MRI Flashcards
What does MRI measure in the body
proton density and T1/T2 properties
How is the MRI signal produced
proton spin is aligned with B0 filed, excited by RF pulse, decays back to normal and emits RF energy in the process. Emitted energy is measured in xy plane, and inversion to null signal from one tissue, time readout when signal from one tissue has decayed back to zero
what plane is emitted energy measured in
xy plane
what are the key formulas
o W = γB
o FOV = BW / 2piγ*G
o Pixel = Δf * matrix / BW
o T = Ny * NEX * TR o T(inv) = ln2 T1
what are the key components of the system
B0 magnet, RF coils, gradient coils
what parameters can we control
TE-time until
TR
pulse sequence
gradients
what are the image quality parameters
spatial resolution, contrast, temporal resolution
what is bright in T1, and what is the TR and TE
fat is bright, and short TR and TE
what is bright in T1, and what is the TR and TE`
T2 has water is bright and long TR and TE
what does a steeper gradient mean
smaller slice thickness
what are the trade-offs with long TR
good SNR, but longer scan and decreased T1
what are the trade-offs with long TE
T2 weighting but decreased SNR
what are the trade-offs with increased NEX
good SNR of all tissue, reduced flow artifact, slower scan
what are the trade-offs with increased slice thickness
increased SNR, increased coverage, decreased spatial resolution, and partial volume
what are the trade-offs with increased received bandwidth
decreased miniumum TE, decreased chemical shift, but decrease SNR
what are the bioeffects of MRI
metal heating, big magnet that can pull stuff out, and no radiation J
how is the image in MRI reconstructed
for each phase gradient, it fills up one row of kspace, and one kspace is full, trasnform into spatial domain.
what are the strengths of MRI
good picture quality, and good for soft tissue, can emphasize tissue of interest.
what are the weaknesses of MRI
expensive, noise, uncomfortable, can’t fit obese people, and no metal
what is fast MRI
it is fast, and ends up T2 weighted. One 90 pulse and many 180 to keep echoes going. Longer train length decreases SNR and resolution, but it is faster
what is a gradient echo
no 180 pulse, so there is reversal of f. Gradient allows rephasing and echo
what is fMRI
functional MRI, so it is used to look at uptake in the brain
what changes are needed for pediatrics
◦Safe for kids because it is magnets
◦Bad because it is loud and takes forever
◦Kids have to be sedated for MRI, which increases injury or risk
◦Need to be still as well because of it
◦Takes a long long time.
what do you do for obese patients
•Obese
◦Tube is small
◦Longer scan times without as good of resolution
◦Hard to get as much resolution as well
◦Higher risk of burning people
