An MRI (Magnetic Resonance Imaging) scan uses a powerful magnet, radio waves, and a computer to create detailed images of the inside of your body. An MRI coil is a device that acts like an antenna, receiving the radio frequency signals from the body during the scan. They are placed on or around the part of the body being imaged to help produce clear pictures for diagnosis. The information from the coils is then sent to a computer to construct the final images.
How an MRI Coil Works
An MRI coil’s primary function is to transmit and receive radiofrequency (RF) signals. When you are inside the MRI machine, a powerful magnetic field causes the protons in your body’s water molecules to align in the same direction. The MRI machine then sends a radiofrequency pulse from a transmit coil that knocks these protons out of alignment. As the protons realign, they release energy in the form of RF signals.
The coil detects these faint signals. The quality of an MRI image depends on the signal-to-noise ratio (SNR), which is the strength of the signal from the body compared to background noise. A coil placed close to the body part being scanned can pick up a stronger signal, improving the SNR. This is because the signal strength decreases with distance from the coil.
This improved signal clarity allows the MRI system to generate high-resolution images. Different coils are designed to maximize this effect for specific body parts.
Common Types of MRI Coils
MRI coils come in various shapes and sizes, each designed to fit a specific part of the body to ensure high image quality. The design of the coil is related to its intended application, as a snug fit improves the signal reception.
For brain imaging, technologists use a head coil, which resembles a helmet or a cage-like structure that fits over the patient’s head. This design ensures the antenna elements are uniformly close to the entire brain. For joints like the knee or wrist, a cylindrical coil is used, where the patient slides their limb into the device.
Shoulder coils are designed to be draped over the shoulder and upper arm, conforming to the complex shape of the area. Other coils, known as surface or flex coils, look like flexible pads that can be wrapped around a body part or placed directly on the skin over the area of interest, such as the spine.
The Patient Experience with a Coil
During an MRI scan, a technologist places the appropriate coil on or around the body part being examined. For a knee MRI, they will have you lie on the scanner table and then fit the coil around your leg.
Coils are designed for a snug fit to ensure high-quality images while being as comfortable as possible. While some, like a head coil, might feel restrictive, they are not meant to be painful and are generally lightweight.
The technologist is trained to position the coil correctly to be secure but not uncomfortably tight. If you experience pain or discomfort from the coil, it is important to let the technologist know so they can make adjustments.