Magnetic resonance imaging

What is a magnetic field?

Region of space in which magnetic forces are experienced

Strength measured in Teslas

What is a magnetic moment?

Quantity that represents the direction and strength of the magnetic field

What is the MRI magnet?

Superconducting electromagnet with a stronger magnetic field

What is an electromagnet?

Supercoiled wire with a current passed through it

What are the properties of the superconducting magnet?

High resistance increases the coil temperature

Cooled by pumping liquid helium to lower temperature

What are radiofrequency transmitter coils?

Receive radiofrequency pulses administered to the patient

In order to produce an image

Located in the main magnet

What are receiver coils?

Coils deigned to be places over specific imaged areas that receive the MRI signal

What is the motion in the atom?

Protons and neutrons spin in opposite directions

Atoms with equal amounts of each have no net spin

What is an MRI active nuclei?

Atoms with unequal amounts of protons and neutrons have a net spin

Tend to align their rotation to an applied magnetic field

Magnetic moment

What is the MRI active nuclei used in medical imaging?

Hydrogen atoms due to its abundance

What happens when a magnetic field is applied to MRI active nuclei?

Magnetic moments align with the magnetic field

Low-energy nuclei align parallel

High energy nuclei align anti-parallel

What is a net-magnetic vector?

Parallel nuclei in magnetic moments

What is precession?

Additional spin caused by magnetic field strength

What is the precession frequency?

The speed at which hydrogen nuclei spin

What is resonance?

Occurs when object exposed to external once with similar frequency

Nuclei gain energy and resonate (move)

What is excitation?

Application of radiofrequency pulse that causes resonance

What happens when radiofrequency pulse is applied?

Net magnetic vector (created by parallel nuclei) move out of alignment with magnetic field

What is in phase?

Resonance causes magnetic moments of hydrogen nuclei to move in phase with each other

When does the MRI signal occur?

When in phase magnetism occurs across the receiver coil

When moving transverse magnetisation produces magnetic field fluctuations inside the coil, inducing electric voltage in the coil

What is relaxation?

When the signal is switched off, high energy nuclei lose energy and their net magnetic vectors realign with the magnetic field

What are the three main hazards of MRI?

Static magnetic field

Time varying magnetic field gradients

Radiofrequency magnetic fields

What are three magnetic field strength hazards?

Sensory effects

Projectile effects (loose objects containing ferromagnetic material)

Torque (metal implants and foreign bodies)

What is a quench?

Emergency demagnetisation, causing liquid helium to evaporate

Results in irretrievable damage to the MR

What is a spontaneous quench?

Where helium gas escapes into the room instead of through the quench pipe

What is the purpose of oxygen monitoring?

Alerts is oxygen deficient atmosphere is reached

When oxygen in the room is forced down