Radiologists, imaging scientists and clinical providers interested in understanding the MR imaging appearance of bone marrow, osteonecrosis, and features that may help distinguish among bone marrow edema etiologies.

At the conclusion of the talk, the participant should be able to:

· Recognize the normal appearance and maturation of bone marrow on MR imaging

· Identify MR imaging appearance of bone marrow edema

· Describe several imaging characteristics that can help distinguish among causes of bone marrow edema

To review the expected MR appearance of bone marrow and features of bone marrow edema that will help to differentiate among etiologies.

o Bone marrow signal on MRI is based on fat and water content

· Red (cellular) marrow has low T1 signal

· Yellow (fatty) marrow has high T1 signal

o Bone marrow maturation occurs in a recognizable pattern

· Fatty replacement of red marrow as we age

· Occurs distal to proximal

· Axial and proximal appendicular skeleton maintain red marrow into adulthood

o Bone marrow edema

· Intermediate to high signal on fluid sensitive sequences

o Osteonecrosis

· Recognizable by “double line” sign on fluid sensitive sequences

Bone marrow is the fourth largest organ in the body and contains the hematopoietic elements, stroma, and supporting trabeculae providing both structural and hematologic support for the body. MR imaging appearance of bone marrow is primarily dependent on fat and water signal, which differs between the red marrow, containing the hematopoietic elements, and white marrow, containing mostly fat. Specifically, red marrow constitutes 40% fat and 40% water, while yellow marrow contains 80% fat [1]. This results in ample contrast on MR imaging, especially on T1 where fat signal has a short T1 relaxation resulting in bright signal on T1 imaging, whereas the red marrow signal has increased water content due to greater cellularity that results in intermediate signal on T1, which is generally higher than muscle signal, since it still contains 40% fat [1]. To evaluate marrow on MR imaging, one must recognize the normal expected appearance of marrow, which changes as we age and is dependent on signal contribution from red and yellow marrow as discussed above. When humans are born, the entire skeleton is filled with red marrow. As we age, much of the red marrow converts to yellow marrow and this occurs in a characteristic pattern [1, 2]. Generally, this follows a distal to proximal or appendicular to axial pattern though timing may differ among individuals. For example, the hands and feet are the first body parts to convert to yellow marrow occurring by age 10 followed by the tibia/fibula and radius/ulna. By the age of 25, most individuals are considered skeletally mature with persistent red marrow in the axial skeleton, which includes the head and spine, sternum, ribs, and proximal humeri and femurs [1, 2].Pathologic states can be identified generally by a deviation from the normal maturation process of marrow or focal bone marrow edema. Deviation from the normal bone marrow maturation is best identified on T1 MR imaging because of the high content of fat in bone marrow, especially in areas of yellow marrow. Generally, increased cellular marrow will have lower signal intensity on T1 imaging and decreased cellular marrow will produce much higher T1 signal (more fat). A distinction that can help in identifying the etiology is whether the pathologic process replaces the fat or infiltrates the bone marrow. Bone marrow replacing processes are typically neoplastic or infectious in etiology [1-11]. On conventional imaging sequences, these processes result in low T1 signal, lower than that of muscle or vertebral disc, and intermediate to high signal on fluid sensitive sequences, such as short inversion tau recovery (STIR), T2 imaging with fat suppression (T2fs), or intermediate weighted proton density with fat suppression (PDfs). More infiltrative processes that do not replace all fat and trabeculae in the marrow tend to have intermediate T1 signal with retained trabeculae and intermixed fat. Many processes can result in this pattern, including hematologic malignancy, red marrow reconversion, transient bone marrow edema, transient osteoporosis, contusion or fracture, osteoarthritis, and inflammatory arthropathy [3-6, 9, 10, 12-17]. Depending on the pattern of edema or characteristic features, the etiology may be apparent with MR imaging [9, 13, 17-20]. For example, the “double line” sign is a classic imaging finding with osteonecrosis and a linear focus of low T1 signal with adjacent edema in a subchondral location is characteristic of a subchondral fracture. In addition to bone marrow replacing and infiltrative processes, some processes may make the bone marrow less cellular and result in increased marrow fat content on imaging. For instance, poor nutrition, osteoporosis, or radiation treatment can result in less cellularity in the marrow.For problem solving, chemical shift imaging (CSI) or diffusion weighted imaging (DWI) may be helpful to distinguish some etiologies [4, 7, 11, 21, 22]. With CSI, a bone marrow lesion that loses greater than 20% of its signal from in-phase images to out-of-phase images is a benign process. Using DWI, a lesion that has diffusion restriction is either neoplastic or infectious.