The MRI equipment we are familiar with mainly consists of a main magnet system, gradient system, RF system, computer system, and other auxiliary equipment. The coil runs through the entire MR equipment, such as: uniform field coil, gradient coil, RF coil, etc.
What is the function of an RF coil? Send radio frequency (RF) pulses to the patient and receive the signal. From the perspective of the function of RF coils, they can be divided into two categories – transmitter and receiver coils and receiver coils.
The T/R coil, as the name suggests, integrates the transmitting and receiving coils, which can emit and receive radio frequency pulses. Common transmitting/receiving coils in our daily use include a body coil embedded in the aperture of the magnet (referred to as Q-body coil or QBC coil), a head coil (transmitting/receiving coil), and a knee joint coil. This type of coil does not work simultaneously in receiving and transmitting, but through electronic circuits, it quickly switches between transmitting and receiving to achieve the function of transmitting radio frequency pulses and receiving signals.
The receiving coil is only used to receive magnetic resonance signals, and this type of coil is commonly found in surface soft coils, body phased array coils, and some limb coils. For this type of coil in use, the task of emission and excitation is entrusted to the body coil installed inside the magnet aperture to complete.
If classified according to the scope of application, coils can be divided into the following five categories based on their usage range: full volume coils, partial volume coils, surface coils, intracavity coils, and phased array coils.
Full volume coils generally refer to cylindrical coils that can wrap the entire or a certain position. These types of coils are mainly used to transmit radio frequency pulses and receive MR signals from tissues, such as head coils and body coils. The full volume coil of the head, also known as the bird cage coil, has two functions: transmitting and receiving. This type of coil, due to its large space, can generally be used for head positioning and gamma knife positioning.
Partial volume coils are coils formed by combining full volume coils with surface coils, which typically have two or more imaging planes.
Surface coil, a receiving coil that can be placed on the surface of an imaging body. The surface coils have different shapes, such as flat rectangles and circles. Due to the close proximity of these coils to the imaging site, there is a high degree of freedom when placing the coils, which is commonly used in shallow tissues or organs. Due to the single number of surface coil channels at the beginning, parallel acquisition technology cannot be used for accelerated scanning. In order to improve the functionality of surface coils and expand their application range, new surface coils, such as phased array coils, were developed later.
A phased array coil is a coil array composed of two or more coil units that are connected to form a larger imaging range. Each coil unit can be separated from each other, and each coil unit can be considered as an independent coil. Due to the fact that each coil unit can transmit signals through a separate channel, resulting in multi-channel transmission, parallel acquisition technology is compatible during imaging, greatly improving the efficiency of scanning. The acceleration factor of parallel acquisition is related to the number of coil channels in that direction. If the number of coil channels in a certain direction is 5, the limit of the parallel acquisition factor in that direction is 5.
Intraluminal coil is a small coil that must be placed in the human body cavity when used. It can perform high-resolution imaging of certain structures in the body, such as rectal coils. In principle, the body cavity coil is still a surface coil.
Microscope coil is a special type of surface coil, also known as a microscope coil due to its very small diameter.
There are many other names and classifications for coils, such as orthogonal coils (birdcage coils), which are the transmission/reception coils we introduced earlier. Some people also classify them according to polarization mode, main magnetic field direction, or winding form, but no matter how they are classified, there are currently only so many coils that we commonly use