低成本64通道磁共振成像发射器：模块化设计与创新技术

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本文档主要探讨了在磁共振成像（MRI）领域的一项重要进展，即一种低成本且高度集成的64通道发射机设计。MRI技术依赖于精确控制射频（RF）信号的传输，以获取高质量的图像。传统的多通道发射机由于成本和可用性问题，在实际应用中受到了限制，尤其是在科研和临床环境中。当前的研究者们正在探索利用多通道发射机进行诸如传输SENSE（Sensitivity Encoding）和B1磁场校准等先进成像技术，这些技术能够提高图像分辨率和扫描速度。然而，高昂的成本和复杂的设计使得这些技术难以大规模普及。该论文介绍的64通道发射机系统旨在解决这一问题。它具备以下关键特点： 1. 高集成度：系统设计紧凑，每个通道都能提供高达100瓦的峰值输出功率，这在提升成像效率的同时，也降低了设备的总体尺寸，有利于安装和维护。 2. 模块化设计：系统采用四通道模块化结构，这种设计灵活性使得用户可以根据需要轻松扩展或缩减通道数量，适应不同规模的研究或临床应用需求。 3. 技术创新：为了简化设计并降低复杂性，文中提到采用了矢量调制器替代传统的I/Q直接数字调制器。矢量调制器能够在保持性能的同时，减少电路元件的数量，从而降低成本。 4. 功率放大器：使用了100瓦的MOSFET射频放大器，结合部分微带匹配技术，实现了高效能与高效率的平衡，同时确保了信号的稳定输出。整体而言，这篇论文的成果对于推动MRI技术的普及和应用具有重要意义，因为它提供了一种经济高效的解决方案，使更多研究机构和医疗机构能够利用多通道技术进行深入的MRI研究，从而推动MRI技术的发展和临床实践的进步。

2152 IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 59, NO. 8, AUGUST 2012

A 64-Channel Transmitter for Investigating Parallel

Transmit MRI

Ke Feng, Neal A. Hollingsworth, Student Member, IEEE, Mary Preston McDougall, Member, IEEE,

and Steven M. Wright

∗

, Fellow, IEEE

Abstract—Multiple channel radiofrequency (RF) transmitters

are being used in magnetic resonance imaging to investigate a num-

ber of active research topics, including transmit SENSE and B

shimming. Presently, the cost and availability of multiple channel

transmitters restricts their use to relatively few sites. This paper

describes the development and testing of a relatively inexpensive

transmit system that can be easily duplicated by users with a rea-

sonable level of RF hardware design experience. The system de-

scribed here consists of 64 channels, each with 100 W peak output

level. The hardware is modular at the level of four channels, easily

accommodating larger or smaller channel counts. Unique aspects

of the system include the use of vector modulators to replace more

complex IQ direct digital modulators, 100 W MOSFET RF am-

pliﬁers with partial microstrip matching networks, and the use of

digital potentiometers to replace more complex and costly digital-

to-analog converters to control the amplitude and phase of each

channel. Although mainly designed for B

shimming, the system is

capable of dynamic modulation necessary for transmit SENSE by

replacing the digital potentiometers controlling the vector modula-

tors with commercially available analog output boards. The system

design is discussed in detail and bench and imaging data are shown,

demonstrating the ability to perform phase and amplitude control

for B

shimming as well as dynamic modulation for transmitting

complex RF pulses.

Index Terms—B

shimming, parallel transmission, radiofre-

quency (RF) ampliﬁer, transmit SENSE, vector modulation.

I. INTRODUCTION

AGNETIC resonance imaging (MRI) systems have in-

creasingly moved to higher main magnetic ﬁelds in an

effort to obtain an increased signal-to-noise ratio (SNR) [1]–[3]

with associated potential beneﬁts [4]–[6]. The correspondingly

higher frequencies and shorter wavelengths, however, have

led to well-known radiofrequency (RF) inhomogeneity prob-

lems [7]–[11]. Parallel transmission in MRI has been explored

in recent years as a method to address this problem, as well

Manuscript received June6, 2011; revisedAugust 24,2011; acceptedSeptem-

ber 27, 2011. Date of publication April 27, 2012; date of current version July

18, 2012. This work was supported in part by the National Institutes of Health

under Grant 1R21EB005695 and Grant 1R01NS05876. Asterisk indicates cor-

responding author.

K. Feng and N. A. Hollingsworth are with the Department of Electrical and

Computer Engineering, Texas A&M University, College Station, TX 77801

USA (e-mail: jnfengke@gmail.com; nealaholl@gmail.com).

M. P. McDougall is with the Departments of Biomedical Engineering and

Electrical and Computer Engineering, Texas A&M University, College Station,

TX 77801 USA (e-mail: mpmcdougall@tamu.edu).

*S. M. Wright is with the Department of Electrical and Computer Engi-

neering, Texas A&M University, College Station, TX 77801 USA (e-mail:

smwright@tamu.edu).

Digital Object Identiﬁer 10.1109/TBME.2012.2196797

as to accelerate spatial-spectral RF pulses, and even poten-

tially reduce speciﬁc absorption rate (SAR) [7]–[9], [12]–[17].

Much of the early research in the ﬁeld used simulations due

to the lack of multiple channel hardware systems [15], [16].

Other studies combined several commercially available trans-

mitters [16]–[18]. This approach is limited by high cost and

complexity, making such parallel systems impractical for most

researchers. Commercial solutions are becoming available but

remain relatively expensive, though Vaughn et al. successfully

implemented a 16-channel parallel transmit system [19]. We

report here the design, construction, initial testing, and results

from a 64-channel parallel transmitter that is both scalable and

of low cost. This system is capable of not only traditional parallel

transmission MRI applications, but also additional possibilities

enabled by the high channel count. In particular, the system was

developed for rapid imaging using a transmit/receive version of

single echo acquisition (SEA) imaging [20].

In this paper, we focus on the design and implementation

of the 64-channel parallel transmitter system. A system-level

overview is presented with details of each subsystem (mod-

ulation, control system, RF ampliﬁers, transmit/receive (T/R)

switches, and preampliﬁers), followed by a discussion of the

integration of the parallel transmitter with an existing MR sys-

tem. We conclude with results from bench testing and imaging

to demonstrate the functionality of the system, and a discussion

of possible future modiﬁcations.

II. M

ETHODS

A. System Overview

Each channel in the parallel transmit system had an indepen-

dent modulator and a two-stage power ampliﬁer. An overview of

a single channel is shown in Fig. 1. The interface to the MR sys-

tem consisted of a single digital line to gate the transmit system

and the RF signal, which was split 64 ways in the transmitter sys-

tem to each of the modulators. Each modulator utilized a single

integrated circuit (IC) vector modulator (described later) with

additional gain and signal conditioning on board. The modulat-

ing signals were provided by digitally controlled potentiometers

for “static modulation,” which is well suited to B

shimming

applications. These can be easily replaced with analog sources

to allow for the true RF pulse modulation needed for transmit

SENSE, which is also demonstrated. The modulators were fol-

lowed by a two-stage, noise-gated RF power ampliﬁer system

capable of 100 W peak output per channel. Finally, active T/R

switches and preampliﬁers were located near the magnet bore

to interface to the coil array and the parallel receive system.

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