FAT AND WATER DO NOT SOUND EXACTLY ALIKE
Fat protons resonate at a slightly lower frequency than water protons.
Electrons around fat shield its hydrogen protons from a small part of B0. Fat therefore experiences a slightly lower effective magnetic field and precesses slightly slower than water.
MRI uses frequency to help decide position. If the system reads fat's lower frequency as a different location, fat signal can shift away from its true position.
TYPE 1: POSITION SHIFT
Fat moves along the frequency-encoding direction.
Fat and water meet at one boundary.
A bright band forms on one side and a dark band on the other.
This first kind of chemical shift appears in the readout, or frequency-encoding, direction. The amount of pixel shift increases when the frequency difference is large or the receiver bandwidth per pixel is small.
TWO RELATED APPEARANCES
Recognize whether the problem is displacement or cancellation.
A bright edge and a dark edge appear on opposite sides of a fat-water boundary in the frequency direction.
On opposed-phase gradient-echo images, fat and water within the same voxel can cancel and create a dark outline called the India-ink artifact.
Opposed-phase signal loss can help identify microscopic fat. Before calling it unwanted artifact, check the sequence, TE, anatomy, and clinical purpose.
HEAR THE NOTE, CHECK THE DIRECTION
Use F-A-T to guide your first response.
Type 1 displacement follows the readout direction.
A higher receiver bandwidth reduces the pixel shift but also reduces SNR.
In-phase or opposed-phase timing determines whether mixed fat and water add or cancel.
Remember: Fat speaks at a lower frequency, so MRI may shift its address.
CHECK YOUR UNDERSTANDING
Match the appearance to the physics.
A bright band and a dark band appear on opposite sides of a kidney. Which artifact fits best?
Type 1 chemical shift. Fat is displaced relative to water along the frequency-encoding direction.
What protocol change commonly reduces type 1 chemical shift?
Increase receiver bandwidth. This reduces the fat-water pixel shift, although SNR decreases.
A thin dark outline surrounds an organ on an opposed-phase gradient-echo image. What caused it?
Fat and water share boundary voxels and are out of phase, so their signals partially cancel. This is the type 2, or India-ink, appearance.
LESSON 31 COMPLETE
You can recognize chemical shift and choose a correction that matches its cause.Educational references
Artifact direction, severity, and correction depend on sequence, field strength, receiver bandwidth, TE, anatomy, and vendor implementation. Follow approved protocols and scanner-specific guidance.