Short Note: Magnetic Resonance Imaging (MRI)

MRI produces detailed images of soft tissue without ionizing radiation, using a strong magnetic field and radio waves.

Principle

1. Hydrogen nuclei (protons) in the body have a magnetic moment; in a strong field $B_0$ (1.5 T or 3 T) they align and precess at the Larmor frequency $f = \gamma B_0/2\pi$ (about 63.9 MHz at 1.5 T).
2. An RF pulse at this frequency tilts the magnetisation.
3. When the pulse stops, protons relax and emit a signal, detected by receiver coils.
4. Gradient coils add position-dependent field changes for slice selection and spatial encoding; a Fourier transform builds the image.
5. Contrast depends on relaxation times T1, T2 and proton density.

Main parts

Superconducting magnet (cooled with liquid helium), gradient coils, RF coils, shim coils, computer and console, RF shielded room.

Applications

Brain and spine, joints and soft tissue, heart, tumours, MR angiography, functional MRI.

Advantages and cautions

No radiation and excellent soft-tissue contrast; long scan time, high cost, contraindicated with pacemakers and metal implants, noisy, claustrophobia.

Exam tip

Write the Larmor frequency, T1/T2 contrast and the block diagram.