Magnetic Resonance Imaging (MRI) is a powerful medical tool that uses a strong magnetic field and radio waves to generate remarkably detailed images of the body’s soft tissues and organs. While the procedure is non-invasive and painless, the experience inside the scanner is often characterized by extremely loud, repetitive noises. These sounds are a completely normal consequence of the machine’s complex physical operations. Understanding the nature of the noise, its source, and the safety measures in place can help alleviate the anxiety associated with the scan.
The Auditory Experience
Patients undergoing an MRI scan are exposed to a variety of distinct sounds that change depending on the specific imaging sequence being performed. The most noticeable and intense sounds are often described as a rapid, repetitive banging, clanking, or jackhammering noise. This loud acoustic output occurs when the machine is actively acquiring the data needed to construct the images. The pattern of this noise follows the structured electrical pulses of the imaging protocol.
Other sounds include high-pitched whirring or beeping noises, which may resemble a rapid clicking or thumping sound. The acoustic profiles of these noises vary significantly in frequency and pitch, creating a dynamic auditory experience. These intense sound sequences, which can last from a few seconds to several minutes, are separated by brief periods of relative quiet. The sounds are further amplified by the confined, cylindrical space of the machine’s bore, making the environment much louder for the patient inside.
The Physics Behind the Noise
The loud acoustic noise is a direct result of the machine’s process of localizing and encoding the image data. The mechanism involves the rapid, pulsed switching of powerful electrical currents within specialized components called gradient coils. These coils are essentially electromagnets nested inside the main bore of the scanner. The main magnet maintains a static, uniform magnetic field, which is necessary for the initial alignment of the body’s protons.
When the electrical current is quickly turned on and off in the gradient coils, the static magnetic field exerts a physical force on the coil wires. This phenomenon is known as the Lorentz force. Because the current is switched extremely fast—often thousands of times per second—the Lorentz force causes the gradient coils to vibrate and physically expand and contract. This rapid mechanical movement generates the strong acoustic pressure waves heard as the loud banging and thumping noises. The specific pitch and frequency of the sound are determined by the rate at which the electrical current is switched, which varies with each imaging sequence.
Managing the Noise and Safety Precautions
The noise levels produced by a modern MRI scanner are substantial, often reaching between 110 and 120 decibels, which is comparable to the volume of a jackhammer or a loud rock concert. Exposure to sounds at this intensity necessitates mandatory safety precautions to prevent potential hearing damage. The primary safety measure involves the provision and required use of hearing protection for all patients.
Patients are typically given a combination of foam earplugs and non-metallic earmuffs or specialized headphones, which work together to significantly reduce the sound exposure. The International Electrotechnical Commission (IEC) recommends that this protection reduce the effective exposure level to below 99 decibels.
In addition to sound protection, the technician operating the scanner maintains contact with the patient using a two-way communication system, often integrated into the headphones. This system allows the technologist to check on the patient and provide instructions over the substantial acoustic noise. Some newer machines also offer “silent scan” or acoustic reduction modes, which use specialized hardware and software to dampen the sound, though this may result in slightly longer scan times.