How Does an MRI Scan Work?

Magnetic resonance imaging is one of the most impressive technologies in modern medicine. Without any radiation, an MRI scanner produces extraordinarily detailed images of the inside of your body — from delicate brain structures to the smallest tears in a knee ligament. But how does it actually work? This guide explains the science in plain terms, so you understand what's happening during your scan.

The Basic Principle: Magnetism and Water

Your body is roughly 60% water. Water molecules contain hydrogen atoms, and each hydrogen atom has a single proton at its centre. These protons have a property called "spin" — they behave like tiny bar magnets, constantly rotating on their axes.

Normally, these proton magnets point in random directions and their effects cancel each other out. But when you enter the powerful magnetic field of an MRI scanner (typically 1.5 or 3 Tesla — that's 30,000 to 60,000 times stronger than the Earth's magnetic field), the protons line up with the field, like compass needles pointing north.

Step by Step: How an MRI Creates Images

Step 1: Alignment

When you lie inside the scanner, the strong magnetic field forces the hydrogen protons in your body to align in the same direction. They don't all align perfectly — a tiny majority point one way, creating a small but measurable net magnetisation. It's this slight imbalance that the scanner detects.

Step 2: Excitation (the Radio Pulses)

The scanner sends short bursts of radiofrequency (RF) energy — essentially radio waves tuned to exactly the right frequency to interact with hydrogen protons. This is the "resonance" in magnetic resonance imaging. The RF pulses knock the protons out of their aligned position, tipping them to a different angle. The exact frequency depends on the magnetic field strength: at 1.5T, hydrogen resonates at 63.87 MHz; at 3T, it's 127.74 MHz.

Step 3: Relaxation and Signal Emission

When the RF pulse stops, the protons gradually return to their original alignment — a process called relaxation. As they do, they release the energy they absorbed as faint radio signals. Receiver coils positioned around the body part being scanned pick up these signals.

Here's what makes MRI so diagnostically powerful: different tissues relax at different rates.

• Fat relaxes quickly (short T1 time)
• Water and fluid relax more slowly (long T1 and T2 times)
• Dense tissues like cortical bone produce very little signal (low hydrogen content)
• Abnormal tissues — tumours, inflammation, oedema — often have different relaxation times from surrounding healthy tissue

By measuring these timing differences, the scanner distinguishes between muscle and fat, healthy and diseased tissue, fluid collections and solid structures.

Step 4: Spatial Encoding (Why It's So Loud)

To build an image rather than just a single blob of signal, the scanner uses additional magnetic fields called gradient coils. These slightly vary the field strength across your body so that protons in different locations resonate at slightly different frequencies. By analysing which frequencies come from where, the computer builds a map of signal intensity across a slice of your body.

The gradient coils switching on and off thousands of times per second cause mechanical vibration — that's the source of the loud knocking, buzzing, and tapping you hear during the scan. Different sequences use different gradient patterns, which is why the sounds change throughout.

Step 5: Image Reconstruction

A powerful computer processes the millions of signals using mathematical algorithms (primarily Fourier transformation — the same maths used in audio processing and signal analysis). The result is a series of highly detailed cross-sectional images — slices through your body in any plane: axial (horizontal), coronal (front-to-back), or sagittal (side-to-side). Modern scanners can also create 3D volume datasets that can be rotated and resliced in any direction.

T1, T2, and Other Weightings Explained

If you look at your MRI report, you'll see terms like "T1-weighted" and "T2-weighted." These refer to the two main types of relaxation the scanner can measure:

• T1-weighted images — Fat appears bright white, water appears dark. Excellent for showing anatomical structure and detail. Often used after gadolinium contrast injection, because gadolinium shortens T1 relaxation time, making enhanced areas appear bright.
• T2-weighted images — Water and fluid appear bright white, fat appears darker grey. Excellent for detecting oedema (swelling), inflammation, cysts, and many types of pathology. Most abnormalities "light up" on T2 because diseased tissue tends to contain more water.
• FLAIR (Fluid-Attenuated Inversion Recovery) — Similar to T2, but the signal from cerebrospinal fluid (CSF) is suppressed (appears dark). This is particularly useful in brain imaging — small lesions near the fluid-filled ventricles that would be hidden on standard T2 become clearly visible.
• Diffusion-weighted imaging (DWI) — Measures the random movement of water molecules in tissue. Restricted diffusion (where water can't move freely) shows up as bright signal. This is the sequence that detects acute ischaemic stroke within minutes of onset — it's one of the most clinically valuable advances in MRI.
• STIR (Short Tau Inversion Recovery) — Suppresses fat signal, making fluid and oedema stand out. Widely used in musculoskeletal imaging to detect bone marrow oedema, stress fractures, and soft tissue inflammation.
• MR angiography (MRA) — Specialised sequences that highlight blood vessels, often without needing contrast injection. Time-of-flight (TOF) MRA is commonly used for brain vessels; contrast-enhanced MRA is used for larger arteries.

What Makes MRI Different from Other Imaging?

No Ionising Radiation

Unlike X-rays and CT scans, MRI does not expose you to ionising radiation. The magnetic fields and radio waves used have no known harmful effects at clinical strengths. This makes MRI particularly suitable for children, pregnant women (after the first trimester), and patients needing repeated imaging over time — for example, monitoring MS lesions or tumour response to treatment.

Superior Soft Tissue Contrast

MRI provides far greater contrast between different types of soft tissue than any other imaging method. A knee MRI can distinguish between articular cartilage, meniscal fibrocartilage, the anterior cruciate ligament, bone marrow, muscle, and subcutaneous fat — all in a single image. No other modality comes close for soft tissue discrimination.

Multiplanar Imaging

MRI can acquire images in any orientation without moving you. The scanner simply adjusts its gradient fields to slice through the body at whatever angle gives the best view. This flexibility lets radiologists see structures from the optimal angle for diagnosis.

MRI Scanner Strengths: 1.5T vs 3T

MRI scanners are classified by their magnetic field strength, measured in Tesla (T):

• 1.5 Tesla — The clinical workhorse. Provides excellent image quality for most routine scans. Generally better tolerated by patients with MR-conditional implants (many implants are only approved for use at 1.5T). Produces fewer susceptibility artefacts near metal and at air-tissue interfaces (sinuses, skull base).
• 3 Tesla — Double the field strength means roughly double the signal-to-noise ratio, producing higher-resolution images. Particularly valuable for detailed brain imaging, small joint imaging, prostate MRI (PI-RADS scoring), and cardiac MRI. Scans can sometimes be completed faster. However, the higher field can increase artefacts in certain areas and some patients report more sensation (warmth, peripheral nerve stimulation).
• Open/low-field (0.2-1.0T) — Used in open MRI scanners. Lower image quality but much more comfortable for claustrophobic patients. Adequate for many musculoskeletal examinations.

Both 1.5T and 3T scanners are available across the Lola Health scanning network.

The Role of Contrast (Gadolinium)

Gadolinium-based contrast agents are sometimes injected intravenously to improve the visibility of certain structures. Gadolinium is a paramagnetic substance that shortens the T1 relaxation time of nearby protons, making enhanced tissues appear brighter on T1-weighted images. Read our full gadolinium contrast guide.

Contrast is particularly useful for:

• Characterising tumours and distinguishing them from surrounding tissue
• Assessing blood vessel abnormalities (aneurysms, stenosis, malformations)
• Evaluating inflammatory and infectious conditions
• Post-surgical imaging to tell scar tissue from recurrent disease
• Detecting active demyelination in MS

Frequently Asked Questions

Is the magnetic field in an MRI scanner dangerous?

The magnetic field itself is not harmful to biological tissue. However, it can attract metallic objects with enormous force — a 1.5T scanner can pull a steel oxygen cylinder across a room. That's why all metal must be removed before entering the scanning room, and patients with certain metallic implants may not be able to have an MRI safely.

Why is an MRI scanner so loud?

The gradient coils inside the scanner switch on and off thousands of times per second to create the varying magnetic fields needed for spatial encoding. The rapid switching causes the coils to vibrate against their mountings, producing sounds of 80-100 decibels — comparable to a lawnmower. Different sequences produce different rhythms and pitches, which is why the sound pattern changes throughout your scan.

Can an MRI scan see everything?

MRI is excellent for soft tissues, the brain, spine, and joints, but it's not the best tool for every situation. Lung tissue (low water content, constant respiratory motion), cortical bone detail, and calcifications are better seen on CT. Your doctor or radiologist will recommend the most appropriate imaging for your specific clinical question.

How detailed are MRI images?

Modern MRI scanners can resolve structures as small as 0.5mm at 3T (with specialised coils and sequences). Standard clinical resolution is typically 0.5-1.0mm in-plane, with slice thicknesses of 2-5mm. This allows radiologists to identify subtle changes — small meniscal tears, early cartilage loss, tiny brain metastases — that other imaging methods would miss.

Does the MRI scan damage cells or DNA?

No. There is no evidence that the magnetic fields or radio waves used in clinical MRI cause any damage to cells or DNA. MRI has been used safely for over 40 years, with extensive research confirming its safety. The energy levels involved (radiofrequency, not ionising) are fundamentally different from those that cause cellular damage (X-rays, gamma rays).