Chapter 5 p. 2 Flashcards by Julia Boudreau

SSFP stands for

steady state free precession

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the SSFP method overcomes the problem of __ (__) in GE sequences and allows one to obtain __

inhomogeneity artifacts (T2* weighting); true T2 images

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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

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repositioning is achieved by applying a __, which speeds up __ so that the __ occurs sooner

rewinding gradient; rephrasing; stimulated echo

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SSFP: the resulting signal has more __ than regular GE

T2 weighting

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there are 2 Te values

the actual TE (from echo to next RF); effective TE (from echo to RF pulse which created the FID)

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TEeff = __

2TR - TE

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image from slide 35

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the longer the __ TE the lower the __ TE

actual; effective

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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

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disadvantages of SSFP (3)

susceptibility to artifacts; image quality may be poor; loud gradient noise due to very frequent gradient application

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images from slide 37 (not coherent or spoiled images)

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coherent GE sequences sample __ to produce T1 or T2 weighting depending on __

both the FID and the stimulated echo; the TE

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incoherent (spoiled) GE sequences sample __ to produce mainly __ images

the FID only; T1-weighted

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SSFP sequences sample __ which is done before the __ to produce images which are more __

only the stimulated echo; FID; T2-weighted

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image from slide 38 (same as slide 37)

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in balanced GE, a sequence of gradients with __ is used to correct for __ in __ (2)

opposite polarities; phase inhomogeneities; CSF or flowing blood

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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

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balanced GE: the gradients are applied in the __ directions

slice and frequency encoding

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balanced GE: larger __ and shorter __ are used compared to in coherent GE, resulting in higher __ and shorter __

flip angles; TR values; SNR; scan times

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image from slide 39

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balanced GE: to avoid saturation occurring due to __ (which increases __ contrast), the excitation phase pulse is __

large flip angles; T1; changed every TR

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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

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balanced GE: alternating pulses allows the transverse magnetization to __ after each repetition, and thus __ is avoided

have opposite phases; saturation

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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)