MXR Imaging acquired Oxford Instrument Healthcare in May 2020

What Everything You Need to Know About MRI Coils

Your Go-To Guide for MRI Coils

MRIs are used by many hospitals to capture high-quality diagnostic images of a patient's internal organs and structures. They produce magnetic fields to capture these images. There are many components an MRI needs to generate a magnetic field. In addition to the magnets themselves, some of the most important components include the MRI coils. The following is a breakdown of what MRI coils are, why there are so many different types, and what their function is.

​An Overview of MRI Coils

MRI coils function similarly to antennas. A single coil consists of one or more loops of conductive wire looped around the coil's core. The coils are vital to capturing diagnostic images since they are what help to create the magnetic field through the voltage that's induced in the wire. The typical MRI system consists of a variety of different coils, some of which are built within the machine itself and some of which circle the patient's body.

Why Are There So Many Kinds of Coils?

An MRI system requires the use of multiple coils to create a uniform magnetic field to enable the capture of the highest quality diagnostic image possible. There are several different types of coils within the MRI system itself that serve different functions. Coils include gradient coils, radiofrequency coils, surface coils, shim coils and more.

The Coils in The Machine

The following are the different types of MRI coils built within the actual system:

Shim Coils

Shim coils only carry a small current, which is used to provide an auxiliary magnetic field. These coils are meant to help adjust the homogeneity of the magnetic field being produced by the MRI. They are typically found in the cryostat, which contains the system's liquid helium, or they are mounted within the inner walls of the scanner. Some MRI systems have shim coils in both locations. Both types of shim coils require their own individual power supplies and are controlled by special circuitry.

Gradient Coils

Gradient coils produce a secondary magnetic field that purposefully distorts the main magnetic field to allow for spatial encoding of the MRI signal. There are three sets of gradient coils built into the MRI system. The coils are made by winding thin strips of aluminum or copper into a specific pattern. The size of these coils is also what determines the width of the bore through which the patient is placed. When turned on, the strength of the magnet on one side of the bore will be less than the static main magnetic field, while the strength on the other side will be greater. There are several types of gradient coils, including figure 8 coils, Golay coils, Helmholtz coils, Maxwell coils, paired saddle coils, and shielded gradient coils.

Radio Frequency (RF) Coils

It is the RF coils that function as the antennas of the MRI system. They both broadcast the RF signal to the patient and receive the return signal. If body coils are used to transmit the RF signal, the RF coils will only be used to receive the return signal. When they are used as transmitters, they will produce a rotating oscillating magnetic field that is perpendicular to the static magnetic field. Modern MRI systems generally use separate transmit and receive systems.

The Coils For The Body

In addition to the MRI coils built into thesystem, there are numerous coils that are used on the patient when capturing images of specific parts of the body. The following are a few of the different coils commonly used on the patient:

• Surface coils - Surface coils can only receive MR signals. They are known for providing excellent signal-to-noise ratio for tissues that are positioned adjacent to the coil. Surface coils are commonly used for capturing detailed images of a patient's spine, temporomandibular joints, shoulders, and smaller body parts. Surface coils extend over a large area. They include array coils, body wrap around coils, linearly polarized coils, and saddle coils.
• Volume coils - Volume coils are designed to surround the patient's entire body, although they can also be used to surround specific regions, such as the head. Volume coils provide a better RF homogeneity than surface coils and can both transmit and receive. They include birdcage coils, circularly polarized coils, crossed coils, Helmholtz pair coils, paired saddle coils, quadrature coils, and single turn solenoids.
• Extremity coils - Extremity coils are used for exactly what they're named for -- the extremities of the patient, such as the wrists, elbows, shoulders, ankles, and more.

What Kind of Repair Will Coils Need?

Coils can malfunction or experience physical damage, which will require you to have your coils inspected and either repaired or replaced. An MRI coil can experience two main categories of problems--electronic problems and cosmetic problems.

Electronic

Electronic problems can occur for a variety of reasons. They can simply happen because the coil has failed or malfunctioned due to no fault of the operator (often as a result of regular use and aging) or they can happen due to improper use. For example, when a transmit/receive coil is used in conjunction with a body coil, it can result in the transmit/receive coil's electronics breaking down due to the increase of focused energy coming from the body coil.

Some of the signs that your MRI coils are experiencing electronic issues include a loss of signal, a low signal, a grainy image, poor contrast, a coil ID problem, poor uniformity, fat saturation, and artifacts in the image (such as localized artifacts, bright areas, or dark holes).

Cosmetic

Cosmetic issues refer to physical damage that your MRI coils can experience. In some cases, physical damage may not affect the electronic function of the coil, but it should still be addressed and repaired (or replaced if needed). Any kind of cosmetic damage should be taken seriously since it could eventually affect its function if it hasn't already, and you don't ever want to risk the quality of the image captured or the safety of the patient by using a damaged coil.

Coils are very sensitive, which means that they must be handled with care. Damage can occur if the coil is dropped, which can cause damage to the cables, the coil loops, the connector pins, or the solder joints. Impact damage can also result in the components of the coil to become loosened, which can result in failure. If a coil is dropped, and physical damage is noticeable, it should be inspected. Coils should regularly be evaluated for potential damage as well, such as damage to the housing, output cable, or connector.

What You Can Do To Prolong the Life of Your MRI Coils?

The best way to ensure that your coils last for as long as possible is to take care of them. Make sure that they are handled carefully so that they aren't dropped or physically damaged. It's also important that you have your MRI system inspected regularly. This allows your coils to be evaluated. Your coils may have experienced some damage as a result of wear and tear that can still be repaired. By repairing minor issues as soon as possible, you can prevent them from developing into major problems that could drastically reduce their lifespan.