Bone marrow hyperplasia

This condition is an incidental finding on MR imaging of the knee, with a frequency varying from 0.7% to 35% (1-3). Generally the adjacent epiphysis does not contain haematopoietic tissue and this feature allows the differential diagnosis between benign bone marrow hyperplasia and the same condition of hyperplasia secondary to bone marrow disease.

Bone marrow hyperplasia is more frequent in females (1,2), the obese (3), smokers and long distance runners. In smokers bone marrow hyperplasia is probably caused by a chronic tissue hypoxia due to increased haemoglobin carboxylation, while in long distance runners it is due to chronic anaemia caused by mechanical haemolysis (4), although bone marrow hyperplasia is a more frequent finding than anaemia in this group of patients. Bone marrow can also induce some modifications of the signal intensity which are not related to a real anatomical modifications, but are due to artifacts (5). The chemical shift artefact is generated by different speed in the rotatory precession movement of protons in water and those in fat. The difference is 2.4 MHz in a 0.5 T magnet, and 7 MHz at 1,5 T. At the interface of tissues containg fat and water this difference in precession speed cause an apparent displacement of lipid protons in relation to those of water. Another artifact which may be present in MR images of bone is the magnetic susceptibility artifact (see images 1a and 1b). This is caused by inhomogeneities produced by small magnetic fields (generated by fixed dipoles of osseous tissue) present at the surface of bony trabeculae. These inhomogeneities are essentially visible in gradient images, especially in high magnetic fields (> 0.5 T).