With the continuous improvement of magnetic resonance imaging equipment performance, the application of magnetic resonance imaging in clinical practice is becoming increasingly widespread. As is well known, MRI equipment mainly consists of auxiliary equipment such as main magnet system, gradient system, RF system, computer system, etc. The coil runs through the entire MR equipment, such as the shim coil, gradient coil, RF coil, etc.
The editor will introduce you to RF coils. 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 functions of RF coils, they can be divided into two categories: transmitter/receiver coils and receiver coils.
The transmit/receive coil (T/RCoil), as the name suggests, integrates the transmit and receive coils together, which can emit and receive radio frequency pulses. The commonly used transmit/receive coils in our daily use include body coils embedded in magnet apertures, head coils (transmit/receive coils), and knee joint coils. This type of coil does not receive and transmit simultaneously, but quickly switches between transmission and reception through electronic wires, thereby emitting radio frequency pulses and receiving signals.
The receiving coil is only used to receive magnetic resonance signals. This type of coil is commonly found in surface soft coils, body control array coils, and partial limb coils. For this type of coil, the task of emission and excitation is entrusted to the body coil installed within the aperture of the magnet during use.
According to the scope of application, it can be classified into five categories: 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 coils are mainly used for transmitting radio frequency pulses and receiving MR signals from tissues, including head coils, body coils, etc. The head full volume coil is generally referred to as a bird cage coil, which has two functions: transmitting and receiving. Due to its large space, this type of coil can generally be used for head positioning and gamma knife positioning.
A partial volume coil is a coil composed of a full volume coil and a surface coil, usually with two or more imaging planes.
The surface coil can be placed in the receiving coil on the surface of the imaging body. The surface coils have different shapes, commonly known as flat rectangles and circles. Due to its proximity to the imaging part, this type of coil has a high degree of freedom when placed and is typically used for shallow tissues or organs. Due to the limited number of surface coil channels at the beginning, parallel acquisition technology cannot be used to accelerate scanning. Later, in order to improve the functionality of surface coils and expand their application range, some new surface coils, such as phased array coils, were developed.
A phased array coil is a coil array composed of two or more coil units. These arrays are interconnected 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, forming multi-channel transmission, parallel acquisition technology is compatible during the imaging process, greatly improving scanning efficiency. 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.
The coil inside the body cavity is a small coil. When used, it must be placed inside the human body cavity, which can elevate certain structures inside the body, such as rectal coils. In principle, the body cavity coil is still a surface coil.
MicroscopeCoil is a special type of surface coil, also known as a micro coil, due to its small diameter.
There are many ways to name and classify coils, such as orthogonal coils (birdcage coils), which we previously introduced as transmit/receive coils. Some people classify according to polarization, main magnetic field direction, or winding form, but regardless of the classification, there are currently only so many commonly used coils.