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Coil Elements (coil + element)

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Medical Sciences100%

Selected Abstracts

Improving SNR of RF coils using composite coil elements

NMR IN BIOMEDICINE, Issue 9 2009
Zhiyue J. Wang
Abstract A composite coil element consists of up to three independent orthogonal loops. It improves the flexibility in shaping the radio frequency (RF) field in its vicinity, compared with a single-loop coil element. Computer simulations were conducted to explore the potential advantages of this type of coil configuration for improving the signal-to- noise ratio (SNR), including the intrinsic SNR (ISNR) and the realistic SNR, when the effects of resistive loss of the coil were included. A ,half-space' model was considered, with a variable B0 direction relative to the surface of a large conductive medium. The SNR performance of a square single-loop coil parallel to the surface of the medium was compared with that of a composite coil element where up to two additional orthogonal square loops of the same size were added to the single coil loop. The SNR performances of coil arrays consisting of single-loop elements and composite elements were also evaluated. The RF field was calculated using the finite-difference time-domain method. The results show that the composite coil element has a substantially better ISNR at all depths from the surface than that of a single-loop element covering the same surface area. Furthermore, the ISNR of a composite element is not sensitive to the surface orientation relative to the B0 field. The computer simulation also revealed that at 128,MHz, the resistive loss from the copper coil loops standing upright on the surface at room temperature is substantial compared to the loss in the medium. Consequently, the realistic SNR is significantly lower than ISNR at 128,MHz for a composite coil element. The coil loading by the medium becomes more dominant at 170 and 298,MHz, and the differences between the realistic SNR and ISNR become smaller at these higher frequencies. Copyright © 2009 John Wiley & Sons, Ltd. [source]

96-Channel receive-only head coil for 3 Tesla: Design optimization and evaluation

MAGNETIC RESONANCE IN MEDICINE, Issue 3 2009
Graham C. Wiggins
Abstract The benefits and challenges of highly parallel array coils for head imaging were investigated through the development of a 3T receive-only phased-array head coil with 96 receive elements constructed on a close-fitting helmet-shaped former. We evaluated several designs for the coil elements and matching circuitry, with particular attention to sources of signal-to-noise ratio (SNR) loss, including various sources of coil loading and coupling between the array elements. The SNR and noise amplification (g -factor) in accelerated imaging were quantitatively evaluated in phantom and human imaging and compared to a 32-channel array built on an identical helmet-shaped former and to a larger commercial 12-channel head coil. The 96-channel coil provided substantial SNR gains in the distal cortex compared to the 12- and 32-channel coils. The central SNR for the 96-channel coil was similar to the 32-channel coil for optimum SNR combination and 20% lower for root-sum-of-squares combination. There was a significant reduction in the maximum g -factor for 96 channels compared to 32; for example, the 96-channel maximum g -factor was 65% of the 32-channel value for acceleration rate 4. The performance of the array is demonstrated in highly accelerated brain images. Magn Reson Med, 2009. © 2009 Wiley-Liss, Inc. [source]

Spectral phase-corrected GRAPPA reconstruction of three-dimensional echo-planar spectroscopic imaging (3D-EPSI)

MAGNETIC RESONANCE IN MEDICINE, Issue 5 2007
Xiaoping Zhu
Abstract MR spectroscopic (MRS) images from a large volume of brain can be obtained using a 3D echo-planar spectroscopic imaging (3D-EPSI) sequence. However, routine applications of 3D-EPSI are still limited by a long scan time. In this communication, a new approach termed "spectral phase-corrected generalized autocalibrating partially parallel acquisitions" (SPC-GRAPPA) is introduced for the reconstruction of 3D-EPSI data to accelerate data acquisition while preserving the accuracy of quantitation of brain metabolites. In SPC-GRAPPA, voxel-by-voxel spectral phase alignment between metabolite 3D-EPSI from individual coil elements is performed in the frequency domain, utilizing the whole spectrum from interleaved water reference 3D-EPSI for robust estimation of the zero-order phase correction. The performance of SPC-GRAPPA was compared with that of fully encoded 3D-EPSI and conventional GRAPPA. Analysis of whole-brain 3D-EPSI data reconstructed by SPC-GRAPPA demonstrates that SPC-GRAPPA with an acceleration factor of 1.5 yields results very similar to those obtained by fully encoded 3D-EPSI, and is more accurate than conventional GRAPPA. Magn Reson Med 57:815,820, 2007. © 2007 Wiley-Liss, Inc. [source]

Improving SNR of RF coils using composite coil elements

NMR IN BIOMEDICINE, Issue 9 2009
Zhiyue J. Wang
Abstract A composite coil element consists of up to three independent orthogonal loops. It improves the flexibility in shaping the radio frequency (RF) field in its vicinity, compared with a single-loop coil element. Computer simulations were conducted to explore the potential advantages of this type of coil configuration for improving the signal-to- noise ratio (SNR), including the intrinsic SNR (ISNR) and the realistic SNR, when the effects of resistive loss of the coil were included. A ,half-space' model was considered, with a variable B0 direction relative to the surface of a large conductive medium. The SNR performance of a square single-loop coil parallel to the surface of the medium was compared with that of a composite coil element where up to two additional orthogonal square loops of the same size were added to the single coil loop. The SNR performances of coil arrays consisting of single-loop elements and composite elements were also evaluated. The RF field was calculated using the finite-difference time-domain method. The results show that the composite coil element has a substantially better ISNR at all depths from the surface than that of a single-loop element covering the same surface area. Furthermore, the ISNR of a composite element is not sensitive to the surface orientation relative to the B0 field. The computer simulation also revealed that at 128,MHz, the resistive loss from the copper coil loops standing upright on the surface at room temperature is substantial compared to the loss in the medium. Consequently, the realistic SNR is significantly lower than ISNR at 128,MHz for a composite coil element. The coil loading by the medium becomes more dominant at 170 and 298,MHz, and the differences between the realistic SNR and ISNR become smaller at these higher frequencies. Copyright © 2009 John Wiley & Sons, Ltd. [source]