MRI Flashcards
Larmor
Precession frequency = ?
frequency = w = gamma x field strength
gamma = proportionality constant = gyromagnetic ratio
T1 term
spin lattice relaxation
T1 time
Time at which longitudinal magnetization is 63% of its final value
Greater field strength = longer T1
T2 fancy term
spin spin relaxation
T2 time
time at which signal has decayed to 37 % of its original value
what causes T2 decay (loss of transverse mag)
external field inhomogeneity
inhomogeneities in local field, within tissues. This is why pure things (water) take longer to decay and are therefore bright!
which is faster T2 or T2*
T2* decays faster
Tissue spin interaction + field inhomogeneity
TR =
time between initiation of two successive RF pulses
T2 or T2* decay (downslope)
180 degree RF
FID free induction decay
NO FID with a 180 pulse, just inverts longitudinal mag
otherwise all flip angles cause them
180 RF, T2 and T2*
after a 90 produces transverse mag, as T2 decay is happening, hit with a 180 degree RF pulse
this second, 180 pulse 1 clears out inhomos turning T2* into T2 and creates an echo
180 RF generates what?
turns T2* to T2 and generates echo
echo is some time after the 180 where transverse mag signal has refocused and peaks again in uniformity, then trails off again
When to deliver 180 RF
180 RF given at 1/2 time to echo
1/2 TE = 180 RF
Rough long and short TR
Long TR = > 2000ms
Short TR = 250-700 ms
long and short TE
Long TE = >60ms
Short TE = 10-25ms
Proton density times
MINIMIZE BOTH T1 and T2 effects
Long TR and Short TE
K space trivia
what’s at the center
what’s at the periphery
Center = information about gross form and contrast
Periphery = information about spatial resolution
Spatial encoding
slice select
encode spatial information along rows
encode spatial information along columns
localizing gradients
identical properties, applied at different times and different directions
Selecting desired slice
SSG
placed perpendicular to desired slice plane
Selective pulse
on top of a slice select gradient
RF pulse applied at same frequency as protons in slice being sampled, only protons in this plane will be affected
effect of 180 RF on gradient in spin echo
180 RF pulse applied after the 90 fucks up the field, so before and after the 180RF, you place two identical gradients to cancel each other out and correct for errors
gradients on either side of 180 RF
phase encoding
encoding spatial info in vertical direction
phase encode gradient causes protons in same row perpendicular to the gradient to have same phase. All protons will have same frequency at this point
phase vs frequency encoding
phase encoding is much longer. This is why it’s done on the thinner portion
DURATION of a 2d imaging sequence
phase or frequency encoding
DEPENDS ON PHASE ENCODING
Duration = TR x Phase encode steps x number of excitations