The theory and application of array coils are reviewed in the context of phased array spectroscopy. The optimization of the signal-to-noise ratio from an array of coils is developed by considering the efficiency of a phased array transmit coil. This approach avoids the need to consider noise correlation, and should be useful in future considerations of transmit phased array coils for MR spectroscopy. Methods to characterize array coil performance, including fields and coupling are briefly summarized, along with methods to minimize the effects of mutual inductance. The signal-to-noise advantages of array coils over single coils are examined for both planar and cylindrical arrays. Numerical simulations of planar arrays of 2×2, 4×4 and 8×8 elements and constant overall dimension are compared to a single coil of the same size. The results demonstrate a significant improvement in sensitivity near the array coil. Although the benefits of the array decrease as a function of distance from the array, the array sensitivity never drops below that of a single coil with the same overall dimensions, or that of a single element of the array. Similar results are obtained for a sixteen element cylindrical array, which is compared to a standard quadrature birdcage coil using both computational methods and phantom measurements. The phased array techniques reviewed are demonstrated with proton spectroscopic images of the brain. © 1997 John Wiley & Sons, Ltd.

Theory and application of array coils in MR spectroscopy

Magnetic resonance imaging (MRI) is a powerful new imaging method, which produces cross-sectional tomographic and three-dimensional images similar to those of x-ray computed tomography (CT). However, rather than relying on harmful ionizing radiation, MRT is based on the interaction between RF fields and certain atomic nuclei in the body, when they are in the presence of a strong magnetic field. An MRI system is one of the few complete systems in which the design relies heavily upon a knowledge of electromagnetics. We give a tutorial on the electromagnetic analysis and design of three key components of an MRI system, namely, the magnet, the gradient coil, and the radiofrequency (RF) coil. We also discuss the analysis and characterization of the interactions of RF electromagnetic fields with biological subjects.

Electromagnetics in magnetic resonance imaging

Conventionally, MR images are formed by applying gradients to the main static magnetic field (B0). However, the B0 gradient equipment is expensive, power-hungry, complex, and noisy and can induce eddy currents in nearby conducting structures, including the patient. Here, we describe a new silent, B0 gradient-free MRI principle, Transmit Array Spatial Encoding (TRASE), based on phase gradients of the radio-frequency (RF) field. RF phase gradients offer a new method of k-space traversal. Echo trains using at least two different RF phase gradients allow spin phase to accumulate, causing k-space traversal. Two such RF fields provide one-dimensional imaging and three are sufficient for two-dimensional imaging. Since TRASE is a k-space method, analogues of many conventional pulse sequences are possible. Experimental results demonstrate one-dimensional and two-dimensional RF MRI and slice selection using a single-channel, transmit/receive, 0.2 T, permanent magnet, human MR system. The experimentally demonstrated spatial resolution is much higher than that provided by RF receive coil array sensitivity encoding alone but lower than generally achievable with B0 gradients. Potential applications are those in which one or more of the features of simplified equipment, lower costs, silent MRI, or the different physics of the image formation process are particularly advantageous. Magn Reson Med, 2010. © 2009 Wiley-Liss, Inc.

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MRI using radiofrequency magnetic field phase gradients

The concept of simultaneous image acquisition as originally conceived by Hyde has been implemented by Roemer et al. using a multiple receiver system. This article describes an alternative technique that uses time domain multiplexing (TDM) to acquire simultaneous images using a single receiver channel. This method requires few modifications to the existing receiver and has been shown to be equivalent to a true four channel receiver in most applications. The multiplexing technique was implemented and tested on a standard commercial scanner that has also been equipped with a four channel receiver. Signal-to-noise results demonstrate that four independent images can be multiplexed through a single receiver channel with no degradation in image quality. Additionally, spine images that were obtained from a normal volunteer with both a multiplexed single channel receiver and a true four channel receiver system are presented.

A four-channel time domain multiplexer: A cost-effective alternative to multiple receivers

Flow-sensor arrays uncover the potential to measure spatio-temporal flow patterns rather than flow measurements at just a single point. We present in this paper the developments in design, fabrication and interfacing of biomimetic flow-sensor arrays, inspired by flow-sensitive organs (cerci) of crickets. For the purpose of high-resolution flow field visualization by our artificial hair flow-sensor arrays, various array-interfacing schemes are discussed and compared. Frequency division multiplexing (FDM) is shown to be an attractive method for efficient interrogation of capacitive array sensors. Using silicon-on-insulator technology with deep trench isolation structures, hair-based flow-sensors with differential capacitive read-out, arranged in single-chip arrays, have been successfully fabricated. FDM is implemented and used to interrogate individual hair sensors providing simultaneous real-time flow measurements from multiple hairs. This powerful approach is demonstrated by reconstruction of the field of a harmonic dipole field at the position of the hairs and by localizing this dipole source relative to the array elements.

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Towards a high-resolution flow camera using artificial hair sensor arrays for flow pattern observations.

A new technique for parallel acquisition of magnetic resonance images is described. Two RF coils are time multiplexed into a single receiver channel, halving the effective image acquisition time with no degradation in signal-to-noise ratio. Noise filtering is performed with RF bandpass filters. In principle, the method can be extended to additional coils.

S.W. Wright

Papers

Parallel acquisition of MR images using time multiplexed coils