Experimental verification of SNR and parallel imaging improvements using composite arrays

Abstract

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.

Evaluation of short folded dipole antennas as receive elements of ultra‐high‐field human head array

“…Thus, simulations seem to overestimate the central SNR of the dipole array relative to that of the vertical loops array. This most likely reflects the fact that SNR of an array depends not only on profile of individual elements, but also on coupling between them and, very important, on the hardware connecting the element and preamplifier . Figure D shows the ratio of the SNR maps of the 16‐element dipole array to that of the array of 8 surface loops only.…”

Double‐row 18‐loop transceive–32‐loop receive tight‐fit array provides for whole‐brain coverage, high transmit performance, and SNR improvement near the brain center at 9.4T

“…Another study, which may shed light on the problem of central SNR improvement, also deals with a spherical phantom and arrays, which consist of circular surface loops and so‐called composite elements . Each composite element contains a surface loop and 2 orthogonal “vertical” loops, positioned perpendicularly to both the surface loop and each other.…”

“…In this work, the authors demonstrated that the composite array slightly improves SNR near the center of the spherical phantom compared to an array of surface coils with the same total number of loops (both surface and vertical). Interestingly, the SNR enhancement was substantially greater (up to ~30% for 56‐element arrays) when noise from both the matching networks and preamplifiers was taken into account . Therefore, not only the realistic voxel model and array holder, but also realistic schematics of matching networks and preamplifiers may need to be taken into account.…”

“…Another study, which may shed light on the problem of central SNR improvement, also deals with a spherical phantom and arrays, which consist of circular surface loops and so-called composite elements. 42,43 Each composite element contains a surface loop and 2 orthogonal "vertical" loops, positioned perpendicularly to both the surface loop and each other. In this work, the authors demonstrated that the composite array slightly improves SNR near the center of the spherical phantom compared to an array of surface coils with the same total number of loops (both surface and vertical).…”

Comparison of high-density composite and surface coil arrays for MRI of spherical imaging volumes

“…Various coil types provide these patterns, including upright coils [5]. Composite coils are the combination of a surface coil and two orthogonal upright coils into a triplet [6], [7]. They have been shown to better approximate the electromagnetic fields required to approach the uSNRJ3] [5].…”