MRI Flashcards
MRI details
- no IR
- uses magnetic properties of hydrogen
- structural and functional modality
- great soft tissue imaging
- great image quality in head and neck, pelvic and hip bones
- can view in all planes
- uses NMR
what can’t occur
LOCALISATION: as the treatment position is altered due to the magnetic coils
what is NMR
the magnetic resonance transition of nuclear spin states when a radio frequency signal is applied in the presence of a magnetic field
why is there a net spin
due to an uneven number of protons and neutrons, hydrogen atoms align the precession of the spin occurs at a characteristic frequency
what happens to the protons in hydrogen when there is a stronger magnetic field
they will temporarily change direction, the protons will align with the external magnetic field
what is the measurement of the protons relaxing
T1 and T2
what is T1
time constant for the time of the longitudinal magnetism to return - relaxation phase
describe the alignment etc process T1
- changes in the magnetic field causes changes in the electrical field
- the coil creates an electrical field which runs through
- a measurement is taken for the change from longitudinal to transverse
- the energised protons lose energy from the RF pulse relaxing back into the longitudinal arrangement
- transverse component decays
- longitudinal component is re-established
- exponent; effect
describe T2
- protons experience interface from another spin magnet nearby which then becomes affected
- which decays the transverse magnetic component
- orientation becomes random
- exponential with time
what are the 4 principles in MRI
- the nucleus
- magnetism
- placing the body in a magnetic field
- introducing a RF pulse
describe the nucleus step within the principles
- H has an uneven spin
- velocity is constant
- lamor frequency = number of spins
- the direction of axis spin can change
- proton spin produces an electrical current
describe the magnetism stage
- hydrogen has the highest magnetic signal
- the normal state has no magnetisation
- random alignment
- MRI uses the combined effect of a group of protons
describe the placing the body in a magnetic field stage
- alignment occurs in one direction
- along the long axis there is a small magnetisation
- the precessing spin magnets emit RF waves
- signal is weak
- temporary effect under the influence of the magnet
describe the introduction of the RF pulse phase
- the RF pulse frequency = number of protons in hydrogen
- introducing an RF pulse then changes the orientation
- RF is perpendicular to the long axis of the magnet
- if the Lamar frequency is equal to the hydrogen protons. then they change direction
- spin magnets flip 90
what happens when the protons flip 90 back
they reduce net magnetisation to 0 and some magnetisation is established in the transverse direction. They resonate, precess in synchrony, which causes the transverse magnetic signal to become strong
