Chapter 7 Flashcards
phase mis-mapping (aka __) refers to the __ across an image in the __ direction
ghosting; replication of moving anatomy; phase-encoding
ghosting is typically due to __ (4)
periodic or quasi-periodic movements of the patient during scan; pulsatile movement of vessels and CSF; swallowing; eye movement
when ghosting is present in __ imaging, the __ direction can be determined by __
sequential; phase-encoding; visual inspection
ghosting occurs along the __ direction because that gradient has a __ whereas the __ gradient has __
phase-encoding; different amplitude during each TR; frequency encoding; same value during each TR
(ghosting) as anatomy moves, the scan is __ in the __ direction when each line of k space data are __
misplaced; phase-encoding; acquired within one TR
image from slide 3
— (he’s going to give an image and ask us to name the artifact)
when signal readout and frequency are done __ (which is often the case), there is a time delay between __ and __
simultaneously; phase encoding and signal readout
if anatomy has moved between __ and __, that movement will affect how the signal __
phase encoding and readout; is read into k space
no ghosting occurs in the __ direction because that type of encoding is done __
frequency encoding; simultaneously with the data readout and digitization processes
image from slide 4
one common remedy for ghosting involves swapping __ so that the direction of __ is not the same as the direction in which movement occurs
phase and frequency; phase encoding
in sagittal spine imaging, __ and __ along the phase encoding gradient produce ghosting
swallowing and pulsatile motion of the carotid arteries
in sagittal spine imaging, frequency encoding is usually performed by the __ gradient because this is the __
Z; longest axis of the patient in the sagittal plane
in sagittal spine imaging, freq encoding = Z, phase encoding is then along the __
anterior-posterior axis (Y)
in sagittal spine imaging, ghosting artifacts can be alleviated if the __ gradient is chosen to perform frequency encoding, and the __ gradient is chosen to perform phase encoding
Y; Z
images from slide 6
pre-saturation nulls the signal from __ so that __ are minimized
flowing nuclei; flow effects
pre-saturation: first a __ pulse is delivered to a volume of tissue __
90-degree; outside the FOV
nuclei which flow into the FOV following the pre-saturation pulse receive a 90-degree pulse and are thus __, producing __
saturated; no measurable signal
pre-saturation: if the second pulse is delivered within the FOV, __ will have their signal nulled
any flowing nuclei in it
__ produces ghosting along the __ axis in sagittal scans of the spinal cord
swallowing; phase
(sagittal scan of spinal cord): applying a pre-saturation pulse over the __ reduces __
throat; ghosting due to swallowing and blood flow
pre-saturation is most effective when __
placed between flow origin and FOV
image from slide 7
aliasing (aka __) occurs when __
wrap-around; anatomy outside the FOV is folded into the FOV
in sagittal imaging of the brain, the FOV in the __ direction is smaller than the __ dimension of the head, and signal __ the FOV is aliased
phase; anterior-posterior; outside
aliasing: anatomy outside the FOV still __. this must still be __ if it is sampled, i.e. __
produces signal if it is very close to the receiver coil; encoded; allocated a pixel position
aliasing: if data __ occurs, the signal is mis-mapped __ rather than __
under-sampling; into the FOV; outside
aliasing can occur along both __ axes
frequency and phase
image from slide 8
frequency wrap is __
aliasing along the frequency encoding direction
frequency wrap is caused by __
under-sampling frequencies present in the echo which are caused by signal recorded either inside or outside the FOV
if Nyquist’s theorem is not obeyed and __, signal from anatomy __ the FOV is mapped __ the FOV
frequencies are undersampled; outside; inside
frequency wrap can be corrected by __ so that __
increasing the sampling frequency; Nyquist’s rule is obeyed
image from slide 9
phase wrap is __ and it is caused by __
aliasing along the phase encoding direction; under-sampling along this direction
the signal associated with each __ must be mapped into the FOV, and signal are allocated __ according to their __
precessional phase value (from 0 to pi radians); phase values; position along the phase curve
image from slide 10
phase wrap: because phase repeats, signal originating __ the FOV in the __ direction are allocated __ which have already been assigned to signal which originates __ the FOV (on the opposite side)
outside; phase direction; phase values; inside
the rate of phase repeat is __
the pseudo-frequency
in abdomen imaging, the FOV in the __ axis of the image (__) is smaller than the anatomy
phase; right to left
abdomen imaging: signal originating in anatomy to the left of the FOV has the same phase as __. signal originating in anatomy to the right of the FOV has the same phase as __. this is why these regions are mapped __ rather than not mapped at all
the rightmost portion of the FOV; the leftmost portion of the FOV; within the FOV
images from slide 11
aliasing along the frequency direction can be reduced by __(4)
increasing the FOV; pre-saturating the areas outside FOV to null signal; increasing the sampling rate so that all frequencies are digitized properly; applying a digital filter so that only frequencies of interest are digitized, and frequencies outside the FOV are ignored
one way to reduce frequency aliasing is increasing the __ but this causes a loss of __
FOV; spatial resolution
one way to reduce frequency aliasing is to __ areas outside the FOV to __, but this is __
pre-saturating; null signal; time-consuming
one way to reduce frequency aliasing is to increase the __ so that all frequencies are __, but this causes a decrease in __ due to increase in __
sampling rate; digitized properly; SNR; noise
one way to reduce frequency aliasing is to __ so that only frequencies __ (__) are digitized, and frequencies __ are ignored. analogy: __. drawback: __
apply a digital filter; of interest (due to signals within the FOV); outside the FOV; filtering bass and treble on a music system using an equalizer; Gibbs ringing artifact
apply a digital filter to reduce freq. aliasing can cause a __ artifact at __, resulting in artifactual increases in __ at the __ of the FOV
Gibbs ringing artifact; edges of the filter; image intensities; boundaries
image from slide 12
phase anti-aliasing is also called __ (3)
no-phase wrap, phase over-sampling, or anti-foldover
phase anti-aliasing involves __ the original FOV to __ and prevent aliasing
extending; encompass all anatomy
phase anti-aliasing: because enlarging the FOV while keeping all else constant results in loss of __, the __ is increased to keep this constant
spatial resolution; number of phase encodings
obvious disadvantages of increasing the number of phase encodings in phase anti-aliasing (2)
increase in scan time; and if you want scan time to be constant you must decrease the NEX
phase anti-aliasing: if NEX is reduced, only the __ is kept
anatomy image associated with the original FOV**
phase anti-aliasing: reducing the NEX results in more __… there is no __
motion artifact; silver bullet
images from slide 13
truncation artifact produces a __ at the interface between __ (i.e. at the __)
banding effect; high- and low-intensity signal; at the edges of the brain in a T1-weighted scan
the cause of truncation artifact is __, to the effect that __ (__) are poorly represented
under-sampling of data in k-space; contours (lines at the transition from high to low intensity)
truncation artifact involves bands of __ running through __
low-intensity artifact; high-intensity regions
to avoid truncation artifact, the __ must be increased
number of phase encoding steps
image from slide 14
magnetic susceptibility is a __ which indicates how susceptible they are to __
physical property of materials; becoming magnetized
magnetic susceptibility artifacts result in partial or complete __ in the region where __ is located
loss of signal; an object made of magnetically-susceptible material
__ materials have very high susceptibilities and cause __
ferromagnetic; distortion
susceptibility artifacts are more common in __ sequences because __ followed by __ does not compensate for __
GE; gradient dephasing; rephasing; FID-related T2* effects
to reduce susceptibility artifacts, one should __, use __ sequences, and/or decrease __ to minimize __
remove metal objects; SE; TE; dephasing between tissues with susceptibility differences
3 ways to reduce susceptibility artifacts
remove metal objects; use SE sequences; decrease TE
image from slide 15
cross-excitation occurs when __
adjacent slices have different contrasts
adjacent slices have different contrasts because RF pulse is __ and if there is no __ then adjacent RF pulses can __, resulting in __ across adjacent slices (hence the name ‘cross-excitation’)
fully rectangular; slice gap; overlap; excessive excitation
when cross-excitation occurs, the __ of protons at the __ of slices are tipped farther away from the __, which may cause __
magnetization vector; edges; direction of the static field; partial saturation
cross-talk involves nuclei __ during recovery of the __ (due to __ interactions)
losing energy to protons in adjacent slices; longitudinal magnetization; spin-lattice
images from slide 17
cross-talk __ (can/cannot) be eliminated because it is __
cannot; an uncontrollable natural phenomenon
cross-talk — cross-excitation (which can be reduced/prevented?)
cross-excitation
one way of reducing cross-excitation: implement two scans, one where you __ and another when you __ (called __)
excite all odd slices first; excite even slices; interleaving/concatenation
interleaving/concatenation ensures that __
slices do not overlap at all while they are excited
interleaving has the benefit of __ however it results in __
not having to use a slice gap; longer scans
one way of reducing cross-excitation: __ first, then __, the again __, and so on until all volume has been imaged
excite a few odd slices; excite a few even slices; a few odd slices
alternating odd/even slices being excited results in __ because it takes about __ for __
a faster scan; TR/2; cross-excitation of slices to decay
alternating odd/even slices being excited: disadvantage is that __ is needed to reduce cross-excitation effects
a slice gap of at least about 30% the slice thickness
two ways to prevent cross-excitation
interleaving; alternating exciting a few odd slices, then a few even, then a few odd, etc.
zipper artifacts occur when __
a dense line appear in the image at a specific point
the cause of zipper artifact is the presence of __, which allows __ to interfere with the RF pulse emitted by the scanner
a leak in the RF shielding of the MR room; RF energy from electrical sources outside the room
remedy of zipper artifact: __ (may be __)
locate the RF shielding leak and repair it (expensive)
shading refers to __
differences in intensity across the image volume
3 causes of shading
uneven excitation of nuclei across the sample; undesirably close proximity of the anatomy to the receiver coil, causing ‘coupling’ or even contact of the receiver coil and anatomy; inhomogeneities in the magnetic field (especially at high field)
4 remedies of shading
reposition the patient/use padding around the body; apply shimming; ensure that the coil is of appropriate size; apply from pre-scan pulses to determine the correct RF frequency and amplitude
image from slide 20
what is coupling of the coil and anatomy and how does it cause shading?
