Sensitivity calculations and comparisons for shielded elliptical and circular birdcage coils

Zanche, Nicola De; Allen, Peter S.

doi:10.1002/mrm.10039

Cited by 11 publications

(20 citation statements)

References 14 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…86-93], a solution is not obtained merely by ''flattening'' the cross-section using a computer graphics program, since this procedure does not preserve angles. We are investigating a toroid of elliptical cross-section, by analogy with an elliptical birdcage [20,36], using an elliptical conformal transform based on the elliptical coordinate system [34], in which the ellipses are transformed to a rectangular system on which the field solution is obtained. Similarly, a Cassinian oval crosssection may be derived [34, pp.…”

Section: Discussion Applications and Future Directionsmentioning

confidence: 99%

Computing the B1 field of the toroidal MRI coil

Butterworth

Gore

2005

Journal of Magnetic Resonance

View full text Add to dashboard Cite

Section: Discussion Applications and Future Directionsmentioning

confidence: 99%

Computing the B1 field of the toroidal MRI coil

Butterworth

Gore

2005

Journal of Magnetic Resonance

View full text Add to dashboard Cite

“…This indicates that intervoxel mixing of polarization due to differential RF pulsing from B 1 coil homogeneity is likely to be less of an issue than changes due to localized p0 2 and 2D slice profile and diffusion effects. In future studies the use of a coil with improved B 1 homogeneity, such as a whole-body elliptical birdcage (18), would help to remove this source of error due to partial-volume mixing of gas with different RF histories. In such a case the 3D pO 2 methodology described here could be used to examine anatomical and posture-related changes in pO 2 that have been identified in the ADC (12,13).…”

Section: In Vivo Experimentsmentioning

confidence: 99%

3D volume‐localized pO₂ measurement in the human lung with ³He MRI

Wild

Fichele

Woodhouse

et al. 2005

Magnetic Resonance in Med

103

View full text Add to dashboard Cite

A method for 3D volume-localized quantification of pO2 in the lungs is presented that uses repetitive frame 3D gradient-echo imaging of 3 He. The method was demonstrated by experiments on 3 He phantoms containing known concentrations of O 2 and in vivo on a group of three healthy human volunteers. The results were compared with those obtained by equivalent 2D thin-slice and 2D projection methodologies, and were found to be consistent with published results from the 2D projection methodologies (pO 2 ‫؍‬ 0.09 -0.18 bar). Studies performed on the same subject, on three separate occasions, demonstrated a repeatability of pO 2 measurement to within 14% using the 3D technique. Experimental differences between the 2D and 3D methods were substantiated with theoretical and numerical analyses of the signal decay, which took into account the effects of out-of-slice diffusion as a source of error in the thin-slice 2D experiments. It is shown that the 2D thin-slice technique systematically underestimates pO2 when there is significant gas diffusion (factor of 4 underestimate for D ‫؍‬ 0.9 cm MRI of hyperpolarized (HP)3 He gas can provide high spatial and temporal resolution images of gas ventilation in the lungs and airways, as well as useful functional information (1). Potentially one of the most useful functional parameters is an estimate of the partial pressure of oxygen (pO 2 ) in the lungs and the rate of oxygen extraction (2-6). Accurate spatially localized pO 2 measurement with 3 He MRI would be clinically useful as a means of assessing lung ventilation-perfusion (V/Q). When paramagnetic oxygen mixes with polarized 3 He in the lungs, a marked reduction in the 3 He T 1 arises through the electron-nuclear spin dipolar coupling. Saam et al. (7) quantified the dependence of the 3 He T 1 on oxygen concentration with phantom experiments. This O 2 dependence of the 3 He T 1 was used by Deninger et al. (2,3) for in vivo quantification of pO 2 in a series of breath-hold experiments on pigs and humans. These in vivo experiments used images acquired from whole-lung projections (very thick 2D slices). This was done to maximize the signal-to-noise ratio (SNR) and to circumvent diffusion of polarized gas out of the slice during the significant inter-image delay time (typically up to 7 s) as a possible source of change in the time course of the in vivo 3 He signal. Moller et al. (5) estimated the degree of partial volume mixing of polarization due to gas diffusion for a 2D thin-slice experiment in the guinea pig lung. Assuming a 24 s total acquisition time and an apparent diffusion coefficient of D ϭ 0.16 cm 2 s -1 , they predicted that only 5% of the in-slice magnetization would be exchanged between adjacent slices. In addition, the recently published results by Jalali et al. (6) from 2D pO 2 experiments in porcine lungs, with 2-cm-thick slices, are in fair agreement with the results from a 2D porcine study with 18-cm-thick slices (2). These results suggest that diffusion between slices may not be a considerable problem; however, a...

show abstract

“…The chosen rung positions allow uniform fields to be produced with (co)sinusoidal rung currents in the absence of an RF shield , or in the presence of a confocal elliptical outer shield , but not with other shapes such as the 120‐mm circular cylinder. Nonsinusoidal current distributions were, therefore, determined for the two linearly polarized modes by independently minimizing the standard deviation of the field amplitude of each mode within an elliptical region that is confocal with the coil former.…”

Section: Methodsmentioning

confidence: 99%

“…a: Rung current distributions considered for the shielded elliptical coil of Figure . The standard (co)sinusoidal distributions (dashed) would be optimal in the case of an unshielded coil or one with a confocal elliptical shield and are nearly identical to those optimized for maximal field homogeneity in the presence of the circular shield (solid); irregular patterns (thicker solid) were actually used for the prototype to test the algebraic method's ability to produce specified currents. b,c: Magnetic field homogeneity contours of the linearly polarized modes resulting from the irregular patterns in a) (2% intervals from amplitude at centre, empty coil, 2D quasi‐static field approximation).…”

Section: Methodsmentioning

confidence: 99%