Figure 6.

Differences in T₁ between water and fat will lead to bias if the image acquisition is T₁ weighted. Most MRI methods used to acquire fat-fraction images use gradient echo methods, which T₁ weighted using typical flip angles. Since the T₁ of fat is shorter than the water signal in the liver, the use of T₁ weighted imaging will result in an apparent fat-fraction greater than the true fat-fraction. Reducing the flip angle can minimize T₁ bias, although at a cost of reduced SNR performance. It is important to avoid T₁ bias – otherwise the apparent fat-fraction will depend on image parameters such as TR and flip angle, and it is difficult to compare results using different protocols or different scanners. This example of MRI-M demonstrates how reducing the flip angle reduces the T₁ bias (true PDFF ≈ 11%), although also degrades SNR performance.