1 Chemical Shift Image B shows a more marked amount of chemical shift, why can this be? Chemical shift is an artefact which occurs because fat and water.

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Presentation transcript:

1 Chemical Shift Image B shows a more marked amount of chemical shift, why can this be? Chemical shift is an artefact which occurs because fat and water precess at different frequencies B B A A

2 Chemical Shift Phase Frequency

3 Chemical Shift Phase Frequency The range of frequencies along the frequency encoding axis can be compared to the range of notes along a keyboard 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. This is essentially how frequency encoding works, the Fourier Transform converts “chords” into the respective “notes”

4 Chemical Shift 256 Frequency Encodings 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.

5 Chemical Shift 256 Frequency Encodings 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 125 Hz At total receive bandwidth 32 kHz 1.8pixels

6 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 256 Frequency Encodings 62.5 Hz At total receive bandwidth 16Khz 3.6pixels

7 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 256 Frequency Encodings Hz At total receive bandwidth 8kHz 7.2pixels

8 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 256 Frequency Encodings Hz At total receive bandwidth 8kHz