Quadrature detection in the laboratory frame

Abstract

A theory of quadrature detection in the laboratory frame is developed. It is shown that the geometry of the two orthogonal coil systems needed for quadrature detection is radically different from that used with saddle-shaped coils, and that the homogeneity of the B1 field produced upon transmission is marginally better. The opposing quadrature phase shifts needed for transmission and reception are emphasized, and the use of a quadrature hybrid is advocated as a simple and inexpensive means of interfacing the transmitter, probes, and preamplifier. Experimental results are presented which confirm the theoretical predictions, and show that up to a 40% improvement in sensitivity and a twofold reduction in transmitter power are possible, particularly in those instances where the sample is conductive--namely, imaging of humans and in vivo spectroscopy.

Optimized quadrature surface coil designs

“…Quadrature MRI detectors are comprised of elements whose principal radiofrequency (RF) magnetic field components are orthogonal or nearly orthogonal, such that they can directly receive the circularly polarized MRI or MRS signals from a sample [1][2][3][4]. By combining the raw signals from a pair of quadrature elements with a hybrid that provides the appropriate 90°p hase shift, or by means of a root-of-the-sum-of-the-squares combination of the detected signals, a quadrature detector coil can provide up to a √2-fold gain in the signal-to-noise-ratio (SNR) compared to coils of the same geometry used as linear detectors [1,5].…”

“…By combining the raw signals from a pair of quadrature elements with a hybrid that provides the appropriate 90°p hase shift, or by means of a root-of-the-sum-of-the-squares combination of the detected signals, a quadrature detector coil can provide up to a √2-fold gain in the signal-to-noise-ratio (SNR) compared to coils of the same geometry used as linear detectors [1,5]. Quadrature surface coils comprised of a circular loop and a figure-8 or butterfly coil with long axis perpendicular to the main magnetic field, B 0 , have been adopted for both multinuclear MRS, and for commercial MRI phased arrays in applications including the spine [4,6,7], where their use with quadrature hybrids can halve the number of MRI receiver channels required to service the array.…”

“…For the circular loops in these quadrature detectors, the geometry that produces the maximum intrinsic SNR (ISNR), the SNR excluding MRI system losses and conductive losses in the detector [8], at a desired depth d is known from numerical [9] and theoretical [10] analyses in the low-field (low-frequency) limit to have radius: (1) Unfortunately, no similar design rule exists for the construction of a figure-8 or butterfly detector. Consequently, there is neither a design rule for optimizing the ISNR performance of the figure-8 detector by itself, nor a recipe for designing one whose ISNR performance is matched to that of a loop to achieve an appropriately performance-matched quadrature detector.…”

Abstracts (Continue in Part I)

“…The dual-endring double-tuned birdcage design consists of two overlapping coils, A and B each containing 8 [l-6). To obtain maximum sensitivity for clinical applications, it is deslrabk to perform proton detected l 3 C experiments.…”

Technical and experimental features of Magnetic Resonance Spectroscopy of brain glycogen metabolism

“…two physicallydecoupled overlapped 1 H loops) have been widely used both in animal [18,29,57,58,78,80,81] and human studies [21,76,77,82]. Relative to the linear coil design, an overlapped quadrature coil enhances sensitivity by a factor of √2 [79,83], and 13 C-quadrature coils have been designed in combination with either a 1 H-volume coil [84] or a 1 H-quadrature coil [85]. Indeed, an SNR enhancement of nearly √2 has been achieved to detect glycogen 13 C-1 in vivo at 7 T with the latter design when comparing to the linear-13 C/ quadrature-1 H coil [85].…”