Dixon sequence is a water-fat separation method based on chemical shift imaging (CSI),
and can be applied to TSE or GE sequences and to T1-weighted and T2-weighted images.
In only one acquisition 4 sets of images are obtained: water only,
fat only,
in and out of phase.
MRI is an excellent non-invasive modality for evaluating bone marrow and detecting marrow lesions.
MRI with morphological sequences as T1-weighted images has a high sensitivity for detecting both diffuse and focal bone marrow involvement.
In spite of its high sensitivity,
it is of limited specificity in the evaluation of bone marrow alterations.
However,
with CSI from Dixon sequence,
the sensibility and specificity increase.
Focal bone marrow signal alterations,
such as abundant red marrow,
dense bone marrow edema,
transient osteoporosis,
or diffuse osteopenia,
can occasionally mimic marrow-replacing tumors,
due to its typical low signal intensity on conventional T1 sequences.
These scenarios may benefit from the use of CSI.
For bone tumors,
CSI can evaluate the presence of tumor infiltration of the bone marrow and the intramedullary tumor extent,
help to define the best target for biopsy and in the therapy monitoring setting.
Moreover,
it has proven to be useful distinguishing benign from pathological fractures and assessing the different stages of evolution of osteomyelitis.
Marrow infiltrative processes,
such as malignant neoplasms,
tend to replace the fatty marrow components,
whereas most non-neoplastic abnormalities typically do not completely replace the fat.
Thus,
detection of fat within marrow would be expected to make neoplasm a less likely cause of MR signal intensity abnormalities in the marrow.
In phase and out of phase Dixon sequences allow the detection of fat in lesions and helps in the identification of their origin. However,
CSI does not inherently differentiate between benign and malignant marrow-replacing lesions,
it is only useful for differentiating non-marrow-replacing processes (that are usually benign entities) from marrow-replacing processes (usually malignant lesions).
This can be assessed qualitatively,
but it also may be expressed quantitatively as the Signal Intensity Ratio (SIR).
Processes that replace or displace marrow will have a SIR cut-off value > 0.8 or a percentage of signal drop <20%.
Conversely,
non-marrow-replacing processes will show a SIR <0.8 or a percentage of >20% signal loss,
with a 95% sensitivity and an 89% specificity.