Magnetic Resonance Imaging - MRI

Basic Concept

1. Atoms spin in random direction around individual magnetic field

2. MRI produces magnetic field, atoms align either facing North or South poles

   • half atoms go each way, few remain unaligned

3. Radiofrequency pulse applied, unmatched atoms spin the other way

4. As frequency turned off, atoms returned to normal & emit energy

   • converts energy to image

Hydrogen Atoms

• in water molecules throughout tissues

• nuclei of atoms act like compass needle in magnetic field

• MRI is essentially H+ imaging

Magnetism

• Gauss (G) - imperial unit

  • Telsa (T) - metric

• 3 types of magnets:

  • permanent

  • electromagnets

  • superconductive

Magnet

• shims - correct magnetic field within magnet

• field must be homogenous so all H+ atoms synchronize the same

Radio Antenna

• sends/receives radio waves to & from patient

• needs shielding in room from external signals

MRI Signal

• change of magnetic direction of atoms on the return gives off 2 types of signals

  • T1 & T2 Decay

• different tissues have different response to decay

• scanner applies radio waves in pulses

Repetition Time - TR

• time between radio pulses applied to same section

  • Lower TR shows T1 decay

  • Higher TR shows T2 decay

Echo Time - TE

• time between responses showing maximum signal

  • time between delivery of pulse & receipt of echo signal

T1 Weighted Images

• uses short TE & TR times

• CSF is dark

T2 Weighted Images

• longer TE & TR times

• CSF is light

Magnetic Resonance

• can image blood without contrast agent

• used for angiography & venography

Fat Saturation

• suppress fat signal

  • allows pathology to be seen that may have been altered by fat

Gadolinium

contrast agent that is used in MRI

Safety

• projectile effect screening

  • metal objects become projectiles

• ability of patient’s body to dissipate heat

• ear shielding - very loud