RF Arrays for Head & Body
Nikolai I. Avdievich1
1Max Planck Institute for Biological Cybernetics, Tübingen, Germany

Synopsis

Improvement of SNR is a critical step in designing any MRI RF coil. Use of arrays instead of a single coil is a major technique for the SNR enhancement. Designing an Rx-array commonly includes a choice of the type of elements (e.g. loops, striplines, dipoles), number of elements, decoupling method, coverage. The most common element of Rx-arrays is a surface loop, which has been used for human Rx-array designs at lower (1.5T, 3T) and ultra-high field (UHF, >7T). The presentation overviews most important steps of designing RF arrays including an optimization of the individual elements, decoupling, detuning, interfacing, cable routing.

TARGET AUDIENCE

RF engineers, scientists and students interested in development, construction and usage of RF MRI array coils.

INTRODUCTION

Improvement of signal-to-noise ratio (SNR) is a critical step in designing any MRI radio frequency (RF) coil. Use of receive (Rx) arrays instead of a single RF coil, a method [1] suggested almost three decades ago, is a major technique for the SNR enhancement. Designing an Rx-array commonly includes a choice of the type of array elements (e.g. surface loops, striplines, dipoles), number of elements, method of element decoupling, and geometrical coverage. The most common single element of an Rx-array is a surface loop, which has been successfully used for human Rx-array designs at lower (1.5T, 3T) and ultra-high field (UHF, >7T) [2-20]. The presentation overviews most important phases of designing RF arrays including an optimization of the individual RF coils as well necessary steps while combining RF elements in an array, e.g. decoupling, detuning, interfacing, cable routing.

Optimization of an individual RF coil

According to reciprocity principle [21], the optimization of SNR of any RF coil is similar to the optimization of the transmit (Tx) efficiency, i.e. B1/√P, which is ~√Qη, where Q is the quality factor and η is the filling factor. Filling factor gives a ratio of the RF energy delivered to the sample over the entire energy of the resonance system. Thus, optimization includes two most important steps, i.e. the maximization of the filling factor by optimizing the RF coil geometry, and maximization of the Q-factor by minimizing losses, which define the noise level. Since the sample noise is unavoidable, it is desirable that most of the noise comes from the sample, i.e. providing so-called sample noise domination. Thus, the objective is to identify the various loss mechanisms and minimize noise contributions. Sample noise domination can be evaluated by measuring the ratio of loaded to unloaded Q-factors, i.e. QU/QL=1+RS/RC, where RS and RC are sample and intrinsic coil losses, respectively. According to previous works, QU/QL of 3÷4 and above suffices to optimize SNR [22].

Phased Array Types

There are three major types of MRI RF array coils, i.e. Transmit-only/ Receive-only (ToRo) [23], transceiver (TxRx), and Receive-only. Transceiver and Receive-only arrays are mainly discussed in this presentation. The most common single element of an Rx-array is a surface loop, which has been successfully used for human Rx-array designs at lower (1.5T, 3T) and ultra-high field (UHF, >7T) [2-20]. It is known that increasing the number of smaller surface loops in an Rx-array, while keeping the same coverage, improves mostly the peripheral SNR. At the same time, SNR near the center doesn’t improve substantially. This effect was demonstrated both experimentally [3,6] and theoretically [24,25].

Building Block – a Single Element of an Array

This part of the presentation overviews the most important components of a single element of Rx and TxRx arrays including matching [26], cable traps, detuning [23,27-29], decoupling [1,30-36], and a T/R switch [37].

Decoupling of Array Elements

Decoupling, i.e. eliminating the cross-talk between elements of the same type of the array, e.g. Rx, TxRx, is one of the most important steps of array designing. The most common method of decoupling adjacent loop elements of an Rx array is overlapping the loops [1]. In addition, this method allows increasing the size of individual loops while keeping the same coverage. This also increases the penetration depth and loading, thus, improving the sample loss domination regime. Non-adjacent elements of an Rx-array are commonly decoupled using low-impedance preamplifier [1]. This part of the presentation also overviews some other methods of decoupling [30-36].

Detuning (Decoupling of Tx and Rx Coils)

To avoid interaction between Tx-coil (array) and Rx-array as well as the noise injection from RF high power amplifiers connected to the Tx-coil, interaction between elements of the Tx-coil (array) and Rx-array must be cancelled or minimized. A method providing such cancellation is often referred as detuning. Active detuning, which implies using an electrical signal activating detuning, is commonly used in the ToRo array design [23]. This part of the presentation overviews the most common methods of active detuning using PIN diodes connected with RF elements in series or using trap circuits.

Miscellaneous

Finally, the presentation briefly overviews other important aspects of array designing, i.e. cable routing [1], interfacing, as well as one of the most important application, i.e. parallel reception [39-40].

Acknowledgements

No acknowledgement found.

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Proc. Intl. Soc. Mag. Reson. Med. 30 (2022)