In an adult, hypointense marrow foci are a common finding, and they are often due to rests of hematopoietic marrow. The signal intensity of such red marrow foci is higher than that of adjacent muscles or nondegenerated intervertebral discs, although, occasionally, it may be equal to them, particularly in young adults (Fig. 4.8). Compared to malignant bone marrow lesions, foci of red marrow have less distinct borders. A focus of fat in the center of a hypointense focal bone marrow lesion (bull’s eye sign) is characteristic of a red marrow rest as it reflects the centrifugal pattern of red to yellow marrow conversion, at cross section [7].

An atypical hemangioma is a diagnosis applied to an incidental, solitary, hypointense focal marrow lesion, when all other diagnoses have been excluded and the patient has no known history of malignancy (Fig. 4.9). Other benign primary bone tumors, such as osteoblastoma, aneurysmal bone cyst, and enchondroma among others, may present as hypointense foci. These tumors, as well as most primary malignant bone tumors, usually have characteristic findings on conventional radiographs, CT, and MRI and rarely present as a diagnostic dilemma.

Bone metastases and hematologic malignancies are the most common malignant causes of focal hypointense patterns; such lesions are usually well-defined, with a sharp transition zone to adjacent uninvolved marrow, and they are hypointense to disc and muscle. They occur more often in older individuals, in whom fatty marrow is abundant in the central skeleton, and thus are easily depicted as “holes in the marrow” (Figs. 4.10 and 4.11).

The signal intensity of signal-void focal lesions is lower than that of hypointense focal bone marrow lesions, similar to that of cortical bone, and it may be produced by osteosclerotic processes or air. Signal-void focal lesions have well-defined borders, and they remain of low signal intensity on all MR sequences due to the absence of mobile protons.

Enostosis (bone island) is a common cause for a benign signal-void focal lesion (Fig. 4.12). Rare causes of signal-void focal lesions include mastocytosis and osteopoikilosis. Intravertebral gas is specifi c for osteonecrosis complicating a benign vertebral compression fracture and manifests as a signal-void lesion [8] (Fig. 4.13).

Malignant focal lesions with signal void include osteoblastic metastases, osteoblastic lymphomatous focal lesions, and rare sclerotic myelomatous lesions more often seen in association with POEMS syndrome (Fig. 4.14). A high-intensity halo on T2-weighted images (T2 halo sign) has been described in patients with prostate cancer as a helpful sign in differentiating osteoblastic metastases from benign bone islands [7] (Fig. 4.15).