MRI (Magnetic Resonance Imaging) and CT (Computed Tomography) are two of the most commonly requested imaging investigations in medicine. Both produce highly detailed cross-sectional images of the inside of the body — but they work in completely different ways, take different amounts of time, and are better suited to different clinical questions.
How MRI works
MRI uses a powerful magnetic field and radio waves to generate images. When you lie inside the scanner, the magnetic field causes hydrogen atoms in your body (primarily in water molecules) to align. Short pulses of radio waves then knock them out of alignment, and as they realign they emit signals that are captured and converted into images. MRI uses no ionising radiation, making it safe for repeated use and particularly appropriate for children, pregnant women, and younger patients.
How CT works
CT uses a rotating X-ray beam and detectors to capture hundreds of images from different angles, which a computer assembles into a detailed 3D picture. CT is much faster than MRI — a chest CT takes under 10 seconds — and is excellent for bone, lung, and vascular imaging. CT does use ionising radiation, though modern scanners are optimised to keep doses as low as reasonably achievable.
MRI is usually the better choice for:
- Brain and spinal cord conditions
- Joint injuries (knee, shoulder, hip, ankle)
- Soft tissue tumours and masses
- Pelvic conditions (prostate, uterus, ovaries)
- Liver and abdominal organ assessment
- Nerve problems (sciatica, carpal tunnel)
- Patients who need repeated scans (no radiation)
CT is usually the better choice for:
- Chest and lung conditions (cancer, pulmonary embolism, infection)
- Abdominal emergencies (appendicitis, kidney stones)
- Bone fractures and trauma
- Vascular conditions (aortic aneurysm, arterial blockage)
- Cancer staging (fast, whole-body assessment)
- Situations requiring rapid results
Which costs more?
CT scans are generally less expensive than MRI because the scanners are faster and the imaging time per patient is shorter. At ScanBook partner centres, CT scans start from £185 and MRI scans from £275, though complex protocols with contrast agents or multiple body areas will cost more.
