Size-adaptable "Trellis" structure for tailored MRI coil arrays

Abstract

Purpose: We present a novel, geometrically adjustable, receive coil array whose diameter can be tailored to the subject in order to maximize sensitivity for a range of body sizes.

Theory and methods: A key mechanical feature of the size-adaptable receive array is its trellis structure that was motivated by similar structures found in gardening and fencing. Our implementation is a cylindrical trellis that features encircling, diagonally interleaved slats, which are linked together at intersecting points. The ensemble allows expansion or contraction to be controlled with the angle between the slats. This mechanical frame provides a base for radiofrequency coils wherein approximately constant overlap, and therefore coupling between adjacent elements, is maintained when the trellis is expanded or contracted. We demonstrate 2 trellis coil concepts for imaging lower extremity at 3T: a single-row 8-channel array built on a trellis support structure and a multirow 24-channel array in which the coil elements themselves form the trellis structure.

Results: We show that the adjustable trellis array can accommodate a range of subject sizes with robust signal-to-noise ratio, loading, and coupling.

Conclusion: The trellis coil concept enables an array of surface coils to expand and contract with negligible effect on tuning, matching, and decoupling. This allows an encircling array to conform closely to anatomy of various sizes, which provides significant gains in signal-to-noise ratio.

Keywords: coupling; phased-array; preamplifier noise; stretchable; trellis.

FIGURE 1

A, Trellis structure used as the holder for the 8-ch array, the use of 10 nylon strips per coil element width allows maintaining a constant 10% overlap between neighbor coils to assure inductive decoupling when the coil size changes. In fact, when the width of each coil element is w₁, the overlap is 0.1 × w₁, and when the width of the coil element is squeezed to w₂, the overlap becomes 0.1 × w₂. B, Schematic drawing of each coil element in the 8-ch array

FIGURE 2

A, Layout of the 24-ch trellis knee coil. The PCBs (yellow) are linked at pivot points by rivets (white circles), whose positions were selected to optimize geometric overlap between neighbor elements for decoupling. The PCBs include gaps (white stripes) for tuning and matching capacitors. Flexible wires (black arcs) were used to link the PCB pieces such that electrical connections were maintained during trellis flexion. B, The 3 basic PCB pieces used to create the array. C, Schematic drawing of each trellis coil element

FIGURE 3

A, photographs of the 8-channel trellis array adapted for 3 phantoms with diameters equal to 134, 165, and 195 mm. B, Parameters of a pair of coil elements in the 8-channel array on 134, 165, and 195 mm phantoms

FIGURE 4

The SNR of the 8-channel trellis array wrapped tightly around phantoms of different size (first 3 columns) and surrounding the smallest phantom, but with the diameter expanded to fit the largest phantom (fourth column), is compared with the SNR of the commercial 15-ch knee array surrounding the smallest phantom (fifth column). Images in SNR units are shown for a central slice in the transversal plane (first row) and coronal plane (second row). The noise coefficient matrixes for each case are shown in the third row

FIGURE 5

S parameters of a 7-element cluster within the 24-ch trellis array

FIGURE 6

SNR of the 24-channel trellis array adapted to the diameter of different phantoms (first~third columns) and expanded to largest phantom, but used on the smallest phantom (fourth column) is compared to the SNR of the QED 15-channel knee coil for the smallest phantom (fifth column) and Siemens 18-channel flexible body array for the largest phantom (sixth column)

FIGURE 7

Inverse g-factor (1/g) maps at various 1D and 2D accelerations are shown for the 24-channel trellis arrays for the smallest (first row) and largest (third row) phantom. For comparison, the corresponding maps for the QED 15-channel knee coil and the Siemens 18-channel body coil are shown in the second and fourth row, respectively. Maximum g-factor values within the phantom are reported at the bottom of each map

FIGURE 8

In vivo images in SNR units for 3 orthogonal slices in the knee for the trellis 24-channel array (top row) and the QED 15-channel array (bottom row)

FIGURE 9

In vivo images in SNR units for 2 orthogonal slices in the thigh (same subject as in Figure 8) for the trellis 24-channel array (top row) and the combination of the system-flexible body with spine array (bottom row)