Diagnosis

Extramedullary hematopoiesis. Conventional spin echo MRI demonstrated the masses and the marrow of the adjacent vertebral body to have the same signal characteristics on all pulse sequences. This indicates they are composed of identical material. Conventional spin echo imaging (as opposed to fast spin echo imaging) was used to better demonstrate the signal of the fatty elements in the marrow and the masses.

Discussion

Patients do not have significant extramedullary hematopoiesis at birth. However, when stimulated, such as during severe hemolytic anemia, Hodgkin's disease, or bone marrow failure (from metastatic disease, poisoning, myeloid metaplasia, irradiation and infection), mesenchymal cells resume active hematopoiesis.¹ The most common sites are in the liver, spleen, and lymph nodes.² Most often these present radiographically as asymptomatic masses. Nuclear medicine bone marrow imaging studies may suggest this diagnosis, but MRI, due to its multiplanar capability, variable pulse sequences, and the use of contrast agents, is much better at characterizing the masses and differentiating among the radiographic possibilities. MRI can show enhancement and possible neural foraminal extension in neural tumors, the non-enhancing characteristics of cysts, the adjacent inflammation related to abscesses, the unique signal characteristics of hematomas, and the communication with the dural sac in pseudomeningoceles. In this case, the history, and slight heterogeneity of the masses (suggesting the presence of fat) on CT suggested the diagnosis. MRI narrowed the differential diagnosis to EMH and diffuse involvement of bone marrow and paraspinal lymph nodes by metastatic disease. The lack of a known primary tumor, the lack of brightening on the T2-weighted images, and the suggestive clinical history lead to the diagnosis of EMH. As expected, follow-up at 5.5 months demonstrated no interval change. MRI is helpful in confirming this diagnosis and avoiding the possible complications during biopsy of these vascular masses.

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REFERENCES

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