Fast Imaging & Artifacts

126 Pages·2017·29.3 MB·English

by  Daniel Ennis

MRI Basics - Fast Imaging &
Artifacts
Daniel B. Ennis, Ph.D.
Magnetic Resonance Research Labs
[email protected]
310.206.0713 (Office)
http://mrrl.ucla.edu
UCLA
Radiology
Spatial Localization
Spatial Encoding
• Three key steps:
– Slice selection
• You have to pick slice!
– Phase Encoding
• You have to encode 1 of 2
dimensions within the slice. Steps required to
– Frequency Encoding (aka readout) acquire k-space data.
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• You have to encode the other
dimension within the slice.
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Radiology
Where am I in k-space?
RF Phase
Cycling
RF
Spoiler
Gradient
Slice
Select
k (t)
pe
Phase
Encode
k (t)
ro
Freq.
Encode
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Gradients move the acquisition through k-space.
Radiology
How do we calculate scan time?
T = T R N N N
Scan y,pe z,pe avg
· · ·
• TR – Repetition time
– Time between excitation pulses
– Typically one echo per TR (but not always!)
• Multi-echo imaging is faster…
• N – Number of in-plane (y) phase encodes
y,pe
– Required for 2D and 3D imaging
• N – Number of slice (z) phase encodes
z,pe
– Only used for 3D volume imaging
• N – Number of averages
avg
– Averages are repeated acquisitions that boost SNR
• T =1000ms•256•1=4:16 [mm:ss]
Scan
– MRI scanning can be slow.
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Radiology
k-space
What is k-space?
• k-space is the raw data collected by the scanner.
– A point in k-space tells us about the presence/absence of a
spatial frequency (pattern) in the acquired image.
– Each echo measures many of the spatial frequencies that
comprise the object.
– k-space has units of cm-1 or mm-1
– Audio signals have units of Hertz (s-1)
• Gradients
– Help extract spatial frequency information
– Move us around in k-space
• A line of k-space is filled by an echo
• 2D FT of k-space produces the image
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Radiology
What is k-space?
k-space image space
➨
Fourier
Transform
k-space is the raw data collected by the scanner.
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Radiology
What is k-space?
k-space image space
➨
Fourier
Transform
One echo fills a line of k-space.
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Radiology
What is k-space?
Contrast
Information
Points in k-space represent different patterns in an image.
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Radiology