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MR TOPICS
Magnetic Resonance Imaging of Bone Marrow1
Bone marrow is the fourth largest organ in the body by weight, following bone, muscle, and fat. The cancellous bone matrix of primary and bridging secondary trabeculae constitutes the osseous component of marrow and provides architectural support for the hematopoietic, osteoid, and fatty marrow elements.
Progressive conversion of hematopoietic marrow to fatty marrow is seen with age. The conversion process continues until the adult pattern is reached by the age of 25 years.
Marrow Edema
Marrow edema is a generalized response to either trauma or stress of the bone marrow. Bone marrow edema may be the first recognizable MR finding in many conditions, including transient synovitis, trauma, stress, infection, ischemia, tumor and reflex sympathetic dystrophy. MR imaging is exquisitely sensitive for detecting the presence of marrow edema.
Hematopoietic Malignancies
Leukemia
The changes seen in the marrow secondary to leukemia are due to malignant proliferation of cells originally native to bone marrow tissue rather than malignant proliferation of cells transported from other sites.
Bone Marrow Metastases
The vascular nature of red marrow accounts for the high frequency of bone metastases. Because metastases grow first in the marrow and then spread to the trabecular and cortical bone, lesions can often be detected on MR images before appearance of significant bone loss on a plain radiograph, or even a nuclear bone scan.
Researchers at the University of Iowa, under Dr. William Yuh, found MRI can determine if cancer has spread to bone faster than bone scintigraphy. The study found that bone scans detected more than half of the large lesions but only 3.4% of the small ones, which had affected the cortical bone and which were identified by MRI2.
Disorders of Hemoglobin and Hematopoiesis
Chronic Anemia
Marrow abnormalities on MR images in patients with chronic (inherited) anemia are common. Marrow changes can be due to:
1. The patient's underlying anemia,
2. The patient's recurrent marrow ischemia,
3. Hemosiderin deposition from recurrent transfusions.
The diffuse marrow hyperplasic changes seen in patients with chronic anemia (sickle cell anemia, thalassemia, spherocytosis) relate to the degree of underlying anemia and are more pronounced in older patients who have had the disease for the longest period and in patients with the most severe anemia.
Before MR imaging, bone marrow infarction was diagnosed with the use of conventional radiography and isotope bone marrow scanning. Conventional radiographs show no acute changes and areas of marrow ischemia are present only on follow-up examination. Scintigraphic studies may not allow differentiation of acute marrow infarction from chronic marrow infarction. On MR, acute marrow images infarcts are often seen as focal marrow abnormalities.
Marrow Depletion States
Aplastic anemia is characterized by acellularity or marked hypocellularity of the bone marrow. Drugs, viral infections, toxins and hepatitis have all been associated with the development of aplastic anemia, but the disorder is often idiopathic. Pathologically, the marrow is hypocellular, with primarily fatty marrow and areas of fibrosis.
On MR images, untreated aplastic anemia displays increased signal which represents the diffuse replacement of hematopoietic marrow by fatty marrow.
Myelofibrosis is characterized by progressive splenomegaly and a diffuse stromal marrow reaction, with replacement of the normal medullary fat and marrow elements of fibrosis. Myelofibrosis can be the result of myeloproliferative syndromes, as well as a sequela of treatment for leukemia, lymphoma, Gaucher's disease, metastatic carcinoma or infection.
Gaucher's Disease
MR changes can be seen in lipid storage disease specifically Gaucher's disease. Marrow involvement generally follows the distribution of red marrow and appears to occur first in regions of the marrow normally occupied by hematopoietic marrow (spine, pelvis, proximal femoral metaphysis) and then progresses in a proximal-to-distal manner in the appendicular skeleton. Vertebral marrow demonstrates uniform and widespread abnormality at all levels with replacement of the normal fatty marrow by infiltrated marrow.