Magnetic Resonance Imaging (MRI) is a powerful diagnostic tool that creates detailed images of internal body structures, including organs, bones, muscles, and blood vessels. Unlike X-rays or CT scans, MRI does not use ionizing radiation, relying instead on strong magnetic fields and radio waves. While providing invaluable diagnostic information, a common experience for patients undergoing an MRI scan is the significant noise generated during operation. This inherent loudness can be surprising and often a source of concern.
The Physics Behind the Noise
The primary source of the loud sounds in an MRI machine comes from its gradient coils. These coils are essential for creating the detailed images by rapidly changing magnetic fields within the scanner. Inside the MRI scanner, three sets of wire coils modulate the main magnetic field by carrying electrical currents in different directions. Gradient amplifiers supply these coils with rapid pulses of high voltage electricity, causing them to expand and contract very quickly.
This rapid expansion and contraction of the gradient coils is a direct result of the Lorentz force. This force describes the interaction between electrical currents in the coils and the MRI machine’s powerful main magnetic field. As these currents rapidly switch, they generate significant force, causing the coils to vibrate against their mountings. The resulting mechanical vibrations produce the loud noises, which are further amplified within the scanner’s hollow, tube-shaped design, similar to how sound resonates inside a drum.
Characterizing the Sounds
Patients typically hear a variety of distinct sounds during an MRI scan, including knocking, banging, buzzing, clicking, whirring, clanging, and beeping. These different sounds correspond to the activation of specific gradient coils or the use of various pulse sequences. Each pulse sequence creates a unique sound based on the current’s waveform, with different sequences used to capture images from various angles or with different contrasts.
Sound intensity typically ranges from 65 to 130 decibels (dB), often exceeding 90 to 100 dB. To provide a sense of scale, 70 dB is comparable to an alarm clock, 94 dB to a food processor, 112 dB to a rock concert or chainsaw, and 120 dB to a jet plane takeoff. The loudest sounds often occur with fast gradient echo and echo-planar imaging (EPI) pulse sequences. Some high-field scanners, such as 7-Tesla MRI machines, can produce average sound levels exceeding 105 dB.
Patient Experience and Mitigation
The substantial noise produced by MRI machines can lead to patient discomfort, anxiety, and even claustrophobia. Prolonged exposure to these high decibel levels without protection poses a risk of hearing damage, including temporary threshold shifts in hearing sensitivity. This discomfort can sometimes lead to patient movement, potentially necessitating repeat scans.
To mitigate the noise impact, manufacturers and clinics employ several strategies. Providing hearing protection, such as earplugs or headphones, is standard practice and helps significantly reduce sound reaching the patient’s ears. These protective devices must be non-metallic and approved for MRI use to ensure safety and prevent image artifacts. Some facilities also offer music through headphones to help distract patients and enhance comfort.
Technological advancements have led to the development of “silent” or “quiet” MRI sequences. These sequences reduce the rate at which gradient coils switch, thereby lowering the noise levels. While these quieter sequences can reduce noise by up to 20 dB, they may sometimes increase scan time or slightly alter image quality. Additionally, scanner designs incorporate acoustic insulation within the bore and specialized room construction to minimize noise transmission, including features like sound-absorbing panels, insulated walls, and vibration dampening systems.