Chemical Shift Chemical shift is an artefact which

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 1

Chemical Shift Phase Frequency 2

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 3

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. 5 T fat has a precessional frequency that is 225 Hz lower than water Fat and water molecules in the same voxel. 256 Frequency Encodings 4

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. 5 T fat has a precessional frequency that is 225 Hz lower than water At total receive bandwidth 32 k. Hz 1. 8 pixels 125 Hz 256 Frequency Encodings 5

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. 5 T fat has a precessional frequency that is 225 Hz lower than water At total receive bandwidth 16 Khz 3. 6 pixels 62. 5 Hz 256 Frequency Encodings 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. 5 T fat has a precessional frequency that is 225 Hz lower than water At total receive bandwidth 8 k. Hz 7. 2 pixels 31. 25 Hz 256 Frequency Encodings 7

Chemical Shift This means that on our image (FOV 24) structures that are comprised of fat will appear to be shifted 6. 6 mm from their actual position At total receive bandwidth 8 k. Hz 31. 25 Hz 256 Frequency Encodings 8