Chapter 5 p. 1 Flashcards
in PD + T2 spin echo sequences, which echo happens first?
pd
conventional spin echo has a long __
TR
why is reducing the NEX and TR not a desirable way to reduce scan time in SE?
these affect SNR and T1-weighting
in FSE, scan time is reduced by __, such that we acquire __, thus reducing scan time
performing more than one phase encoding step (different amplitudes) within one TR, more than one line of k space at a time
FSE involves several __ pulses, called a __
180 degree rephasing; echo train
echo train length (ETL) =
number of 180 pulses in the echo train
the echo train length and the matrix size determine __
how scan time is reduced
because FSEs are generated at different values of the TE, collected data have __
variable weightings
FSE: the scanner operator selects an __, which determines the desired weighting of the image and which is a weighting of the __
effective TE; individual TE values associated with each spin echo
FSE: when each phase encoding is performed, __ is varied such that the signal is __
gradient slope; phase-shifted by a different amount and signal amplitude is different
when very steep phase gradients are turned on, __ nuclei are affected by the gradient, so signal amplitude is __
fewer; low
when very shallow phase gradients are turned on, __nuclei are affected and signal amplitude is __
more; high
FSE: scanner software arranges the recorded signals so that __
kspace waveforms with low amplitude (recorded when the phase gradient was steep) are on the upper or lower edges of kspace
FSE sequences result in much __ acquisition times than SE due to the __
shorter; echo train approach to data acquisition
because there are many, closely-spaced RF pulses in FSE imaging, the effects of __ are reduced, resulting in __ appearing bright in T2-weighted FSE images
spin-spin interactions in fat; fat
(FSE) because each line is filled during an echo train in which data with different TE values are acquired, __ may occur at __
image blurring; boundaries between tissues with different T2 values
why does image blurring occur in FSE?
because echos placed on the top and bottom edges of kspace have low signal amplitude – these top/bottom signals determine signal resolution, and so images with long ETLs have increased image blurring at boundaries because resolution is lost when signal amplitude determining resolution is low
image blurring in FSE sequences can be reduced by __ or by __
decreasing the ETL; decreasing the spacing between echoes (reducing the effective TE)
summary of differences between SE and FSE
FSE: shorter scan time, spin-spin interactions of fat are reduced and fat shows up bright in t2 images, but image blurring at boundaries between tissues of different t2 values
the __ application of FSE is SS-FSE, where __
most extreme; the entire kspace is filled in one TR (in a single shot) (but really its only half)
in SS-FSE, __ is acquired in one TR and then __
half of kspace; the rest is obtained by flipping the acquired data (making use of the conjugate symmetry property of kspace
SS-FSE makes use of what property of k space
the conjugate symmetry
advantage of SS-FSE
FAST
disadvantages of SS-FSE
decrease in SNR (due to partial k space acquisition), body temperature increases (due to many 180 degree pulses transferred a lot of energy to sample)
to prevent SAR increases in SS-FSE, __, though this results in __
the 180 pulse may be replaced with a 120 pulse; decrease in SNR
pi =
180 degree
all the 180 pulses in SS-FSE cause __
an increase in SAR of the body, and subject can overheat
in T1 and PD-weighted FSE imaging, __ ETLs introduce too much T2-weighting, so _ ETLs should be used
long; shorter
in T2-weighted FSE imaging, __ ETLs are more useful because __
long; T2 contrast is stronger than T1 contrast
using FSE, scan times are __ in T2 imaging than in T1 imaging
shorter
Driven-equilibrium FSE (DRIVE) applies a __ to avoid having to wait for __
reverse flip-angle excitation pulse at the end of the echo train; longitudinal magnetization to recover
(DRIVE) because water has the longest relaxation constants, most of the magnetization affected by the reverse pulse is __
due to water and thus water has high intensity in DRIVE images
DRIVE imaging produces increases in signal intensity in __ structures when using TRs which are __ than traditional FSE
CSF-rich; shorter
inversion recovery sequences:
begin with a 180 degree inversion pulse that saturates the sample
IR: some time is allowed to elapse after 180 saturating pulse and before 90 pulse as M0 begins to __
recover its direction along Bo
(IR) the time from inversion (Ti/tau)
the time between the 180 saturation/inversion pulse and the 90 excitation pulse
(IR) after the 90 excitation pulse, the resulting signal is then __ and then it is __ to produce a spin echo at the echo time
allowed to experience FID; rephrased by a 180 pulse
in IR sequences, contrast is primarily dependent on the value of __
the Ti/tau
in IR, the TE is the time between
the 90 excitation pulse and the echo
(IR) if the 90 pulse is applied after the magnetization vector has relaxed through the transverse plane, image contrast depends on __; resulting image is __-weighted because the 180 pulse achieves full saturation and ensures a large contrast difference between fat and water
the amount of longitudinal recovery in each vector; T1-weighted
(IR) if the 90 pulse is applied after the magnetization vector has achieved full recovery (in longitudinal plane), image contrast depends on __ because the magnetization vectors of both fat and water have fully relaxed; resulting image is _-weighted because signal amplitude is dependent on density of spins rather than on the difference in longitudinal magnetization between fat and water
proton density; PD
advantages of IR sequences
can get large contrast between fat and water so excellent T1-weighting, can get pathology weighting, excellent SNR (because TR is long)
IR is a sequence of choice for what type of weighting
T1
we can get pathology-weighting if the echo time is __ in IR, because tissues with __ appear bright
long; long T2 decay constants
the TR should always be long in IR to allow __
full recovery of longitudinal magnetization before next inversion pulse
FIR involves the use of __
an echo train of 180 pulses after the initial 90 pulse
FIR allows us to fill __
multiple rows of kspace at once, allowing shorter scan times
FIR is typically used in conjunction with __, so that __ appear bright
T2-weighting; fluid and pathology
the two main FIR sequences are __
short-tau inversion recovery (STIR) and fluid-attenuated inversion recovery (FLAIR)
in STIR, the Ti is set to __
the amount of time needed for fat to recover into the transverse plane
in FIR (STIR/FLAIR), what is the null point?
the amount of time needed for that tissue to recover into the transverse plane (null point of fat, CSF, gray matter, white matter)
because of nulling _-rich tissues appear dark in STIR
fat
main uses of STIR
(nulls fatty bone marrow) often used to identify bone bruises, fractures, and tumors
FLAIR - Ti/tau is equal to __
the time needed for CSF to recover into the transverse plane (from 180 to 90 degrees)
nulling: when the 90-degree excitation pulse is applied, whatever matter is nulled has no signal because
the transverse component becomes a completely longitudinal component and there is no signal
FLAIR is used to suppress __ signal in __-weighted images so that __-adjacent pathology (__) can be seen as hyperintense
CSF; T2; CSF; edema
uses of FLAIR imaging
useful for TBI, visualizing periventricular and spinal cord lesions, MS plaques, sub-arachnoid hemorrhages and brain swelling due to meningitis
there are other types of FIR sequences, aimed at nulling signal due to __
white matter, muscle, etc.
FIR sequences which null white matter are used to detect __
periventricular leukomalacia, a type of white matter necrosis which can occur in preterm infants
GE: instead of using 180 pulse like spin echo to dephase/rephase signal, __
first dephase using a freq encoding gradient, then flip the gradient polarity to product the echo
because GE sequences can involve small flip angles, __ images can be acquired using __ TR values
T2* and PD; short
fundamental innovation of GE sequences
use variable flip angles and gradient to produce dephasing and rephasing faster than SE, reduce scan time
GE: __ determines the degree of saturation and therefore T1 weighting
flip angle and TR
GE: to prevent saturation, which is necessary in __ -weighting, the flip angle should be __ and the TR __
T2* and PD; short; long enough to permit full recovery (with a short flip angle, this “long” TR is still shorter than SE)
GE: if saturation and therefore _-weighting is required, the flip angle should be __ and the TR __, so that __
T1; large; short; full recovery cannot occur
GE: the TE controls the amount of __
T2* dephasing
GE: to minimize T2*, the TE should be __
short (to maximize it should be long)
GE T1 weighting: __ flip angle, __ TR, and __ TE
large (to maximize saturation); short (to maximize saturation); short (to minimize T2*)
GE T2* weighting: __ flip angle, __ TR, and __ TE
small (to minimize saturation); long (to minimize saturation); long (to maximize T2*)
GE PD weighting: __ flip angle, __ TR, and __ TE
small (to minimize saturation); long (to minimize saturation); short (to minimize T2*)
the the steady state, the amount of energy imparted to a system is __
equal to the amount of energy flowing out of the system
in GE imaging, the __ (2) determine the amount of energy imparted to the protons as well as the amount of energy allowed to dissipate into the lattice, so one can __
flip angle and TR; choose the right combination of flip angle and TR so that the spin-lattice system remains in a steady state as a function of time
RF pulses have __ energy, ad thus __ values of the TR are needed to achieve a steady state
low; short
because TR is short in steady-state sequences, there is always __ at the end of each repetition
residual transverse magnetization
in steady-state sequences, the TR is so short that __
the magnetization does not have enough time to reach its T1 and T2 values before the end of each repetition
steady-state image contrast is not due to difference in __ between tissues, but rather __
T1 or T2; the ratio of T1/T2
in steady-state images, tissues where T1 and T2 are very similar (like water) will appear __
bright
in the human body, the T1/T2 ratio is high for __ (2) compared to other tissues, so they appear brighter
water and fat
because the Tr is very short in steady-state sequences, __
most GE sequences use a steady-state design
GE sequences are classified depending on whether __
the residual transverse magnetization of protons at the beginning of each repetition is in phase (coherent GE) or out of phase (incoherent GE)
in steady state, residual transverse magnetization is __
rephrased by an RF pulse to generate a SE, which occurs at precisely the time of the next RF pulse because it takes as much time to rephrase spins as it does to dephase them
the TR in steady state is equal to
the tau of the spin echo
in steady state, the __ occurs as a result of the initial pulse, and the __ occurs as a results of the second pulse
FID; Hahn or stimulated echo
two consecutive RF pulses produce a __ in the steady state
stimulated echo
in steady state, because the TR between RF pulses is shorter than __, there is residual magnetization after each RF pulse, and this magnetization is __
the T1 and T2 of the tissues; excited subsequent to each TR
(steady state) the first RF pulse excited nuclei __
regardless of its net amplitude
(steady state) the second RF pulse __
rephases the FID nuclei and the residual magnetization to generate a SE
(steady state) if both RF pulses have a flip angle of 90 degrees, they generate __ (otherwise they produce __)
Hahn echoes; stimulated echoes
coherent GE sequences use an RF pulse with a __ followed by __ to produce a GE
variable flip angle; gradient rephasing
coherent GE sequences involve a steady state because __ and there is __
the TR is short; residual transverse magnetization when the next excitation pulse is delivered
(coherent GE) the residual magnetization is kept in phase (coherent) via __ , where __
rewinding; the slope of the phase encoding gradient is reversed after readout
rewinding results in the __ so that __
rephasing of residual magnetization; in phase (coherent) at the beginning of each repetition
4 advantages of coherent GE
very short TR and thus scan time; water is bright so good T2*; useful for angiographic, myelographic, or arthorgraphic imaging; excellent for determining whether brain vessels are occluded or whether there is fluid accumulation in certain brain areas
3 drawbacks of coherent GE
SNR is low when images are acquired 2Dlly; increased magnetic susceptibility artifacts; loud (due to gradient noise)
spoiled/incoherent GE sequences involve a __ and __
variable flip angle excitation pulse and rephrasing
residual magnetization is spoiled so that __
its effect on image contrast is minimal
RF spoiling: when the signal produced by the second pulse is rephrased, __
there is some residual signal from the first pulse
the two overlapping signals in RF spoiling are kept separate because __, and the signal due to the dephasing residual magnetization is not recorded
the two signals are at different frequencies and phases
RF spoiling allows only information from the __
most recent signal to affect image contrast
4 advantages of spoiled GE
accommodates 2D and 3D imaging; short TR so short scans; good SNR and anatomical detail; amenable to the use of contrast agents
2 drawbacks of spoiled GE
low SNR when images are acquire 2Dlly (same as coherent GE); loud (due to gradient noise) (also same as coherent GE)
what disadvantage does coherent GE have that spoiled GE does not?
greater magnetic susceptibility artifacts
