MRI components, image detection, MRI vs CT

MRI (Magnetic Resonance Imaging) produces images from the radio signals emitted by hydrogen nuclei (protons) of the body when placed in a strong magnetic field and excited by RF pulses.

System components

 +--------------------------------------+
 | Main magnet (0.5-3 T) + shim coils   |
 |  gradient coils (X,Y,Z)  RF coil     |
 |       [ patient on table ]           |
 +--------------------------------------+
   ^ gradient amps ^ RF transmitter   | signal
   |               |                   v
 [Computer / control] <- [RF receiver, ADC]
        |
 [Display, storage]

1. Main magnet: superconducting (or permanent/resistive) magnet giving a strong uniform field B0, to align the proton spins.
2. Shim coils: make the field homogeneous.
3. Gradient coils (X, Y, Z): give a linear field variation for slice selection and position (frequency and phase) encoding.
4. RF coils and transmitter: send an RF pulse at the Larmor frequency (f = 42.58 MHz/T × B0) to flip the spins.
5. RF receiver and image detection system: picks up the signal.
6. Computer, console, display and storage.

Image detection system

• Function: after the RF pulse, the protons return to equilibrium and emit a weak RF signal (FID or echo) which induces a voltage in the receiver coil. The signal is amplified by a low-noise preamplifier, demodulated, filtered and digitised by an ADC. The digital data fill k-space; a 2D Fourier transform converts it into the image, with brightness depending on proton density, T1 and T2.
• Application: gives high-contrast images of soft tissue: brain, spinal cord, joints, heart, tumours, and with contrast agents for blood vessels (MR angiography).

Four differences between MRI and CT