磁共振成像基本原理 9.4 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.
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.
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。
The other requirement of an imaging coil is that the B1 field must be perpendicular to the Bo 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 Bo 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.
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。
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。
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.
Here is a picture of a flat circular surface coil with its connecting cable. The cable will connect the coil to the imager.
This is a picture of a surface coil molded to conform to the back of the knee.
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.
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.
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