MEDRADSC 3DE3 Lecture 9: Artifacts
18 Jun 2018
School
Department
Course
Professor
Introduction
Magnets
Gradients
Patients
Artifact- any feature in an image which misrepresents the object in the field of view
3 categories:
oMotion- show up as ghosts, occur in phase encoding direction
oInhomogeneity
oDigital imaging- systems themselves in Fournier transform, aliasing
Gross Patient Motion
Most common
Generalized blurring
Ghosts- faint images behind the image
Sedation in the form of Adavan (must be prearranged), come back if they are too claustro
oGeneral anaesthetic: will have entire team
Information- how long will it take?
Immobilization straps and foam cushions
Intercom
Repeat and/or reduce scan time
oOne repeat and that’s it
oReduce scan time- less detail
Physiological Motion
Respiration, heart beats, peristalsis, blood flow
Always occurs in phase encoding direction
Phase encoding gradient has different amplitude for each TR
Frequency and slice selection gradient have same amplitude for each TR
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Time between phase gradient and application of frequency gradient also a part of why this
artifact shows up in the phase encoding direction
oThe frequency gradient is on for only 1ms, the phase encoding gradient is on for as long
as the TR requires (much longer than 1ms)
Respiratory Motion
Reduce scan time- less than 25s (one breath hold, but some people can’t hold their breath for
25s)
oIf they can’t we can split it up into “batches”- requires coaching, we want the chest wall
to be in the exact same place each breath hold so that we don’t have ghosting
oEasier to put the diaphragm into the same place on expiration (batches)
oEasier to hold breath longer on inspiration (single breath hold)
Increase NSA or NEX increases the overall time
oStatic anatomy will be in the same place, the motion will be random, the averaging will
reinforce the signal intensity of the real image and reduce the SNR of the ghost (4-6 NSA
for each DR if we want to reduce the artifacts)
Swap phase and frequency
oGood for pulsatile movements
Use pre saturation pulse to null specific frequencies
oShoulders right next to lungs, we wall to null the frequencies from the lung, we can lay
down a presaturation pulse/band and get rid of the saturation in that entire area
Respiratory compensation- Respiratory Ordered Encoding (ROPE)- waveform from bellow,
reorders the filling of K space to match the respiratory waveform, removes breathing artifact,
lengthens scan time
oUses bellows that contract and relax with each breath, causes air to move back and
forth in the bellows, connected to tubing, tubing connected to the transducer in the
system and the transducer is going to convert the mechanical motion of the breathing
to an electrical siangl, amplitude of this corresponds to max and min of each position of
the chest wall. The cost of this is the number of slices for each TR (may be reduced) the
system itself will map out the shallow breath in the center of the k space first to provide
contrast and map the max breath hold in the outer k space. Separating the two
positions reduce the ghosting
Respiratory gaiting or triggering- times excitation pulse off specific phase of respiration
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oUses bellow, time the RF pulse with a certain phase resp. each slice taken at the same
position of the chest wall. The tr and contrast not affected by breathing rate but the
scan time will be increased
Navigator echoes- no bellow, rectangular window placed at right hemidiaphragm, extra RF
pulse, no PE, strong contrast between dark lung and brighter liver, series of linear navigators
acquired, many lines, scanner software detects moving diaphragm, gates the acquisition
oUses a ROI, on abdo
oWe look at signal intensity
oPlace it so that we have half the signal from the dark lung and half the signal from the
bright abdomen. Each time the patient breathes for a while, the system will watch it
until it gets a sense of having dark lung tissue and bright liver tissue.. when it is equal, RF
pulse will be triggered
oMust take regular, steady breaths
oCan be a very long scan
yellow: phase gradient / green:
frequency
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Artifact- any feature in an image which misrepresents the object in the field of view. 3 categories: motion- show up as ghosts, occur in phase encoding direction, inhomogeneity, digital imaging- systems themselves in fournier transform, aliasing. Sedation in the form of adavan (must be prearranged), come back if they are too claustro: general anaesthetic: will have entire team. Repeat and/or reduce scan time: one repeat and that"s it, reduce scan time- less detail. Phase encoding gradient has different amplitude for each tr. Frequency and slice selection gradient have same amplitude for each tr. Reduce scan time- less than 25s (one breath hold, but some people can"t hold their breath for. Swap phase and frequency: good for pulsatile movements. Respiratory gaiting or triggering- times excitation pulse off specific phase of respiration: uses bellow, time the rf pulse with a certain phase resp. each slice taken at the same position of the chest wall.
