1. Chemical Shift To do with the different electron environments of protons Results in a shift of resonant frequency which affects the image e.g. fat (CH 2 ) vs water (H 2 O) ? © D M Higgins Since the secondary fields are as a result of the application of B 0, they are proportional to B 0. There’s nothing you can do about them! Spatial position is frequency encoded in one direction in the imaging process, and so signals from fat and water in the same spatial location will be assigned to different spatial locations on the image. This is called misregistration, and has nothing to do with relaxation times. The external applied magnetic field is modified by the local structure of the molecule a proton resides in. Local variations in field mean different resonant frequencies (c.f. Larmor equation). This is sometimes stated as shielding by the electron cloud of the molecule. Shielding refers to the induction of secondary magnetic fields that oppose B 0 by the movement of electrons due to B 0. Let’s look at how this happens. This tutorial is about chemical shift artefact of the first kind, which produces “incorrect” placement of signal intensities in the image. [There is also chemical shift artefact of the second kind – phase cancellation artefact, which is to do with specific echo times at which fat and water signals from the same voxel are either in or out of phase. The second kind of chemical shift artefact will not be addressed here.]

2. H protons resonate at a particular frequency which depends on the main magnetic field B 0 water fat frequency encoding gradient We then vary the magnetic field across the patient so that frequency relates to position The H protons in fat (unfortunately) resonate at a slightly different frequency The position of fat in the image is slightly shifted 0 This is chemical shift: the difference in resonant frequency. Chemical shift artefact is a consequence of chemical shift. Since the frequency difference will vary with external magnetic field strength (B 0 ), we express it in a way which is fixed: parts-per- million (ppm). The chemical shift between water and fat is 3.5 ppm. From this value we work out the frequency shift for a specific B 0. © D M Higgins

3. frequency encoding direction fat water Frequency Relates to Position superposition of signal signal void object image The signal from fat is placed correctly in the image according to its frequency. Unfortunately this is not where the signal came from. fat © D M Higgins

4. tray of water plastic cup of vegetable oil frequency encoding direction A Real Example Low receiver bandwidth High receiver bandwidth Notice that this artefact occurs in the frequency encoding direction only. © D M Higgins

5. How big is the shift on the image? How many pixels? (receive) bandwidth frequency matrix chemical shift for your magnet e.g. ±16 kHz= 32 kHz e.g. 256 32 x 10 3 (Hz) 256 (pixels) 125 Hz / pixel 3.5 ppm 224 Hz (@ 1.5T) = 1.8 pixels (receive) bandwidth frequency matrix chemical shift for your magnet in this example (this is what you change on a GE scanner) (this is what you change on a Siemens scanner) (this is what you change on a Philips scanner) © D M Higgins