Towards a complete coil array

Wang, Zhiyue

doi:10.1016/j.mri.2008.02.002

Cited by 12 publications

(15 citation statements)

References 19 publications

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“…As predicted by previous simulations (15), capturing the components of flux parallel to the surface results in SNR gains and increased sensitivity in regions where surface coils alone have low sensitivity. Instead of using upright coils this behavior could also be achieved with coils such as the butterfly coil (48) (essentially an unfolded, two-turn upright coil) or the "wheel-and-spoke" coil (13,28) (a butterfly coil combined with a loop), which have similar field patterns.…”

Section: Discussionsupporting

confidence: 74%

“…In this study the complete coil system suggested by Wang is approximated by an array of composite elements , each consisting of three orthogonal rectangular loops arranged according to Fig. (A).…”

Section: Theorymentioning

confidence: 99%

“…A composite array triplet can produce linearly or elliptically polarized RF magnetic fields in three orthogonal orientations at any spatial location. Simulations have shown that an array of such composite triplets provides a better approximation of a complete basis set than surface coil elements alone, in part because of the additional degrees of freedom in the direction of the magnetic fields produced and also because the number of independent loops per unit surface is tripled.…”

Section: Theorymentioning

confidence: 99%

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Experimental verification of SNR and parallel imaging improvements using composite arrays

et al. 2014

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Composite MRI arrays consist of triplets where two orthogonal upright loops are placed over the same imaging area as a standard surface coil. The optimal height of the upright coils is approximately half the width for the 7 cm coils used in this work. Resistive and magnetic coupling is shown to be negligible within each coil triplet. Experimental evaluation of imaging performance was carried out on a Philips 3 T Achieva scanner using an eight-coil composite array consisting of three surface coils and five upright loops, as well as an array of eight surface coils for comparison. The composite array offers lower overall coupling than the traditional array. The sensitivities of upright coils are complementary to those of the surface coils and therefore provide SNR gains in regions where surface coil sensitivity is low, and additional spatial information for improved parallel imaging performance. Near the surface of the phantom the eight-channel surface coil array provides higher overall SNR than the composite array, but this advantage disappears beyond a depth of approximately one coil diameter, where it is typically more challenging to improve SNR. Furthermore, parallel imaging performance is better with the composite array compared with the surface coil array, especially at high accelerations and in locations deep in the phantom. Composite arrays offer an attractive means of improving imaging performance and channel density without reducing the size, and therefore the loading regime, of surface coil elements. Additional advantages of composite arrays include minimal SNR loss using root-sum-of-squares combination compared with optimal, and the ability to switch from high to low channel density by merely selecting only the surface elements, unlike surface coil arrays, which require additional hardware.

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Section: Discussionsupporting

confidence: 74%

Section: Theorymentioning

confidence: 99%

Section: Theorymentioning

confidence: 99%

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Experimental verification of SNR and parallel imaging improvements using composite arrays

et al. 2014

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“…This can be accomplished, for example, by combining surface loops with dipole antennas, 18-20 a recently proposed method. [21][22][23][24] Using the UISNR concept, 17,[25][26][27] it has been demonstrated that the combination of loops and dipole elements within the same array structure can provide for better SNR near the center of a human head over an array of surface loops alone. 27 However, the size of the dipole antenna, e.g.…”

mentioning

confidence: 99%

Combination of surface and ‘vertical’ loop elements improves receive performance of a human head transceiver array at 9.4 T

et al. 2017

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Ultra-high-field (UHF, ≥7 T) human magnetic resonance imaging (MRI) provides undisputed advantages over low-field MRI (≤3 T), but its development remains challenging because of numerous technical issues, including the low efficiency of transmit (Tx) radiofrequency (RF) coils caused by the increase in tissue power deposition with frequency. Tight-fit human head transceiver (TxRx) arrays improve Tx efficiency in comparison with Tx-only arrays, which are larger in order to fit multi-channel receive (Rx)-only arrays inside. A drawback of the TxRx design is that the number of elements in an array is limited by the number of available high-power RF Tx channels (commonly 8 or 16), which is not sufficient for optimal Rx performance. In this work, as a proof of concept, we developed a method for increasing the number of Rx elements in a human head TxRx surface loop array without the need to move the loops away from a sample, which compromises the array Tx performance. We designed and constructed a prototype 16-channel tight-fit array, which consists of eight TxRx surface loops placed on a cylindrical holder circumscribing a head, and eight Rx-only vertical loops positioned along the central axis (parallel to the magnetic field B ) of each TxRx loop, perpendicular to its surface. We demonstrated both experimentally and numerically that the addition of the vertical loops has no measurable effect on the Tx efficiency of the array. An increase in the maximum local specific absorption rate (SAR), evaluated using two human head voxel models (Duke and Ella), measured 3.4% or less. At the same time, the 16-element array provided 30% improvement of central signal-to-noise ratio (SNR) in vivo relative to a surface loop eight-element array. The novel array design also demonstrated an improvement in the parallel Rx performance in the transversal plane. Thus, using this method, both the Rx and Tx performance of the human head array can be optimized simultaneously.

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“…We have previously shown that an array of composite elements can be configured to approximate a 'truncated complete coil' system, i.e. a coil system capable of generating every RF-field mode in a multipole expansion, up to a certain order (8). The capability of generating flexible field patterns is associated with improvement in SNR (9)(10)(11), based on the principle of reciprocity (12).…”

Section: Introductionmentioning

confidence: 99%

Improving SNR of RF coils using composite coil elements

Wang

2009

NMR in Biomedicine

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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 B(0) 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 B(0) 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.

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Towards a complete coil array

Cited by 12 publications

References 19 publications

Experimental verification of SNR and parallel imaging improvements using composite arrays

Experimental verification of SNR and parallel imaging improvements using composite arrays

Combination of surface and ‘vertical’ loop elements improves receive performance of a human head transceiver array at 9.4 T

Improving SNR of RF coils using composite coil elements

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