MR imaging uncovers new territory in assessing bone marrow edema

Bone marrow edema produces characteristic alterations in signal (low on T1- and high on T2-weighted MR images), but its pattern of presentation is highly nonspecific, posing a challenge for radiologists. Edema-like bone marrow patterns generally are reflected by ill-defined increased signal changes on fluid-sensitive sequences such as short-tau inversion recovery or fat-suppressed T2-weighted.

Images demonstrate an osteoid osteoma. It is visible on CT with a characteristic nidus (arrowhead), while on MRI only an uncharacteristic bone marrow edema pattern is visible (arrow). Additional bone scintigraphy may be also used when osteoid osteoma is suspected or when a multilocalized condition (e.g., metastatic disease) is suspected. (Provided by M. Zanetti)

The presence of bone marrow edema changes may not even be clinically relevant. Signs are often adjacent to obvious degenerative disc disease, but even incidental findings should drive further investigation if they suggest underlying infection, inflammation, or fracture.

"When bone marrow edema patterns are encountered, causes like bone contusions and infections have to be ruled out with clinical information. When such conditions are absent, mechanical stress reactions (initial stages of stress fractures) and initial stages of bone marrow necrosis are most commonly considered," said Dr. Marco Zanetti of Uniklinik Balgrist in Zurich, Switzerland, at a special focus session.

Bone mineral density changes that are part of the spectrum of abnormalities associated with a suspected disease in the vertebrae should be classified by their distribution, shape, and relationship to other local or distant abnormalities, said Prof. Frederic Lecouvet of Saint-Luc University Hospital in Brussels.

Sagittal T1-weighted (A) image of thoracic spine shows acute compression fracture of vertebral body (arrow). Posterior bulging of posterior vertebral wall (arrowhead) is suggestive of malignant origin. Coronal T1-weighted image (B) of pelvis shows evident additional foci, which confirms multifocal metastatic disease (arrowheads). (Provided by F. Lecouvet)

"The major features that will be helpful for this categorization are the distribution of the signal intensity changes within the vertebrae; their extension; their association with alterations of adjacent disc, vertebral endplates, or soft tissues; and the presence of similar changes of other more suggestive abnormalities, such as tumor foci, at other levels," he said.

Bone marrow edema associated with a degenerative disc, for example, has a bandlike appearance next to the endplate and occurs predominantly near the anterior corners of the vertebrae on both sides of the disc space. On the other hand, edema related to traumatic or osteoporotic vertebral factures is linked with vertebral contour deformities and a normal marrow signal distant from the compressed endplate.

In infection, the bone marrow edema pattern is present on both aspects of the disc space and in combination with interruption of the vertebral endplate, loss of height, or an abnormal signal from the disc as well as nearby paraspinal or epidural soft-tissue alterations.

Distinguishing among necrosis, fracture, and edema is especially difficult, Zanetti said. In one case, a 64-year-old woman with a painful hip had bone marrow edema-like changes in the femoral head and neck on coronal T1-weighted on the left side and fat-suppressed T2-weight images on the right side. The images could be classified by radiologists as signs of the initial phase of femoral head necrosis, based on the circumscribed low signal intensity on the T1-weighted image, or as insufficiency fracture, based on a subchondral linear abnormality in the femoral head.

Sagittal T1-weighted (A) and T2-weighted (B) images reveal bone marrow edema located at anterior vertebral angles, adjacent to T11-T12 disc space, highly suspect for inflammatory disease (arrows). Oblique coronal T1-weighted (C) and T2-weighted (D) images show evident bone marrow edema adjacent to both sacroiliac joints, making diagnosis of ankylosing spondylitis certain (arrowheads). (Provided by F. Lecouvet)

Bone marrow edema patterns typically are investigated with fluid-sensitive MRI techniques as well as additional T1-weighted sequences to look for linear signal abnormalities that indicate the presence of an initial stress fracture, he said.

The MR examination may be adapted to increase its sensitivity by incorporating gradient-echo sequences or by injecting contrast material. This approach can reveal necrotic areas within the disc or soft-tissue infection, detect subtle tumor foci in disseminated metastatic disease, distinguish true bone marrow edema from foci of normal hyperplastic bone marrow, and find abnormalities in the posterior elements of traumatic spinal fractures, according to Lecouvet.

Although MRI is the procedure of choice for diagnosing the cause of bone marrow edema, other imaging modalities are sometimes useful. When the differential diagnosis of degenerative disc disease and infectious spondylodiscitis is difficult to make on MRI, x-ray and CT may reveal erosive and osteosclerotic changes that are typical for inflammatory disease in the thoracolumbar junction and sacroiliac joints.

Prof. Frederic Lecouvet.

CT can clearly identify an osteoid osteoma with a distinctive nidus when MRI shows only an uncharacteristic bone marrow edema pattern, Zanetti said.

CT also may help distinguish between benign osteoporotic and malignant vertebral fractures when patterns of bone marrow edema within the vertebrae are diffuse and confusing. Both x-ray and CT recognize benign conditions, such as Paget's disease of the bone, acute intravertebral disc herniation, and angiomas, that are nonspecific on MRI, Lecouvet said.