CSI from Dixon sequence is useful for differentiating non-marrow-replacing processes (that are usually benign) from marrow-replacing processes (that are usually malignant lesions) but it does not inherently differentiate between benign and malignant lesions.
Thus,
it is important to understand the bone marrow composition changes within the different clinical scenarios.
The usefulness of the CSI in bone marrow clinical applications include some of the bone marrow signal alterations that may mimic marrow-replacing tumors in morphologic sequences as T1-W and fluid-sensitive images,
as they are abundant red marrow,
dense bone marrow edema,
transient osteoporosis or diffuse osteopenia.
- Abundant red marrow: there are several clinical scenarios where red marrow is abundant (i.e.
pediatric population,
smokers,
red marrow hyperplasia,
secondary to treatment with growth factors,
or in response to anemia or other co-morbidities…),
showing a similar fat-water proportion than normal red marrow.
Distinction from a marrow-replacing process may occasionally be problematic due to its typical low signal intensity on conventional T1-W sequence.
In these cases,
red marrow may be confluent and similar to or slightly lower in signal intensity than adjacent skeletal muscle,
potentially mimicking a tumor.
On CSI,
there will be a signal drop on OP images compared with IP images.
Quantitatively,
normal and reconverted red marrow usually has a SIR much lower than 0.8 (even when abundant),
whereas tumor has a SIR >0.8,
corresponding with a signal drop of <20%.
Fig 6.
- Dense bone marrow edema,
transient osteoporosis and diffuse osteopenia: secondary signal changes in any of these entities have a similar appearance to those of malignant infiltration on T1-W and STIR sequences.
There is a drop of marrow signal intensity on OP images,
as all of them are non-replacing marrow lesions demonstrating both fat and water components,
whereas marrow-replacing lesions,
such as tumor or metastasis,
show no drop of signal on opposed phase images,
because neoplastic lesions replace normal marrow fat,
resulting in a marrow containing only water.
Fig 7.
- Hemangiomas: are slow growing,
benign neoplasms that are commonly found in the vertebral bodies.
Histopathologically,
they consist of thin-walled,
blood-filled vessels and sinuses that are lined by endothelium,
are interspersed among the bone trabeculae,
and have a variable amount of fat.
Classical hemangiomas do not demonstrate a decrease in signal intensity on CSI,
because they have a predominant fatty content,
so there are not many voxels with water and fat in a similar proportion.
These hemangiomas,
however,
are easily characterized at standard T1-W MRI and do not pose a diagnostic dilemma.
Atypical hemangiomas,
which contain only small or microscopic quantities of fat,
may demonstrate the utility of CSI because they will lose signal intensity in out of phase images.
Such lesions may have otherwise been difficult to distinguish from malignant neoplastic lesions on conventional T1- and T2-W sequences.
Fig 8.
But,
radiologist should know that there are some benign entities that may replace bone marrow elements,
as they are:
- Myelofibrosis: is characterized by the replacement of normal marrow by fibrotic tissue,
which can result in anemia and extramedullary hematopoiesis.
It is associated with chemotherapy or radiation therapy for lymphoma,
leukemia,
multiple myeloma,
and metastatic disease.
Occasionally,
it is a primary disorder.
The characteristic findings on conventional MRI are very low T1-W and T2-W signal in the marrow and slightly hyperintense signal relative to muscle on fat-suppressed images.
An infiltrative process such as fibrosis can be sufficiently dense that it may be incorrectly interpreted as a marrow-replacing tumor.
- Storage diseases: such as Gaucher disease,
which is an autosomal recessive disorder with accumulation of glucocerebrosidase within histiocytes because of decreased levels of the enzyme glucocerebrosidase.
The fat of the marrow is replaced by infiltration of Gaucher cells,
which manifests as T1-W and T2-W hypointensity.
In some cases,
the STIR sequences reveal hyperintense inclusions,
which may indicate acute bone crisis,
occult fracture,
infection,
or bone infarction.
Signal drop-out on OP images compared with IP images may result from the accumulation of both lipids and water and is also attributed to the relatively short T2 of glucocerebrosidase.
Quantitative fat fraction measurements obtained through Dixon imaging can be used to quantify fat content and have a close correlation with the clinical occurrence of skeletal involvement.
Thus,
it may serve as a prognostic marker for the disease,
detecting the increase/reduction in the fat fraction of bone marrow as a response of treatment.
Fig 9.
- Osteomyelitis: CSI has potential utility as an adjunct sequence for identifying and characterizing the different stages of evolution of osteomyelitis.
Signal changes by CSI sequences in the setting of osteomyelitis are variable depending on stages of evolution.
In early osteomyelitis,
there is just some edema without fat marrow replacement,
demonstrating a SIR <0.8.
But as the process worsens,
it may result in lack of signal drop between IP and OP images due to the formation of pus or abscess in the medullary canal.
At that moment,
osteomyelitis may act as a marrow-replacing process by dense inflammatory cells and purulent material (SIR >0.8).
Fig 10.
On the other hand,
CSI is also useful for assessment of malignancies,
such as detection of hematopoietic malignancies,
defining intramedullary tumor extent and biopsy targeting of bone tumors.
Furthermore,
it can be performed in whole-body protocols,
with clinical applications in tumor detection,
staging and treatment monitoring of myeloma multiple and metastatic status.
- Primary bone tumors as well as metastatic disease,
manifest as focal marrow-replacing bone signal abnormalities,
and behave similarly on CSI sequences,
with a characteristic lack of signal drop on OP compared with IP images.
Such lesions are detected by spin-echo T1-W imaging or fluid-sensitive sequences,
and CSI is used to confirm the marrow-replacing nature of the signal abnormality,
with a cut-off value of 0.8. Furthermore,
it may help in the therapy monitoring of bone tumors.
Fig 11.
- Interestingly,
the detection of melanoma metastases is also straightforward on Dixon fat-only T2-W images,
with consistent loss of signal intensity,
whereas it represents a classic pitfall on T1-W images because of the shortening of T1 signal,
according to melanin content.
- Defining intramedullary tumor extent: CSI is a potential alternative technique to T1-W imaging for defining the intramedullary extent of a bone tumor in regions with abundant red marrow or perilesional inflammation,
which may also appear as low signal on T1-W images,
similar to tumor.
Using CSI,
tumor extent is determined by areas where there is a lack of signal drop on OP compared with IP sequences.
However,
CSI sequence is not sufficient by itself,
and requires correlation with standard anatomical sequences,
as the latter offer higher signal-to-noise as well as spatial resolution.
Fig 12.
- On the same basis,
it is also possible to determine the best area for biopsy (focused on the area where there is the major lack of signal drop on OP compared with IP sequences).
Fig 13.
- CSI can differentiate a benign from pathologic fracture.
The spine is one of the most common sites for metastasis,
and several predictors for pathological vertebral fractures have been described for various MRI sequences,
and CSI is among the most useful.
A signal drop on OP images compared with IP images within the vertebral body indicates a benign fracture.
The signal drop is greater when the proportions of fat and water are balanced,
such that benign fractures can appear more hypointense on OP than IP images.
Nevertheless,
quantitative assessment is important in these cases,
as marrow healing,
scar formation,
and edema may result in a relative lack of signal drop by visual inspection,
and may cause false-positive diagnoses.
A SIR >0.8 (<20% signal drop) is characteristic of a pathological fracture in either the vertebrae or long bones,
where normal fatty marrow is replaced with tumor.
Fig 14.
-Therapy monitoring is an interesting application of whole body Dixon MRI.
Fast Dixon-based WB MRI is highly specific and sensitive in the detection of bone metastases.
Its use in substitution of conventional T1-W and fluid-sensitive sequences shortens scan time.
The combined used of Dixon sequences and diffusion-weighted imaging enhances detection of metastases and helps to understand the complex changes with treatment within the bone marrow.
In addition,
Dixon sequences add quantitative information of the fat and water levels in red bone marrow during therapy which are useful as biomarkers for treatment monitoring.
This technique can also detect extra skeletal metastases,
such as those involving the liver or lymph node,
in the same imaging session.
Multiorgan system evaluation for extra skeletal metastases may counteract the expense of MRI by obviating the need for additional imaging studies.
Fig 15.
But CSI from Dixon sequence has some pitfalls and limitations,
as when fatty marrow is almost solely present (commonly found in the appendicular skeleton of adults),
a drop in signal on OP compared with IP images in the normal fatty marrow should not be expected,
as the voxels contain only lipids,
without significant amounts of water,
to result in signal changes between the OP and IP sequences Fig 16.
Furthermore,
false-negative results have been described with CSI sequence interpretation due to infiltrating tumors such as multiple myeloma or leukemia,
although this may depend on the degree of abnormal marrow cellularity.
Similarly,
myeloproliferative disorders such as mielofibrosis,
as well as osteomyelitis or some storage diseases as seen before,
may produce a signal drop on the OP compared with IP images due to infiltrating bone marrow elements,
however they are not malignant tumors.
Renal metastases containing sufficient lipid may in theory result in a false-negative interpretation by CSI.