How does an MRI scan work and why is it noisy?
MRI makes pictures with magnets and radio waves, not X-rays, and its loud knocking comes from vibrations in the scanner itself.
Covers: This page explains the basic physics of magnetic resonance imaging — how strong magnetic fields and radio waves produce images of the body — and the mechanical origin of the loud banging sounds during a scan. It does not cover detailed clinical indications, interpretation of specific scan results, or safety screening for implants.
Also answers: Why is an MRI so loud? · What makes the banging noise in an MRI? · How do MRI machines create images? · What happens during an MRI scan?
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The short answer
Evidence-backed AI-prepared starting mapMRI builds pictures of the body using strong magnetic fields, magnetic field gradients and radio waves, and it does not use X-rays or ionizing radiation, which distinguishes it from CT and PET. It is a medical application of nuclear magnetic resonance (NMR). The loud knocking or banging during a scan is mechanical, not electrical noise in the images: vibration modes are generated by eddy currents in the scanner's cylindrical shell induced by the gradient magnetic fields, and because the scanner wall is typically joined to the gradient spiral cylinder, those vibrations are transmitted to the wall and radiate extra sound waves.12
- Evidence 13
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Be the first to voteIn brief
MRI uses strong magnetic fields, magnetic field gradients and radio waves, and involves no X-rays or ionizing radiation, unlike CT and PET.1
Evidence-backedIt is a medical application of nuclear magnetic resonance and gives better soft-tissue contrast than CT, for example in the brain or abdomen.1
Evidence-backedThe loud knocking is mechanical: eddy currents induced in the scanner's cylindrical shell by the gradient fields excite vibration modes, and vibrations transmitted to the joined scanner wall radiate extra sound.2
Evidence-backedNoise reduction is an active engineering problem; a 20 mm uniform absorber has been shown in numerical work to significantly reduce acoustic noise between 0 and 3 kHz.2
Evidence-backed
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The evidence behind it
3 sources- Reviews of many studies1
- Other studies and data1
- Background1
Published in 2025 and 2026
| Source | Kind | Year |
|---|---|---|
| Magnetic resonance imaging. | Other studies and data | 2026 |
| Magnetic resonance imaging (Wikipedia) | Background | Unknown |
| A Numerical Systematic Review and Meta-Analysis of Diagnosing the Vibration Modes of the Cylindrical Shell in the MRI Machine. | Reviews of many studies | 2025 |
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What it means for you
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If you want to know why the scan is noisy rather than whether it is safe
the noise comes from vibrations of the scanner structure driven by the switched gradient fields, not from the imaging signal itself, so it is a mechanical by-product of how the image is encoded.2
Evidence-backedIf you are comparing MRI with CT or PET
MRI uses strong magnetic fields and radio waves and no ionizing radiation, and it generally gives better contrast in soft tissues such as the brain or abdomen.1
Evidence-backedIf you are interested in how the technique developed
it was first realised over 50 years ago, was established in medical diagnostics within about 10 years, and has since advanced through higher magnetic fields and more affordable, portable instrumentation.3
Evidence-backedThe full story · 2 chapters
01
How an MRI scan produces an image
AI summary:MRI uses strong magnetic fields, gradients and radio waves, with no ionizing radiation, and gives good soft-tissue contrast.
Evidence-backed: MRI is a medical imaging technique used in radiology to generate pictures of the anatomy and the physiological processes inside the body. Scanners use strong magnetic fields, magnetic field gradients and radio waves to form images of the organs. It is a medical application of nuclear magnetic resonance (NMR), the same physical phenomenon used in NMR spectroscopy, and it does not involve X-rays or ionizing radiation, which distinguishes it from computed tomography (CT) and positron emission tomography (PET).1
Evidence-backed: The underlying physics is described as exploring the atomic-scale and mesoscale environments of nuclear spin to produce images, predominantly of soft matter. Compared with CT, MRI provides better contrast in images of soft tissues, such as the brain or abdomen, which is why it is widely used in hospitals and clinics for diagnosis, staging and follow-up of disease.31
Evidence-backed: The technique has a long development history: it was first realised over 50 years ago and was established in medical diagnostics within 10 years of that first realisation. In the early 1970s it was demonstrated by R. Damadian, C.F. Hazlewood and D.C. Chang that measurement of NMR relaxation times can be used to detect cancer, and soon after, P.C. Lauterbur proposed using a magnetic field gradient to generate the NMR image in a 2-D plane. Development has continued to improve image quality and sensitivity, both through higher magnetic fields and by making instrumentation more affordable and portable.31
02
Why the scanner knocks and bangs
AI summary:Eddy currents from the switching gradient fields vibrate the scanner shell and wall, which is heard as knocking.
Evidence-backed: The noise is a mechanical side effect of the way the image is encoded. Vibration modes are generated by eddy currents in the cylindrical shell of the scanner — the space where the patient lies — induced by the gradient magnetic fields. In addition, the scanner wall is typically joined to the gradient spiral cylinder, so vibrations are transmitted to the wall and produce extra sound waves.2
Evidence-backed: Because the gradients are switched on and off rapidly during a scan, these induced currents and the resulting vibrations follow that switching pattern, which is what a patient hears as repeated knocking or banging. Research on the problem has focused on reducing the acoustic noise produced inside the cylindrical shell, using numerical methods and practical solutions for lowering noise in MRI gradient coils. One demonstrated design is a 20 mm uniform absorber, reported as significantly reducing acoustic noise in the frequency range 0 to 3 kHz; numerical analysis of gradient cycles also yields solutions that lower both vibration and noise levels.2
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What to remember
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Noise reduction is an active engineering problem; a mm uniform absorber has been shown in numerical work to significantly reduce acoustic noise between 0 and 3 kHz.
MRI uses strong magnetic fields, magnetic field gradients and radio waves, and involves no X-rays or ionizing radiation, unlike CT and PET.
It is a medical application of nuclear magnetic resonance and gives better soft-tissue contrast than CT, for example in the brain or abdomen.
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- 1Magnetic resonance imaging (Wikipedia)WikipediaPublished Oct 7, 2026Checked Oct 11, 2026
“Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to generate pictures of the anatomy and the physiological processes inside the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio waves to form images of the organs in the body. MRI does not involve X-rays or the use of ionizing radiation, which distinguishes it from computed tomography (CT) and positron emission tomography (PET) scans. MRI is a medical application of nuclear magnetic resonance (NMR), which can also be used for imaging in other NMR applications, such as NMR spectroscopy. MRI is widely used in hospitals and clinics for medical diagnosis, staging and follow-up of disease. In the early 1970s, it was demonstrated by R. Damadian, C.F. Hazlewood and D.C. Chang that measurement of NMR relaxation times can be used to detect cancer. Soon after that, P.C. Lauterbur proposed to use a magnetic field gradient to generate the NMR image in a 2-D plane. Compared to CT, MRI provides better contrast in images of soft tissues, e.g. in the brain or abdomen.”
- 2A Numerical Systematic Review and Meta-Analysis of Diagnosing the Vibration Modes of the Cylindrical Shell in the MRI Machine.Biomedical engineering and computational biology (Mortazavy et al.)Published Jul 10, 2025Checked Oct 11, 2026
“Magnetic Resonance Imaging (MRI) is a non-invasive imaging method that utilizes radio waves and magnetic fields. This study focuses on reducing the acoustic noise produced inside the cylindrical shell of the scanner, where the patient is located. Vibration modes are generated by eddy currents in the cylindrical shell induced by gradient magnetic fields. Additionally, the scanner wall is typically joined to the gradient spiral cylinder, causing vibrations to be transmitted to the wall and thereby producing extra sound waves. The present study investigates methods for mitigating noise from the scanner wall and reducing the transmission noise from the spiral gradient cylinder. Numerical methods and practical solutions for lowering acoustic noise in MRI gradient coils are explored. A 20 mm uniform absorber is demonstrated as an effective design for significantly reducing acoustic noise in the frequency range 0 to 3 kHz. Finally, numerical analysis of gradient cycles yields solutions that lower both vibration and noise levels.”
- 3Magnetic resonance imaging.Science advances (Blümich et al.)Published Sep 16, 2026Checked Oct 11, 2026
“Magnetic resonance imaging explores the atomic-scale and mesoscale environments of nuclear spin to produce images predominantly of soft matter. Within 10 years of its first realization over 50 years ago, it has been established in medical diagnostics. It has undergone continuous development to improve image quality and sensitivity by using both higher magnetic fields and by making instrumentation more affordable and portable. The physical principles and major milestones of MRI are reviewed and concluded with an outlook into the future.”
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Open questions
What sound levels do patients actually experience during routine scans, and how much do absorbers or other mitigations reduce perceived loudness in a real scanner room?
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How do specific pulse sequences and gradient switching patterns map onto the rhythm and pitch of the knocking a patient hears?
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How do the physics and noise characteristics change as instrumentation becomes more affordable and portable?
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