chap 4 Flashcards
what pulse sequences are classified according to whether the residual transverse magnetization is in phase (coherent) or out of phase (incoherent)
gradient
gradients that dephase
spoilers
gradients that rephase
rewinders
disadvantage of gradients
no compensation for magnetic field inhomogenities
T/F: generally speaking gradient echo sequences are faster than spin echo sequences
true
a PD has a ____ TR ______ TE & ____ FA
long , short, small
the phase of the RF - carrier is changed according to a predefined formula from view to view
RF spoiling
is performed by applying slice select gradients with variable amplitudes at the end of each cycle just before the next RF pulse
gradient spoiling
why is RF spoiling superior to gradient spoiling
it does not generate eddy currents & is spatially invariant
fills k space by single or multi shot by oscillating the frequency encoding gradient
EPI
what controls the polarity of a gradient
the direction of the current through the gradient coil
what is used to rephase the FID in gradient sequences
a gradient
a GRE T2 image has a ____ TR _____ TE & _____ flip angle
long, long, small
time between the peak of the gradient echo and the next RF , selected in protocol
actual TE
time from peak of the gradient echo to a previous RF that created its FID , TE that determines the contrast
effective TE
reflects that magnetic field inhomogeneites are not compensated for by gradient rephasing
T2*
increases as the TR decreases
T1
this sequence is principally used in diffusion weighted imaging (DWI)
echo planar imaging (EPI)
utilizes rapidly switching gradients to acquire the entirety of k space within one spin echo
EPI sequence
uses an additional 180 degree pulse to flip protons and generate an echo as they rephase
spin echo EPI
does not use a 180 degree pulse flip, dephasing occurs according to T2* which is much more rapid
GR EPI
rewinder gradients to re position the stimulated echo
reverse echo
as the area of the gradient under the line equals that above the line, moving spins accumulate a zero change as they pass along the gradients
balanced gradient system
coherent gradient echo
rewinding, transverse is rewound at the end of each TR
incoherent gradient echo
spoilers, the system discriminates between residual and the most recently created transverse magnetization because it has a different phase value
which demonstrate T1 anatomy & pathology and after gadolinium
incoherent gradient echo
flip angle that provides optimum signal intensity for a tissue with a given T1 recovery time using a given TR
ernst
control the amount of saturation and thus T1 , maintains a steady state
flip angle
