What is MRI and how does it work?

Most people walk into an MRI scan knowing two things: it's loud, and it involves a tunnel. That's roughly where curiosity ends. The science behind the picture on the screen is genuinely interesting — and worth knowing, because it explains why MRI is so different from a CT scan or an X-ray.

The basic idea

Your body is mostly water. Each water molecule has two hydrogen atoms, and each hydrogen atom contains a single proton. Those protons happen to behave like tiny spinning bar magnets — they have a "north" and a "south," and normally they point in random directions.

When you lie inside the MRI machine, you're inside a very strong magnet. So strong that all those proton-magnets line up neatly with the magnetic field — like iron filings near a magnet.

The MRI machine then sends a brief radio-wave pulse at exactly the right frequency to knock the protons sideways. When the pulse stops, the protons relax back to their aligned position. As they relax, they release a faint radio-wave signal of their own. The machine listens for that signal, and from the timing and strength of the signal it figures out where the water is — and how much of it there is.

Different body tissues hold water differently. Fat, muscle, brain matter, tumour tissue, fluid in a swollen joint — each has its own characteristic signal. The MRI software turns those signals into a greyscale image that the radiologist reads.

What "3 Tesla" means

The Tesla (T) rating tells you how strong the main magnet is. Older machines were 0.5T or 1T. Most general-purpose machines today are 1.5T. A 3T magnet is twice as strong as 1.5T.

Why does it matter? Two practical reasons:

Sharper images, faster scans. A stronger magnet produces a stronger signal, so the scan can either be done in less time or produce a more detailed image in the same time. For brain scans and joint scans this difference is genuinely visible — your radiologist sees small lesions or early ligament tears that lower-strength machines miss.
Shorter time in the tunnel. A typical 1.5T MRI Brain takes about 45 minutes. On a 3T machine, the same scan finishes in 25–30 minutes. That matters if you're anxious about enclosed spaces.

There's no medical downside to 3T for routine imaging.

What MRI is good at — and what it isn't

MRI is the best imaging available for:

Soft tissue: brain, spinal cord, ligaments, cartilage, muscles, tendons
Joints: especially knee, shoulder, ankle ligament and meniscus tears
Pelvic organs: uterus, prostate, ovaries
Detecting tumours: especially in brain, liver, and pelvis

MRI is not the right choice for:

Lungs: air doesn't have hydrogen, so MRI sees lungs poorly — CT chest is the standard
Bone fractures: X-ray or CT is faster, cheaper, and clearer for broken bones
Kidney stones: CT KUB is the gold standard
Emergency trauma assessment: CT is faster and gives a quicker overall picture in critical situations

Safety

MRI uses no radiation — that is its biggest advantage over CT or X-ray. It is considered safe for pregnant women (after the first trimester, where doctors usually still prefer to avoid it unless essential).

The one real concern is metal. The strong magnet will pull anything ferromagnetic — including pacemakers, certain cochlear implants, and some older surgical clips. Always tell us in advance about any implant, even decades old. Most modern implants are MRI-compatible, but we'll check the model and confirm before the scan.

What to expect on the day

You change into a gown — no metal zips or buttons.
You lie on a table that slides into the tunnel.
The technician communicates through a microphone; you can talk back.
The machine knocks loudly during scanning — earplugs and headphones are provided.
You'll need to lie still for 20–40 minutes depending on what's being scanned.

If you're claustrophobic, tell us at booking. We can prescribe a mild sedative (with your doctor's approval) or guide you through what to expect — most people manage it fine once they know what's coming.