MRI Physics Flashcards
(14 cards)
What happens to the behaviour of H atoms in the human body when a large magnetic field passes across the patient (the MRI scanner)?
H atoms influenced by external magnetic field:
- H atoms align with magnetic field
- H atoms precess around their own axis
What is the precession frequency of Hs proportionate to?
(as X increases, the precession frequency increases)
strength of the magnetic field
How do you change the type of scan weighting? eg T1 or T2 etc
by changing the time of echo TE and time of repetition TR
How does a long or a short TR (time between successive RF pulses) change the scan contrast?
short TR = T1 weighted scan
long TR = T2 weighted scan
What happens to protons/magnetisation during a long TR?
“ short TR?
-each tissue’s magnetisation has enough time return to their longitudinal magnetisation
-not all tissue types will reach longitudinal magnetisation
What is TE and TR?
TE time of echo = time between application of the radiofrequency (RF) excitation pulse and the peak of the signal (echo) measured in the transverse plane
TR time of repetition = time between successive RF pulses
What happens during TR?
protons gain longitudinal magnetisation/relaxation (from transverse)
What gives tissue contrast in a T2 image?
tissue types have unique T2 relaxation times. the differences in these T2 relaxation times gives contrast
What are the main pulse sequences used in MRI?
spin echo
gradient echo
inversion recovery
As TE increases what happens to image contrast and image signal?
As TE increase, does the overall image become better?
As TE increases, tissue contrast increases but image signal decreases.
NO! there is a trade off between contrast and signal because of the above.
Which structural scan is a result of these parameters:
Short TR and short TE →
Long TR and long TE →
Long TR and short TE →
Short TR and short TE → T1-weighted
Long TR and long TE → T2-weighted
Long TR and short TE → Proton density-weighted
What does a T1 image show?
What does it show on a proton level?
T1 = (longitudinal relaxation time) = shows differences in T1 relaxation times for different tissues -> tissue contrast
how quickly protons REalign in the direction of of the magnetic field direction of B0 (longitudinal)
How does a T2 image show tissue contrast?
What does it show on a proton level?
T2 = (transverse relaxation time) = shows differences in tissues based on their T2 relaxation times
how quickly protons lose phase coherence in the transverse plane
For the fMRI 1 lecture, on the graph showing the trade off between signal and contrast, what are the x and y axes?
What is contrast on the graph?
y = signal
x = time
contrast = difference between the at rest curve and the task curve
