Transcript chemical_shift

Chemical Shift
Chemical shift is an artefact which occurs because fat and water precess at
different frequencies
Image B shows a more marked amount of chemical shift, why can this be?
A
B
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Chemical Shift
Phase
Frequency
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Chemical Shift
The range of frequencies along the frequency encoding axis can be
compared to the range of notes along a keyboard
This is essentially how frequency encoding works, the Fourier Transform
converts “chords” into the respective “notes”
Phase
This high intensity pixel
could be represented by
striking the corresponding
piano key forcibly.
This low intensity pixel
could be represented by
striking the corresponding
piano key gently.
Frequency
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Chemical Shift
This system would work perfectly if each voxel contained tissue that all
resonated at the same frequency
Unfortunately, due to it’s chemical environment, at 1.5T fat has a
precessional frequency that is 225 Hz lower than water
Fat and water
molecules in the
same voxel.
256 Frequency Encodings
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Chemical Shift
This system would work perfectly if each voxel contained tissue that all
resonated at the same frequency
Unfortunately, due to it’s chemical environment, at 1.5T fat has a
precessional frequency that is 225 Hz lower than water
At total receive
bandwidth 32 kHz
125 Hz
1.8pixels
256 Frequency Encodings
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Chemical Shift
This system would work perfectly if each voxel contained tissue that all
resonated at the same frequency
Unfortunately, due to it’s chemical environment, at 1.5T fat has a
precessional frequency that is 225 Hz lower than water
At total receive
bandwidth 16Khz
62.5 Hz
3.6pixels
256 Frequency Encodings
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Chemical Shift
This system would work perfectly if each voxel contained tissue that all
resonated at the same frequency
Unfortunately, due to it’s chemical environment, at 1.5T fat has a
precessional frequency that is 225 Hz lower than water
At total receive
bandwidth 8kHz
31.25 Hz
7.2pixels
256 Frequency Encodings
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Chemical Shift
This means that on our image (FOV24)
structures that are comprised of fat will
appear to be shifted 6.6mm from their actual
position
At total receive
bandwidth 8kHz
31.25 Hz
256 Frequency Encodings
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