07 MRI
Chemical shift
Water–fat frequency offset from a ppm shift.
Listen
Listen · English
Simulation
Chemical shift — Change the numbers; the scene follows.
Where it works
MRI scanner
Patient in bore
In the tissue inside the bore — relaxation, fat/water, contrast, flow, magic angle.
Open this machineFormula
Variables
Results
Δf
Frequency offset
223.5Hz
Δt
Phase per ms
80.47°/ms
Explanation
What it means
Chemical shift is a ppm offset of Larmor frequency due to electronic shielding. Water–fat is ~3.5 ppm, i.e. Δf ≈ 224 Hz at 1.5 T and 448 Hz at 3 T. That offset is both a diagnostic tool (in/out of phase) and an artefact (India-ink, spatial misregistration). This is a working relation in MRI physics.
Where it is used
Clinically it sits on the MRI scanner — Patient in bore. In the tissue inside the bore — relaxation, fat/water, contrast, flow, magic angle. MRI physics lives in the magnet, the gradient, and the voxel: Larmor, Ernst, diffusion, and SAR. These relations decide whether a sequence is possible, safe, and worth the time.
MRI scanner · Open this machineHow to use it
Enter B₀ and δ. Combine with matrix and bandwidth to get pixel shift (chemical-shift pixels calculator). Silicone, peak of fat (methyl vs methylene) and temperature tweak δ slightly. Change one input and watch the curve and the simulation follow.
Symbols
- δChemical shift3.5 ppm
- B₀Main field1.5 T
- γ/2πGyromagnetic ratio42.577 MHz/T
Worked example
A typical case from the default values: δ = 3.5 ppm (Chemical shift); B₀ = 1.5 T (Main field); γ/2π = 42.577 MHz/T (Gyromagnetic ratio). Substituting into the relation gives Δf = 223.5 Hz; Δt = 80.47 °/ms. These are teaching numbers — align them with your machine.
Typical values give
- Δf = 223.5Hz
- Δt = 80.47°/ms
Where it comes from
The displayed formula is the working relation. Water–fat frequency offset from a ppm shift. Usual reference: McRobbie. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: McRobbie
Assumptions & limits
Single peak vs water. Fat has several spectral lines; Dixon methods model that. Susceptibility (air/tissue) is a separate off-resonance.
Pitfalls
γ for ¹H is not γ for ¹³C. Ernst angle needs the true T1 at that field, not a 1.5 T table used at 3 T. SAR scales with B₀² and flip² — a 3 T copy of a 1.5 T protocol is not automatically legal. Single peak vs water. Fat has several spectral lines; Dixon methods model that. Susceptibility (air/tissue) is a separate off-resonance.
Keep this
Water–fat shift ≈ 3.5 ppm.
In this specialty