Sequences Flashcards
What causes contrast in MRI?
Differences in T1,T2 relaxation times
Water content
Blood flow
Diffusion
Depends on the sequence you are using
Water content(%) T1(s) T2(ms)
Grey matter
white matter
CSF
Q: fill in table
Q: What are these values dependent on?
Water content(%) T1(s) T2(ms)
Grey matter 70 1.1 90
white matter 85 0.56 80
CSF 99 2.1 600
NB// these are dependant on field strength
Define
Which is important for T2 contrast, which for T1?
Echo Time
Repetition Time
Echo Time - time between FR pulse and centre of acquisition, manipulate this for T2 contrast
Repetition Time - duration of each repeat, manipulate this for T1 contrast
T2 Relaxation
For a gradient echo sequence:
longer TE gives *** signal
signal **** is due to ******.
2 causes of B-field inhomogeneity?
For a gradient echo sequence:
- longer TE gives lower signal
- signal decrease is due to dephasing of magnetisation in xy plane
Dephasing is caused by magnetic field inhomogeneity:
- Spatial variations - tissue structure inhomogeneity cause fluctuations in B. Differences in T2* for materials of different composition and succeptability.
- Temporal variations - molecules rorate and move past each other experiencing fluctuating magnetic environment. Degree of B inhomogeneity determined by tissue inhomogeneity, e.g. water molecules travel through a larger range of magnetic environments in liver than in brain and spins dephase more rapidly (shorter T2).
How do we do spin echo imaging?
Why do we do it?
SE & GE, which relaxation mechanism? T1,T2,orT2*?
Which is shorter T2 or T2*? What does this mean for SE and GE signal?
Advantage and disadvantage of E
Add a 180 degree pulse after Gz, Gy, and before Gx (readout gradient).
Some dephasing is reversed (runners on trach anaology), more signal than gradient echo. Only the spatial field variations are reversed, temporal cannot be.
- SE = T2
- GE = T2*
T2* is shorter, wich means advantage of SE is that refocussing of dephsed spins (due to spatial inhomogeneities in B) increases amount of signal. Disadvantage is that it takes longer than GE, minimum echo time is longer.
How do you change a pulse sequence to change T2* contrast
Choosing when you accquire the data, i.e. different TE, means that you can choose to maximise the difference in signal between materials with different T2* relaxation times.
Move the whole sewuence after the RF to the right.
Letting the signals decay away at different rates to maximise contrast.
Prepulses
Why do we use them?
Additional RF and gradient pulses prior to imaging sequence used to modulate magnetisation before image acquisition and alter contrast.
Prepulses: Fat Sat
What do you add?
How does it work?
Add additional narrow bandwith RF pulse that excites only LIPID protons (water and fat have different resonant frequencies). Saturate and remove lipid signals from an image.
e.g. you take images with and without fatsat to differentiate what is fat and what is water.
T1 contrast - Inversion recovery prepulse
what do you need?
How does it generate contrast?
how does fluid attenuation inversion recovery work?
180degree pre-pulse and delay prior to imaging.
Invert the magnetisation and wait for recovery, contrast depends on duration of delay and T1 relaxation time. Short T1 component will relax considerably (lots of signal to flip over into xy plane), Long T1 relaxation will not have relaxed much (less signal to flip into x-y plane), Generates contrast.
e.g. FLAIR, exploits difference in T1 decay times, timing the delay so that 90 deg pulse occurs when there is no signal from CSF (Mxy=0).
T1 contrast - Saturation recovery
How does it work?
You choose TR to exploot differences in the amount of T1 relaxation.
Short TR, highly T1-weighted (i.e. you are flipping magnetisation into the x-y plane at a point when some species have had chance to relax back and others haven’t = T1 contrast)
Long TR, lower T1 weighting. (i.e. you have waited so long that all species (long and short T1) have had time enough to erlax maxk along Mz.
How can you produce images faster?
What’s sequence called?
How much faster is it?
Accquire all of k-space in one acquisition
Single initial RF pulse, then repeat the GxGyAcquisition module over and over. Called Echo-Planar imaging.
Images take 40ms down from 1-2 mins
