In the thorax, the most common imaging manifestations are paraspinal masses and rib expansion [3, 4], and these findings are more frequent in β-thalassemia than in other causes of extramedullary hemopoiesis. The paraspinal active hemopoietic masses are well marginated and show mild homogeneous enhancement on contrast-enhanced CT, whereas old, burnt-out lesions may show iron deposition or fatty degeneration [4]. Active hemopoietic masses are serendipitously found overlying the sympathetic chain (Fig. 2A,2B,2C), a location that is also favored by paraspinal neurogenic tumors seen in neurofibromatosis type I; however, neurofibromatosis type I can be easily excluded on the basis of clinical history. Rib or diploic space expansion is not uncommon, especially in thalassemia, and results from the contiguous expansion of the intramedullary marrow (Fig. 3A,3B). Rarely, hemopoietic elements can involve the precardiac and pleural spaces, but patients are usually asymptomatic. Cases of extramedullary hemopoiesis involving the pulmonary interstitium and mimicking an inflammatory or neoplastic diffuse interstitial process have been reported and have occasionally resulted in cardiopulmonary insufficiency [5]. Indirect findings of failing bone marrow or an increased demand for blood elements include cardiomegaly and eventual high-output cardiac failure, a not rare complication in patients with thalassemia. Paraspinal hemopoietic tissues can extend into the central canal, especially in the thorax, and cause neurologic symptoms because of spinal cord compression. MR imaging findings, although nonspecific, can suggest the diagnosis. Suggesting this diagnosis is important because treatment with steroids and external beam radiation is effective and usually long-lasting [6].

Abdominal involvement usually recapitulates fetal development, with the most commonly involved organs being the liver and spleen. The classic imaging finding is that of hepatosplenomegaly [1] (Figs. 4A,4B and 5). Involvement of these organs is usually diffuse, but, as illustrated in Figures 6A,6B and 7A,7B, masslike foci of hemopoiesis can be seen [7, 8] that may be confused with a neoplastic process. Despite the sometimes massive enlargement of these organs, their function is rarely affected. Patients usually present with findings of organomegaly or with vague abdominal symptoms on physical examination. Patients who require multiple transfusions (i.e., those with thalassemia) can develop hemosiderosis or hemochromatosis. As a result, on CT the parenchymal density of the liver (Fig. 6B) and spleen is increased, whereas on both T1- and T2-weighted MR images signal is lost because of the deposition of iron. Extramedullary hemopoiesis can also, rarely, involve the kidneys. Parapelvic hemopoietic masses can be seen because this location is active during in utero hemopoiesis. However, occasionally extramedullary hemopoiesis manifests as uniform, enhancing perinephric masses that appear to engulf the kidneys without however distorting their shape (Fig. 8A,8B). This appearance may be confused with bilateral renal lymphoma, and biopsy is necessary to establish the diagnosis. Masses of hemopoietic elements can involve the mesentery (Fig. 9A,9B), presenting as nonspecific lesions that can be mistaken for lymphadenopathy or metastatic disease. Pelvic hemopoietic masses with predilection for the presacral region are rare but, in the proper context, should be included in the differential diagnosis along with other presacral lesions (Fig. 10), including chordoma.