TWO SIGNALS, SLIGHTLY DIFFERENT SPEEDS
Water and fat gradually change direction relative to each other.
Hydrogen in water and hydrogen in fat resonate at slightly different frequencies. After excitation, their transverse magnetization vectors move in and out of alignment as time passes.
At an in phase echo time, water and fat point in the same direction and their signals add. At an opposed phase echo time, they point in opposite directions and can partially cancel inside a voxel that contains both water and fat.
LOOK INSIDE ONE VOXEL
Cancellation needs water and fat in the same voxel.
No fat signal is present to cancel it at the opposed phase time.
No water signal is present to cancel it at the opposed phase time.
The aligned signals add together.
Opposing signals partially or completely cancel.
This is why opposed phase signal loss is most useful at boundaries or in tissue where water and fat share a voxel. A pure fat voxel does not become dark simply because the image is opposed phase.
FROM PHASE DIFFERENCE TO SEPARATE IMAGES
Dixon methods use multiple echo signals to estimate water and fat.
Water and fat contributions are aligned.
Water and fat contributions oppose each other.
Reconstruction estimates the water contribution.
Reconstruction estimates the fat contribution.
Modern Dixon methods may use two, three, or multiple echoes and more advanced reconstruction to account for field inhomogeneity and the complex fat spectrum. Exact echo times depend on field strength, sequence design, and vendor implementation.
DIXON AND STIR ARE NOT THE SAME TOOL
Dixon separates water and fat. STIR nulls short-T1 tissue.
Dixon samples water and fat at different phase relationships, then reconstructs water-only and fat-only images. It preserves the contrast created by the underlying T1- or T2-weighted sequence and often provides uniform fat suppression over a large field of view.
STIR begins with a nonselective inversion pulse and acquires the image when fat is near its zero crossing. It is robust when field uniformity is difficult, but it also reduces signal from other tissues with a similar short T1 and does not produce a fat-only image.
Yes. Dixon can be paired with T1-weighted gradient echo or spin echo methods, T2-weighted fast spin echo methods, and other sequence families. The base sequence creates the weighting. Dixon performs the water-fat separation.
STIR requires a TI near the fat null point. A common teaching range is roughly 150 to 180 ms at 1.5 T and around 200 ms at 3 T, but the correct value is scanner and protocol dependent. Dixon does not use a fat-nulling TI.
Dixon requires two or more appropriately timed echoes. The scanner selects echo times that capture useful water-fat phase differences. Field strength, readout, sequence type, and vendor implementation change the exact values.
For T1 Dixon, short TR and short TE with the selected flip angle commonly support T1 contrast. For T2 Dixon, longer TR and a longer effective TE support fluid-sensitive T2 contrast. Follow the approved scanner protocol rather than copying universal numbers.
T2 Dixon water only is useful when edema or fluid needs to stand out against dark fat. T1 Dixon water only can be acquired before and after contrast to evaluate enhancement, while the fat-only reconstruction can help show fat distribution. STIR is a dependable fluid-sensitive option when uniform spectral fat suppression is difficult.
Do not treat STIR as a substitute for post-contrast T1 fat suppression.
Gadolinium shortens T1. Because STIR suppresses signal according to T1 recovery rather than chemical identity, enhancing tissue may also lose signal. Dixon water-only T1 imaging can preserve enhancement while separating fat, when it is part of the approved protocol.
Dixon can develop water-fat swaps, motion-related misregistration, or separation errors near severe field distortion and metal. STIR generally has lower signal-to-noise and cannot selectively preserve every nonfat short-T1 tissue. Neither method is automatically best for every exam.
Water only images can provide fat suppressed contrast, while fat only images show the estimated fat contribution. In phase and opposed phase images provide a different view of water and fat interaction.
Opposed phase is not the same as water only.
An opposed phase image shows possible cancellation inside mixed voxels. A water only image is a reconstructed separation of the water contribution.
THE TWO ROWERS ANALOGY
Pull together and the boat moves. Pull against each other and effort cancels.
Imagine two rowers. When both pull in the same direction, their efforts add. When they pull in opposite directions, one effort subtracts from the other. Water and fat signal behave similarly inside a shared voxel.
Remember: In phase adds. Opposed phase subtracts. Dixon separates.
CHECK YOUR UNDERSTANDING
Keep phase cancellation separate from water and fat reconstruction.
What happens when water and fat are in phase?
Their transverse signal vectors point in the same direction, so their contributions add within the voxel.
Where is opposed phase signal loss most likely?
It is most likely in a voxel that contains both water and fat, because the opposing contributions can cancel.
Is an opposed phase image the same as a water only image?
No. Opposed phase shows water and fat cancellation behavior. Water only is a reconstructed estimate of the water contribution from multiple acquired signals.
Can Dixon be used for both T1-weighted and T2-weighted imaging?
Yes. The underlying pulse sequence, TR, TE, flip angle, and echo-train design create the image weighting. Dixon adds water-fat separation and can reconstruct water-only and fat-only images from either a T1- or T2-weighted acquisition.
FOCUSED LESSON 41 COMPLETE
You can distinguish in phase, opposed phase, water only, and fat only images.Educational references
Water and fat separation depends on field strength, echo timing, field uniformity, fat spectrum modeling, reconstruction, and vendor implementation. Follow scanner guidance and approved protocols.