Chapter 5 p. 2 Flashcards
SSFP stands for
steady state free precession
the SSFP method overcomes the problem of __ (__) in GE sequences and allows one to obtain __
inhomogeneity artifacts (T2* weighting); true T2 images
in SSFP, signals are digitized only from the __, not from the __; this is down by repositioning the __ so that it does not occur at the same time as the __
stimulated acho; FID; following excitation pulse
repositioning is achieved by applying a __, which speeds up __ so that the __ occurs sooner
rewinding gradient; rephrasing; stimulated echo
SSFP: the resulting signal has more __ than regular GE
T2 weighting
there are 2 Te values
the actual TE (from echo to next RF); effective TE (from echo to RF pulse which created the FID)
TEeff = __
2TR - TE
image from slide 35
the longer the __ TE the lower the __ TE
actual; effective
advantages of SSFP (2)
signal rephasing is initiated by an RF pulse rather than by a gradient, so the images have more true T2 weighting than conventional GE sequences; scans can be 2D or 3D
disadvantages of SSFP (3)
susceptibility to artifacts; image quality may be poor; loud gradient noise due to very frequent gradient application
images from slide 37 (not coherent or spoiled images)
coherent GE sequences sample __ to produce T1 or T2 weighting depending on __
both the FID and the stimulated echo; the TE
incoherent (spoiled) GE sequences sample __ to produce mainly __ images
the FID only; T1-weighted
SSFP sequences sample __ which is done before the __ to produce images which are more __
only the stimulated echo; FID; T2-weighted
image from slide 38 (same as slide 37)
in balanced GE, a sequence of gradients with __ is used to correct for __ in __ (2)
opposite polarities; phase inhomogeneities; CSF or flowing blood
moving H nuclei accumulate a __ as they pass along the gradients, which means that nuclei in __ (2) are __ and thus achieve high __
zero phase change; CSF and blood; coherent; signal intensity
balanced GE: the gradients are applied in the __ directions
slice and frequency encoding
balanced GE: larger __ and shorter __ are used compared to in coherent GE, resulting in higher __ and shorter __
flip angles; TR values; SNR; scan times
image from slide 39
balanced GE: to avoid saturation occurring due to __ (which increases __ contrast), the excitation phase pulse is __
large flip angles; T1; changed every TR
avoiding saturation is achieved by using a flip angle of __ in the first TR and then a __ pulse in the succeeding TR
45 degrees; 90-degree pulse
balanced GE: alternating pulses allows the transverse magnetization to __ after each repetition, and thus __ is avoided
have opposite phases; saturation
balanced GE, avoiding saturation: tissues with __ will be bright, resulting in high __, good __, and short __
T1/T2 ratios approaching unity; SNR; CNR between fat, water, and surrounding tissues; scan times
image from slide 40
balance GE is used for __ (3)
spine imaging; imaging of the auditory canal, and imaging of structures where CSF flow is present
balanced GE is also used to image __
blood vessels in the brain, heart, etc.
balanced GE requires a short __ to reduce __ and __ as well as a long __ to enhance __
TR; scan time and flow artifact; TE; T2* weighting
fast GE sequences are either __ or __, where the __ is substantially shorter than in conventional ones
coherent or spoiled GE sequences; echo time
FGE: only __ is applied, because it takes less time to __, and after this is done only __ (as in __)
part of the RF excitation pulse; turn on and off an RF pulse; part of the echo is read (as in partial echo imaging)
FGE: because only part of the RF pulse is applied during each repetition, __ are applied __ of the sequence in order to __ (__0
some extra pulses; before the beginning; pre-magnetize the tissue (load it with RF energy)
FGE: there are several ways to pre-magnetize the sample (2):
an 180-degree pulse is applied to completely saturate the sample; driven equilibrium approach
driven equilibrium approach
a 90/180/90 degree combination is applied before the pulse sequence
FGE: driven equilibrium approach: the first pulse __, the second pulse __, the third pulse __, so that __ when the pulse sequence begins
generates transverse magnetization; rephases the transverse magnetization; drives the transverse magnetization into the longitudinal plane, so that it is available when the pulse sequence begins
FGE: k space must be filled efficiently to __, and there are 6 types
minimize scan time; centric, keyhole, single-shot, spiral , elliptic, and propeller filling
FGE: in centric filling, k space is filled __. the __ lines are filled first by applying the __ phase gradients, leaving the __ gradients for the __ of the sequence
linearly (line by line); central; shallowest; steepest; end
FGE: centric filling allows both __ and __ to have reasonable quality and is important in FGE because __
signal and contrast; the SNR is comparatively low in FGE
image from slide 43
FGE: in keyhole filling, k space is filled __ but the __ lines are filled __
linearly; central; certain latencies after the beginning of the sequence
FGE: keyhole filling is employed primarily in conjunction with __, because __
contrast agents; one must wait for the agent to reach the tissues whose signal intensity needs to be enhanced
image from slide 44
FGE: when SS imaging (EPI) is done, __
all k-space is filled within one repetition
FGE: in SS (EPI), echo trains may consist of __ (as in __) or a train of __, in a sequence called __
spin echoes (as in SS-FSE); gradient echoes; echo-planar imaging (EPI)
in EPI, __ are generated and each phase is encoded __ to fill all the lines of k space in one repetition
multiple echoes; by a different slope
image from slide 45
FGE: spiral filling is used when __
signal amplitude is paramount
FGE: elliptic filling is used in certain sequences aiming to __
emphasize blood flow
FGE: propeller filling increases __ because __
SNR and CNR; the origin is traversed repeatedly in each repetition
images from slide 46
in EPI, gradient echoes are generated by __
oscillation (ramping up) of the readout gradient
EPI: different contrasts are achieved by either beginning the sequence using __ or with a set of __
a variable RF excitation pulse called a GE-EPI; 90- and 180-degree RF pulses called SE-EPI
GE-EPI begins with a __ followed by __
variable flip angle; phase/frequency encoding
SE-EPI begins with a __, followed by __, and then a __, followed by __
90-degree RF pulse; phase and frequency encoding; 180-degree pulse; echo trains
image from slide 47
because all k space is filled at once during EPI imaging, the repetition time is __. this is also the case for other sequences where all k space is acquired at once
said to be “infinitely long”
EPI: because the TR is infinitely long, __ is not critical and thus the __ ceases to be a parameter which determines __. for this reason, __ weighting are achieved by selecting either a short or a long TE, respectively
the recovery rate of tissues; TR; weighting; PD and T2
EPI: PD requires a __ TE
short
EPI: TE requires a __ TE
long
in EPI, the TE is the time interval between __
the excitation pulse and the moment when the center of k space is filled
in EPI, T1 weighting can only be achieved by first __ before __
saturating the sample; the excitation pulse is applied
why did people try to combine SE and GE?
because they have complementary advantages
GRASE sequences
hybrid sequences combined gradient echoes and spin echoes
GRASE sequences use a __ followed by a __
RF pulse; series of gradient rephrasing steps
benefits of GRASE (2)
ability to use gradient dephasing/rephrasing (as in GE) and the ability to use RF pulses to compensate for T2* effects (as in SE)
GRASE sequences can have scan times __ but __
which are a bit longer than steady state sequences; image quality is often higher
image from slide 49
EPI and GRASE sequences are often used for fMRI because
they are relatively fast and they can be designed to be sensitive to rapidly changing concentration in hemoglobin
the BOLD signal (__) is usually measured using __ sequences, increasing the popularity of these sequences
blood oxygen level dependent; EPI
in EPI, __ can vary so much that __ can occur
gradient polarities and amplitudes; peripheral nerve stimulation
why/how does EPI cause peripheral nerve stimulation?
— changing the gradient polarities and amplitudes could trigger action potentials
in EPI, patients should wear __ because of __
ear plugs and noise-cancelling headphones; very severe gradient noise
other disadvantages of EPI (2)
blurring and ghosting
images from slide 51
parallel imaging (aka __) involves filling kspace __ (as in __)
sensitivity encoding; several lines at a time (as in FSE) — also involves aliasing
the difference between parallel imaging and FSE is that __
different lines of kspace are acquired by different coils in the magnet
parallel imaging: because each coil records __, the scan time is reduced by an amount equal to __, called the __
a different line; the number of coils; acceleration factor
image from slide 52
when each coil acquires a line of k space, the gap (delta k) between lines is __
x (= # of coils) times larger than it would be if the lines were acquired using one coil
the data spacing (gap) parameter delta k is similar to the __ in our chest analogy, which means that the __ has increased by a factor of __
drawer depth; drawer depth; x (= # of coils)
(gap) delta k = __. this means that the __ has narrowed by a factor of _
1/FOV; FOV; x (= # of coils)
because FOV is narrowed, tissue __ the FOV is __ and the image is __
outside; aliased; ‘wrapped around’
aliasing can be corrected via a sophisticated calculation which relates the acquired signal location to the __ and __ signals by a different factor in order to ‘unwrap’ the image
coil position; weighs
parallel imaging techniques: there is a lot of them, they are complex both conceptually and and programmatically, therefore __, but they are __
it can take years for MR physicists and researchers to become familiar with fine details and subtle differences between them; commonly used
image from slide 54
— at least know the first column (the ones we’ve talked about)
