Magnetic Resonance Imaging (MRI) uses strong magnetic fields and radio waves to generate detailed images of organs and tissues inside the body. This non-invasive diagnostic procedure provides clear contrast between different types of soft tissue. However, the procedure is characterized by the extreme, repetitive acoustic noise produced by the machine.
The Mechanism Driving MRI Noise
The loud banging and knocking sounds originate not from the main, static magnet, but from the gradient coils. These coils create temporary variations in the magnetic field necessary for spatial encoding, allowing the machine to pinpoint the origin of radio signals. To capture an image, the electrical current running through these coils must be rapidly switched on and off, often thousands of times per second.
When electrical current passes through the coils within the strong magnetic field, the Lorentz force is generated. This force causes the coils to rapidly expand and contract with the changing current, pushing against their mounting structure. The resulting loud noise is the sound of these powerful, high-speed vibrations resonating within the scanner’s bore.
The noise intensity relates directly to the specific imaging sequence being performed. Different sequences require varying speeds and amplitudes of current switching, which alters the frequency and force of the coil vibrations. Sequences requiring the fastest switching rates, such as Echo-Planar Imaging (EPI), are typically the loudest, producing intense, rapid banging sounds.
Measuring the Intensity and Safety Thresholds
Sound pressure levels inside the MRI bore often range from 110 to 130 decibels (dB) in modern scanners. This volume is comparable to standing near a jet engine during takeoff or being at a loud rock concert. Intensity varies based on the scanner’s magnetic field strength, as 3-Tesla (3T) machines produce louder noise than 1.5T machines due to increased Lorentz forces.
High noise levels risk temporary or permanent hearing damage, necessitating safety regulations. The FDA permits peak acoustic noise levels up to 140 dB, while the International Electrotechnical Commission (IEC) limits time-averaged exposure to 99 dB for up to one hour. Hearing protection is mandated to reduce the sound reaching the patient’s ears.
Unprotected exposure to noise above 100 dB can cause a temporary threshold shift, resulting in a brief dulling of hearing or ringing in the ears. Lasting hearing damage, known as permanent threshold shifts, is associated with noise levels exceeding 130 to 140 dB. Because of these risks, protective measures are required to ensure the patient’s exposure remains below mandated limits.
Strategies for Noise Reduction and Patient Comfort
Patient protection relies primarily on passive methods that attenuate the noise before it reaches the inner ear. Standard procedure involves providing mandatory hearing protection, typically earplugs and/or MRI-compatible headphones. Properly fitted foam earplugs reduce sound exposure by 25 to 30 dB, and combining them with headphones provides an even greater reduction.
Engineers have also developed active technological solutions to reduce noise at its source. Structural modifications, such as incorporating acoustic dampening materials into the scanner’s housing, minimize the transmission of vibrations. Some advanced systems enclose the gradient coils in a vacuum chamber, preventing mechanical vibrations from creating airborne sound waves.
The most significant advancement is the development of “quiet” or “silent” MRI sequences. These sequences modify the gradient waveform or use slower, continuous switching rates to minimize the sudden Lorentz forces that cause the loud banging. Certain “Silent Scan” techniques can reduce the mean noise level from over 100 dB in conventional scans to around 68 dB, bringing the noise down to ambient background levels.