The Dixon method is an MRI sequence based on chemical shift and designed to achieve uniform fat suppression. It has been gaining popularity as it has some advantages over other fat suppression techniques, namely:
- suppression of fat signal is more uniform and less affected by artifacts than many other techniques
- can be combined with a variety of sequence types (e.g. spin echo, gradient echo, and steady state free procession sequences)
- can be combined with a variety of weightings (e.g. T1, T2 and proton density)
- provides images with and without fat suppression from a single acquisition
- not only shows presence of microscopic fat but it can also quantify the amount of fat
The Dixon technique exploits the fact that water and fat molecules precess at different rates. As such, over time, they will alternate between being in-phase and opposed-phase. Acquiring both in-phase and opposed-phase images simultaneously allows the images to be combined mathematically in two ways which result in a total of 4 sequences 1:
- in-phase = (water + fat)
- opposed-phase = (water - fat)
- fat only = in-phase - opposed phase = (water + fat) - (water - fat)
- water only = in-phase + opposed phase = (water + fat) + (water - fat)
The water only image can be used a fat-suppressed image.
The fat only image can then be combined with other sequences of various weightings to give fat suppression. It can also be used for quantification in certain scenarios.
One limitation of this method is that of fat-water swapping artefact which occurs in cases of magnetic field inhomogeneity.
History and etymology
The sequence was first described by WT Dixon in 1984 2. Unfortunately due to limitations of MRI technology at that time, the sequence was prone to significant artefact and was not implemented widely until two decades later when many refinements of the technique were created.