Physics Ch 10 MRI Flashcards
T1 –> represents what?
recovery of longitudinal magnetization: time it takes for proton to normalize (align back to longitudinal magnetization) after knocked down by RF pulse
T1 –> definition?
time at which long magnetization is 63% of final value
“intrinsic T1 shortening” –> T1 signal?
bright
inc magnetic field strength –> what happens to T1?
longer T1
T2 –> represents what?
decay of transverse magnetization
T2 –> definition?
time at which signal has decayed 63% of original value of transverse magnetization
what is time to repetition (TR)?
time bw initiation bw 2 successive RF pulses
inc time to echo (TE) –> what happens to T2 effects?
inc
short TR, short TE –> T1 vs T2 vs PD weighting?
- max long magnetization
- min trv mag
==> T1 weighting
long TR, long TE –> T1 vs T2 vs PD weighting?
- min long mag
- max trv mag
==> T2 weighting
long TR, short TE –> T1 vs T2 vs PD weighting?
- min long mag
- min trv mag
==> PD weighting
k-space –> center vs periphery –> contains what kind of info?
- center: tissue contrast
- periphery: spatial resolution
phase encoding vs freq encoding –> which takes longer?
phase encoding
encoding spatial information in vertical direction –> describes phase or freq encoding?
phase encoding
encoding spatial information in horizontal direction –> describes phase or freq encoding?
freq encoding
thin slices:
- small vs lrg slice selection gradient (SSG)?
- small vs lrg transmit bandwith?
- lrg SSG
- small transmit bandwith
thick slices:
- small vs lrg slice selection gradient (SSG)?
- small vs lrg transmit bandwith?
- small SSG
- lrg transmit bandwith
table time –> formula?
time = (TR) x (#Phase encoding steps) x (#excitations)
better spatial resolution w…:
- small vs lrg voxel
- small vs lrg field of view
- small vs lrg matrix
- thin vs thick slice
- small voxel
- small field of view
- lrg matrix
- thin slice
bigger voxel –> what happens to…:
- spatial resolution
- signal to noise ratio
- spatial resolution –> worse
- signal to noise ratio –> improve
better signal to noise ratio w…:
- small vs lrg voxel
- small vs lrg field of view
- small vs lrg matrix
- thin vs thick slice
- weaker vs stronger magnetic field
- short vs long TR
- less vs more #excitations
- short vs long TE
- small vs lrg receiver bandwith
- lrg voxel
- lrg field of view
- small matrix
- thick slice
- stronger magnetic field
- long TR
- more #excitations
- short TE
- lrg receiver bandwith
Nyquist ghosting –> occurs w what phase seq?
EPI
Nyquist ghosting –> cause?
too high demand for gradient system –> imperfections in gradient performance
Nyquist ghosting –> appearance?
ghost –> shifted by 1/2 FOV
Nyquist ghosting –> remedy? (4)
- apply eddy-current correction
- re-shimming
- reduce echo train length
- lower phase-enc resolution
truncation –> remedy? (2)
- inc matrix –> inc resolution
- apply pre- or post- reconstruction image filtration
spike (herringbone) artifact –> appearance?
dark/bright stripes –> not necess aligned w phase/freq direction
spike (herringbone) artifact –> cause? (2)
- readout process –> corrupted –> falsely detect lrg signal 1 or more times during readout
- RF arcs get produced in scan room (loose electrical connections, vibrating metal)
==> spike in raw k-space data
spike (herringbone) artifact –> remedy?
call field engineer to check for vibrating metal or loose wires
zipper –> cause?
detect extraneous RF signal during readout
zipper –> remedy? (3)
- make sure scanner room door is closed
- scan room equipment –> check for RF emissions
- call physics staff or field engineer to find RF leak in shielding
susceptibility artifact –> common in what seq? (3)
- T2*
- GRE
- EPI
susceptibility artifact –> remedy? (6)
- remv metal
- use non-gradient echo seq
- inc bandwith
- inc matrix
- dec slice thick
- EPI –> use parallel imaging
surface coil artifact –> appearance?
anatomy near surface coil –> stronger signal
surface coil artifact –> remedy? (2)
- shimming
- use bird cage coil
magic angle artifact –> remedy?
lengthen TE
chem shift artifact type 1 –> occur in phase or freq-enc direction?
freq
chem shift artifact type 1 –> remedy? (1)
inc receiver bandwidth
fat saturation failure –> cause?
resonant freq of fat –> not uniform in image:
- poor shimming
- anatomy is distant from isocenter
fat saturation failure –> remedy? (2)
- shim region of interest
- mv anatomy closer to isocenter
dielectric effect –> remedy? (1)
use dielectric pads
motion artifact –> occur in phase or freq-enc direction?
phase
motion artifact –> remedy? (5)
- use flow compenstation or apply saturation bands to lessen flow
- switch freq & phase enc to rotate the artifact
- ask patient to be still
- use motion resistant seq (SSFE, partial NEX, PROPELLER, BLADE, MULTI-VANE)
- use resp-gating or breath-holding
parallel imaging sequences –> undersample data during readout –> what artifact?
aliasing in image center
susceptibility artifact –> worst in which pulse seq?
EPI
chemical shift artifact type 2 –> occurs with spin echo vs GRE vs both?
GRE only
