There is a wide range of MRI coils types available on the market.
They serve different purposes and some of them even have multiple ones.
Therefore, it can be difficult to get an overview.
Radiofrequency (RF) coils have undergone great development over the past few years, from the single loop to a quadrature, and eventually to the phased array coil.
Technology has developed, and the advancement of coils continues.
Without getting too much into that, we would like to give you a simple, but informative introduction to the world of this magnetized copper.
So, what exactly is an MRI coil?
A coil, by definition, is anything wound in a joined sequence of concentric rings. In other words, it is a looped length of wire.
It becomes actively electromagnetic whenever you run an electrical field through it.
Does this all sound familiar?
Well, it might as it is the Faraday’s law that takes you back to your school days.
Receiver coils are responsible for detecting the MR Signal.
The technical design and engineering of each coil differ, and the differences often relate to their functionality.
But even coils with the same functionalities can be different depending on the manufacturer of the coil.
A coil is responsible directly and indirectly for the image created through the resonance of the magnetic field.
The MRI and the coil create images by working together just like in any digital camera.
The MRI external field acts as the light, going through a coupled-charged sensor and the coil acts as the recipient sensor that translates the images into digital pictures.
In the text below, you can read a very simplified version of what happens between the coil and electromagnetic field:
To understand the role of MRI coils in creating an image, there are two concepts of movements we need to explain:
Oscillation and Precession
Firstly, we will talk about oscillation.
If you are a person who enjoys handcrafts, you are likely familiar with power tools, specifically oscillating multi-tools.
Oscillation is the pattern of movement back and forth that power tools simulate to create enough energy and power.
Radiofrequency coils generate an oscillating and rotating magnetic field.
Meeting the main magnetic field with a 90° angle, it will seek to match the natural movement of protons in the external magnetic field.
This movement is called precession and it is the second concept we would like to introduce.
The precession of protons can be described in a very simple way.
Imagine a toothpick with an olive on top of it, the olive is the proton.
If you pin the end of the toothpick to a fixed point and proceed to turn the olive in circles without moving the toothpick away from that point, this movement is the exact definition of proton precession.
Whenever these two movements of precession and oscillation align, energy is deposited into the system to create a signal.
That signal gets transmitted through a coil, then received by a coil, eventually converted into data, and translated through a computer to form the final image.
Nevertheless, every MRI exam is different.
They are very specialized based on the scanner you have, and the MRI coils types you are using.
A coil helps the MRI system capture high-quality images of a part of the body and is essential when generating images.
Meaning, if you do not have a coil, you do not have an image.
What Are the Common Types of MRI Coils on the Market?
Any MRI scanner is made of several coils.
First, there are magnet windings (superconductive coils), then a shim coil for homogeneity that provides equal magnetic strength throughout the system, and lastly, the main coil, which is the gradient coil.
All these coils are shielded together.
In that shield, there is a cryogen of liquid helium that maintains a low temperature that secures superconductivity.
Underneath that shield is another RF shielding with a radiofrequency coil (body coil).
The patient gets inserted from the outside of the MRI Scanner and connected to the patient coil. As you can see, there coils serve different purposes.
Moreover, you can find coils that can be used for different parts of the body.
The MRI coils types that we will describe are the Radiofrequency coils, which are used to acquire images.
Radiofrequency (RF) Coils
RF coils are responsible for transmitting/receiving the signal. These coils are converting the signal into data that get transferred into a computer and processed to form the final image.
The quality of the MRI images depends on the signal-to-noise ratio (SNR) of the acquired signal from the patient.
The received signal is electronically strengthened, to maximize the reduction of the background noise, which increases the quality of the image.
Hence, the higher the SNR, the better. A lower signal-to-noise ratio generally results in a grainy appearance of the images.
RF coils can be divided into three general categories:
Transmit Only Coil
Receive Only Coil
Transmit Receive Coil
Nevertheless, this is not the only distinction of RF coils. It is also possible to divide them into Volume and Surface coils.
Let’s talk about these MRI coils types in more detail.
These coils have been designed to provide a homogeneous RF excitation across a large volume, hence, they are suitable for full-body imaging.
However, they can be smaller and, in that case, used for head and extremities.
The magnetic field homogeneity of volume coils is better than the homogeneity of surface coils.
It is possible to use the same volume coil to transmit and receive simultaneously. Nevertheless, you can also use two separate coils.
The larger volume coil will be the body coil, typically a transmit and receive coil. If you are using two separate coils, the body coil acts as the transmitter while the smaller coil acts as the receiver.
In general, volume coils are great for transmitting, but less ideal when used for small regions of interest.
Among the common volume MRI coils types belong:
Circularly Polarized Coil (photo on the left)
Bird Cage Coil
Helmholtz Pair Coil
Paired Saddle Coil
Single Turn Solenoid
Types of Volume Coils
Circularly Polarized Coil
Circularly polarized coils have been designed to excite or detect electromagnetic fields by means of two orthogonal transmit and/or receive channels.
When it is a receiver coil, it has a better SNR than a linearly polarized coil (which belongs to the surface coils).
Next, we have the quadrature coils. They produce an RF field with circular polarization.
The RF power that is obtained from the RF power amplifier comes in two signals and the RF transmit coil transforms the power into a circularly polarized RF magnetic field.
Finally, quadrature coils can be used as both, transmit and/or receive coils.
These coils provide the best RF homogeneity of all RF coils. Its name comes from the shape of a birdcage.
This coil is commonly used as a transmit-receive coil for imaging the head. Occasionally, it is used for imaging of extremities such as knees.
A crossed coil is an RF pair of coils arranged with their magnetic fields at right angles to each other in a way that minimizes their mutual magnetic interaction.
Helmholtz Pair Coils
These coils consist of two identical circular coils parallel to each other. Usually, they are used as z gradient coils in MRI scanners. However, they are occasionally used as RF coils for pelvis imaging and cervical spine imaging.
Paired Saddle Coil
Paired saddle coils are used as the x- and y-axis of gradient coils as well as extremity coils. This configuration creates a very linear, homogeneous magnetic field along its central axis.
Single Turn Solenoid Coil
These coils are transmit and receive RF imaging coils that generally have a cylindrical shape.
What is the solenoidal configuration of these coils?
It is a further developed planar surface coil.
While surface coils have a strong coupling and high SNR to nearby signals, they exhibit a significant signal loss above one radius from the centre of the coil.
With a solenoidal coil design, the imaged object is placed within one radius of the centre of the coil. That improves imaging of extremities such as wrists or knees.
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Surface coils are the coils with the simplest coil design.
Basically, it is a loop of wire, either circular or rectangular, that is placed over the region of interest (around the surface of a patient) for increased magnetic sensitivity.
You can find the loop in various shapes. It can be also slightly bent to conform to the imaged body part.
The depth of the image of a surface coil is generally limited to about one radius.
As a type of RF coil, they belong to the group of receive-only coils. The surface coils have a great SNR for tissues placed near the coil. Therefore, the further the tissue is from the coil, the less sensitive it is.
Furthermore, the possibilities of usage are wide.
You can use the surface coils for spines, shoulders, the joint of the jaw (temporomandibular joint), and imaging of other smaller body parts.
The common surface MRI coils types are:
Array Coil (photo on the left)
Body Wrap-Around Coil
Linearly Polarized Coil
Types of Surface Coils
These coils merge the benefits of smaller coils (high SNR) with the benefits of larger coils (large measurement field). Moreover, they consist of separate multiple smaller coils that you can use individually or combined.
On the market, you can find several types of array coils such as a coupled array, isolated array, and phased array coils.
Body Wrap Around Coils (BWA)
BWA coils are flexible surface coils for body imaging.
Linearly polarized coils (LP)
These coils are designed to excite or detect electromagnetic fields using one RF transmit and/or receive channel, in contrast to circularly polarized MRI coils which are using two channels.
Primarily, the magnetic field of the LP coils has a single direction.
Saddle coils have cylindrical bodies with one or more turns of wire or foil on each side. The coils with foil are newer versions of saddle coils.