Receive Coil Arrays & SNR
Ye Li1,2 and Xiaoliang Zhang3
1Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, Shenzhen, China, 2The Key Laboratory for Magnetic Resonance and Multimodality Imaging of Guangdong Province, Shenzhen, China, 3Department of Biomedical Engineering, University at Buffalo, the State University of New York, Buffalo, NY, United States

Synopsis

Keywords: Physics & Engineering: Hardware

The receive coil array plays a crucial role in MRI systems, allowing for the reception of MR signals detected from objects. The receive coil array consists of multiple decoupled coil elements that acquire MR signals simultaneously. The performance of receive coil arrays is critical for signal-to-noise ratio, coverage and parallel imaging capability. This presentation aims to introduce element optimization, decoupling methodologies and integration of RF components such as T/R switches, detuning circuits, baluns, etc..

Introduction

Receive coil arrays are the antennas that detect MR signals from objects in the MRI system. Therefore, receive coil arrays have a significant impact on image quality such as signal-to-noise ratio (SNR), image coverage, image uniformity and scan time [1]. The receive coil array consists of multiple decoupled coil elements that acquire MR signals simultaneously. Optimal individual coil elements and decoupling approaches are crucial to improving the performance of receive coil arrays. This presentation will focus on element optimization and decoupling approaches, while also introducing RF components such as T/R switches, detuning circuits, baluns, etc., and system compatibility / RF safety testing.

Design Criteria of Receive Coil Arrays

The coil array design requires consideration of SNR and field of view (FOV), which were introduced as the theoretical framework of phased arrays in 1990 [2]. Receive coil arrays detect MR signals simultaneously from multiple closely positioned RF coils, offering both high SNR from surface elements and large FOV from the data acquired by independent coils. However, the sensitivity of the surface elements is non-uniform and decays with distance from the imaging region, which means that the signal from the surface region is much higher than that from the deep region [3]. Therefore, a tradeoff between peripheral SNR and image uniformity should be considered based on the application requirements. In order to maintain image uniformity, the size of each coil element is similar in most applications. Receive coil arrays with parallel imaging capabilities are becoming new design criteria as parallel imaging algorithms and high acceleration factors are used in a wide range of applications [4].

SNR improvement for a single element of a coil array

The SNR of a receive coil array could be optimized by improving the SNR of a single element. The signal is proportional to the B1- field of a single coil element and the noise is inversely proportional to the square root of the equivalent noise resistance and the temperature. The equivalent noise resistance consists of coil loss, sample loss and radiation loss. Filling factor and Q-ratio (the ratio of unloaded to loaded quality factors) are used to evaluate the amount of the sample loss. When the filling factor and Q-ratio are high, the sample loss is dominated and other losses are negligible. In these cases, the intrinsic SNR, which depends on the coil geometry, could be utilized to evaluate the single-element performance [5-6]. The intrinsic SNR could be calculated by electromagnetic simulation [7-8]. The coil geometry is optimized to achieve the highest possible intrinsic SNR value for a given sample [9-12]. Additionally, custom-made tight-fitting [13-16], ultra-flexible [16-17], and stretchable/interchangeable [18-19] coil arrays are proposed to overcome poor fitting due to different patients, so the sample loss dominance is maintained. These methods are advantageous in fetal [20], pediatric [21], breast [22] and carotid [23] applications. Moreover, the coil loss should be considered and reduced when the sample loss is not dominant. Thus, cryogenic RF coil [24], superconducting RF coil [25], high permittivity material [26-27] and meta-material insert [28-29] have been proposed to decrease the coil loss and improve the Q-ratio/SNR.

Inter-element decoupling approaches

The elements of the receive coil arrays consists of inductors and capacitors which are tuned to the resonant frequency [30]. When the coil elements are close enough to each other, the mutual inductance of the inductors causes signal and noise coupling among the coil elements, which will decrease the SNR and parallel imaging capability of the arrays. High-density arrays increase the challenges of decoupling. Decoupling approaches can be divided into two main categories. The first type involves reducing mutual inductance by optimizing coil geometry, such as quadrature design [31-34], overlapping [35], or self-decoupling [36-38]. The other type involves eliminating currents transferred from other components, such as preamplifier decoupling, LC network decoupling [39], and ICE/magnetic wall decoupling [40-42]

System compatibility / RF safety testing

When the receive coil array is fabricated, system compatibility and RF safety testing should be carried out prior to in vivo scanning. System compatibility includes B0 and B1+ interaction testing. The B0 and B1+ maps with and without the receive coil array should be similar. Meanwhile, the temperature rise on the coil surface should remain within safety limits after high RF duty cycle sequences are performed.

Discussion

There are several novel applications of receive coil arrays besides detecting MR signals, such as iPRES [43] and AC/DC arrays [44-46] for local B0 shimming.

Acknowledgements

This work was supported by Youth Innovation Promotion Association of CAS No. 2017415 and city grant RCYX20200714114735123.

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Proc. Intl. Soc. Mag. Reson. Med. 32 (2024)