variable number of circular coil elements. Beyond that, simulations of the point spread function are performed to investigate the actual image resolution. When higher PI reduction factors are applied at 7 T to address increased image distortions, high-resolution imaging benefits SNR-wise only at relatively low PI reduction factors. On the contrary, it features generally higher image resolutions than at 3 T due to smaller point spread functions. The SNR simulations are confirmed by phantom experiments. Finally, high-resolution in vivo images of a healthy volunteer are presented which demonstrate the feasibility of higher PI reduction factors at 7 T in practice. Magn Reson Med 67:679-690, 2012. V C 2011 Wiley Periodicals, Inc.Key words: high-resolution diffusion-weighted imaging; echoplanar imaging; signal-to-noise ratio; parallel imaging; ultrahigh field strengths Diffusion-weighted imaging is a powerful noninvasive magnetic resonance imaging technique that relies on the measurement of the restricted Brownian motion of water molecules to deduce microscopic tissue structure in vivo. Although the biophysical basis is not clearly understood, quantitative measures can be derived that exhibit biomarkers in various pathological states, the most prominent being ischemic stroke (1). Diffusion-weighted imaging is clinically promising since the acquisition is noninvasive, relatively fast and requires neither contrast agents nor ionizing radiation. In addition, dedicated tracking algorithms have been developed that allow for the reconstruction of virtual fiber pathways in the human brain which enable presurgical planning and image-guided surgery (2,3).Great potential arises for diffusion-weighted imaging at ultrahigh field strengths through the theoretically associated signal-to-noise ratio (SNR) gain which offers the promise of higher achievable image resolution. Recently, ultrahigh field scanners with 7 T have been introduced to the scientific community. Yet, diffusion-weighted imaging at 7 T is hampered by increased B 0 inhomogeneity which scales linearly with the main magnetic field strength as well as faster T 2 and T 2 * decay. Image acquisition typically relies on single-shot diffusion-weighted (DW) echo-planar imaging (EPI) due to its spe...

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Magnetic Resonance with Conducting and High-Dielectric Samples

Hoult

1996

eMagRes

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Generalized Reciprocity

Cited by 37 publications

References 6 publications

Sensitivity calculations and comparisons for shielded elliptical and circular birdcage coils

Sensitivity calculations and comparisons for shielded elliptical and circular birdcage coils

Optimizing signal‐to‐noise ratio of high‐resolution parallel single‐shot diffusion‐weighted echo‐planar imaging at ultrahigh field strengths

Magnetic Resonance with Conducting and High-Dielectric Samples

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