1

Chemical Shift

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

BBAA

2

Chemical Shift

PhasePhase

FrequencyFrequency

3

Chemical Shift

PhasePhase

FrequencyFrequency

The range of frequencies along the frequency encoding axis can be compared to the range of notes along a keyboardThe 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 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 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”This is essentially how frequency encoding works, the Fourier Transform converts “chords” into the respective “notes”

4

Chemical Shift

256 Frequency Encodings256 Frequency Encodings

This system would work perfectly if each voxel contained tissue that all resonated at the same frequencyThis system would work perfectly if each voxel contained tissue that all resonated at the same frequencyUnfortunately, due to it’s chemical environment, at 1.5T fat has a precessional frequency that is 225 Hz lower than waterUnfortunately, 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.

Fat and water molecules in the same voxel.

5

Chemical Shift

256 Frequency Encodings256 Frequency Encodings

This system would work perfectly if each voxel contained tissue that all resonated at the same frequencyThis system would work perfectly if each voxel contained tissue that all resonated at the same frequencyUnfortunately, due to it’s chemical environment, at 1.5T fat has a precessional frequency that is 225 Hz lower than waterUnfortunately, due to it’s chemical environment, at 1.5T fat has a precessional frequency that is 225 Hz lower than water

125 Hz125 HzAt total receive bandwidth 32 kHzAt total receive bandwidth 32 kHz

1.8pixels

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Chemical ShiftThis system would work perfectly if each voxel contained tissue that all resonated at the same frequencyThis system would work perfectly if each voxel contained tissue that all resonated at the same frequencyUnfortunately, due to it’s chemical environment, at 1.5T fat has a precessional frequency that is 225 Hz lower than waterUnfortunately, due to it’s chemical environment, at 1.5T fat has a precessional frequency that is 225 Hz lower than water

256 Frequency Encodings256 Frequency Encodings

62.5 Hz62.5 HzAt total receive bandwidth 16KhzAt total receive bandwidth 16Khz

3.6pixels

7

Chemical ShiftThis system would work perfectly if each voxel contained tissue that all resonated at the same frequencyThis system would work perfectly if each voxel contained tissue that all resonated at the same frequencyUnfortunately, due to it’s chemical environment, at 1.5T fat has a precessional frequency that is 225 Hz lower than waterUnfortunately, due to it’s chemical environment, at 1.5T fat has a precessional frequency that is 225 Hz lower than water

256 Frequency Encodings256 Frequency Encodings

31.25 Hz31.25 HzAt total receive bandwidth 8kHzAt total receive bandwidth 8kHz

7.2pixels

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Chemical ShiftThis means that on our image (FOV24) structures that are comprised of fat will appear to be shifted 6.6mm from their actual position

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 Encodings256 Frequency Encodings

31.25 Hz31.25 HzAt total receive bandwidth 8kHzAt total receive bandwidth 8kHz