[The Basics of MRI 9.4] 磁共振成像基本原理 9.4 射频线圈

• Hardware Overview

• Magnet

• Gradient Coils

• RF Coils

• RF Detector

• Safety

• Phantoms

• Problems

RF Coils

射频线圈

RF coils create the B₁ field which rotates the net magnetization in a pulse sequence. They also detect the transverse magnetization as it precesses in the XY plane. 

RF coils can be divided into three general categories; 1) transmit and receive coils, 2) receive only coils, and 3) transmit only coils. 

Transmit and receive coils serve as the transmitter of the B₁ fields and receiver of RF energy from the imaged object. A transmit only coil is used to create the B₁ field and a receive only coil is used in conjunction with it to detect or receive the signal from the spins in the imaged object. There are several varieties of each. 

The RF coil on an imager can be likened unto the lens on a camera. A photographer will use one lens for a close up shot and a different one for a wide angle long distance shot. Just as a good photographer has several lenses, a good imaging site will have several imaging coils to handle the variety of imaging situations which might arise.

磁共振成像仪的射频线圈可以类比成照相机的镜头。摄影师会使用一个特定的镜头来拍摄特写镜头，另一种不同的镜头来拍摄广角远距离镜头。就好比一个好的摄影师拥有许多镜头，一个好的磁共振成像仪将会配备许多成像线圈，以应对许多不同的检查需求。

An imaging coil must resonate, or efficiently store energy, at the Larmor frequency. All imaging coils are composed of an inductor, or inductive elements, and a set of capacitive elements. The resonant frequency, ν, of an RF coil is determined by the inductance (L) and capacitance (C) of the inductor capacitor circuit.

一个成像线圈必须以拉莫尔频率进行共振才能有效地存储能量。所有成像线圈都由一个电感器（或感应元件）与一系列电容元件组成。射频线圈的共振频率 ν 由电感电容电路的电感 L 与电容 C 决定：

Some types of imaging coils need to be tuned for each patient by physically varying a variable capacitor.

一些类型的成像线圈需要在扫描每一个病人之前都需要通过改变可变电容的值来进行物理调谐。

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【译者注1】

RLC 串联电路的谐振

关于射频线圈的调谐，原文有一详细说明 RF Coil Tuning，在后续的翻译中将进行解读。

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The other requirement of an imaging coil is that the B₁ field must be perpendicular to the B_o magnetic field.

另一个成像线圈所要满足的条件就是，所产生的 B₁ 场必须与 B_o 主磁场垂直。

There are many types of imaging coils. Volume coils surround the imaged object while surface coils are placed adjacent to the imaged object. An internal coil is one designed to record information from regions outside of the coil, such as a catheter coil designed to be inserted into a blood vessel. 

成像线圈有许多种：容积线圈围绕于成像物体周围；表面线圈与成像物体的表面相邻；内置线圈记录线圈外围组织的信息，例如导丝线圈被用来进行血管成像。

Some coils can operate as both the transmitter of the B₁ field and the receiver of the RF signal. Other coils are designed as only the receiver of the RF signal. When a receive only coil is used, a larger coil on the imager is used as the transmitter of RF energy to producing the 90° and 180° pulses. 

一些线圈既可以作为发射线圈用来发射 B₁ 射频场，也可以作为接收线圈来接收射频信号。其他线圈只能作为接收线圈用来接收射频信号。当我们使用 receive-only 线圈时，成像仪内一个更大的线圈充当发射线圈来发射射频能量产生 90° 和180° 射频脉冲。

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【译者注2】

日常工作中使用的线圈大多都只是采集线圈，不具有发射功能。那么射频发射就由封装在磁体内部的 Q-body 线圈来进行射频的激发。 Q-body 线圈既可以作为发射线圈也可以作为接收线圈，比如我们对全身进行成像时就可能需要用到 Q-body 来充当接收线圈。

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The following table is a partial list of the more common imaging coils with the type classifier, the mode of operation (transmit/receive-T/R or receive only-R), a diagram, and a literature reference for the coil. The diagrams show the direction of the B₁ field.

下表给出了部分常见的成像线圈，分别列出了线圈的类型、操作模式（发射/接收-T/R 线圈、只接收-R 线圈）、示意图、以及参考文献。线圈示意图中给出了 B₁ 场的方向。

Surface coils are very popular because they are a receive only coil and have a good signal-to-noise ratio for tissues adjacent to the coil. In general, the sensitivity of a surface coil drops off as the distance from the coil increases.

表面线圈在当下十分流行，它是 receive-only 线圈，在表面线圈附近的组织可以获得良好的信噪比。总之，表面线圈的线圈敏感度随着距离的增加而下降。

Here is an example of an image of the lower human spine obtained with a surface coil.

下图是使用表面线圈所采集的腰椎图像。

Here is a picture of a flat circular surface coil with its connecting cable. The cable will connect the coil to the imager.

下图也是一表面线圈，连有连接电缆。线圈在使用时，连接电缆将连接到成像仪上。

This is a picture of a surface coil molded to conform to the back of the knee.

下面是一幅将表面线圈按膝关节的形状进行折叠包裹起来的图片。

The bird cage coil is the most routinely used volume coil. It is the coil of choice for imaging the head and brain. Here is a picture of the human head in a bird cage coil. All of the head images in this hypertext book were obtained using a bird cage coil.

鸟笼式线圈是最常见的容积型线圈，可以用来对头部与颅脑进行成像。下图就是一鸟笼线圈。这本书里使用的所有头部图像都是用鸟笼线圈扫描的。

The single turn solenoid is useful for imaging extremities, such as the breasts and the wrist. This animation window entry shows a single turn solenoid imaging coil around the human wrist. The detail icon will provide you with more information on the construction of a single turn solenoid.

单匝螺线管式线圈对于四肢的检查十分有用，比如腕关节的检查。单匝螺线管式线圈还可以用于乳腺的检查。下图就是使用单匝螺线管线圈检查腕关节。关于单匝螺线管的更多信息请参照后续的详细内容。