Juvenile or solitary bone cysts are tumorlike lesions of unclear etiology (Table 10.1). They are sharply demarcated and contain fluid, making them appear bright on T2-weighted images (Fig. 10.2). On T1-weighted images, these cysts have intermediate to low signal intensity. Complications such as pathologic fracture or intralesional hemorrhage may result in an inhomogeneous appearance. Fluid levels or septa may also be present. Thinning of surrounding bone is common. A contrast-enhanced examination is only necessary in cases where a solid lesion cannot be ruled out with certainty. Simple bone cysts typically do not enhance (Lokiec and Wientroub 1998).

Hemangiomas are benign tumors composed of small blood vessels that are commonly found in the vertebral bodies (Table 10.2) but also occur in the appendicular skeleton (Rigopoulou and Saifuddin 2012). The vascular components have high T1 and T2 signal intensity, while thickened trabeculae have low signal intensity (Fig. 10.3). The thickened trabeculae are typically oriented vertically, resulting in a corduroy appearance on sagittal images and a polka dot pattern on axial images (Friedman 1996). A fat-suppressed pulse sequence allows differentiation of intraosseous hemangioma from lipoma, which has the same signal intensity as subcutaneous fat (Fig. 10.4) (Williams et al. 1993; Blacksin et al. 1995). Following IV injection of contrast medium, the vascularized components of a hemangioma enhance strongly, while a lipoma does not enhance at all (Williams et al. 1993; Blacksin et al. 1995).

Osteochondroma is a sessile or pedunculated bone tumor composed of normal cortical and medullary bone (Table 10.3). It is typically contiguous with the cortex and medullary cavity of the underlying parent bone (Fig. 10.5a, b), and its bony portions have the same signal intensity as the parent bone (Fig. 10.5c, d) (Greenspan 1989; Unni 2001). Osteochondromas have an overlying hyaline cartilage cap approx. 1 – 3 mm in thickness. The cap has the characteristic MR signal intensity of well-differentiated cartilage tumors, i.e., intermediate signal intensity on T1-weighted images and very high signal intensity on T2-weighted images (Fig. 10.5d). The cap is often surrounded by a delicate band of low signal intensity, which corresponds to perichondrium (Mehta et al. 1998).

Enchondroma is a well-defined osteolytic lesion with a chondroid matrix. It typically contains calcifications arranged in an arc-and-ring pattern, giving rise to the characteristic popcorn appearance on conventional radiographs (Greenspan 1989; Unni 2001) (Table 10.4). On MRI, an enchondroma appears as a sharply demarcated, lobulated lesion of low to intermediate T1 and high T2 signal intensity (Fig. 10.7a). Calcifications within the matrix may appear as signal voids on all pulse sequences. On fat-suppressed T1-weighted images acquired after contrast medium administration, enhancement is predominantly peripheral and occurs in arcs or rings (Fig. 10.7d). This pattern may be obscured by the high signal intensity of hyaline cartilage on these images (Aoki et al. 1991; Geirnaerdt et al. 1993).

Fibrous dysplasia is a benign chronic disorder of bone development in which the normal medullary space is replaced by fibrous tissue, causing expansion and distortion of the bone. Monostotic fibrous dysplasia involves a single long bone and is far more common than the polyostotic form (85 vs. 15 %). The latter typically affects the skull and facial bones (Ippolito et al. 2003) (Table 10.5). On MRI, fibrous dysplasia is visualized as an expansive mass in the medullary space. The mass has homogeneous low signal intensity on T1-weighted images (Fig. 10.8b) and intermediate to high signal intensity on T2-weighted images (Fig. 10.8a). It is surrounded by a low-signal-intensity rim. Active portions show strong homogeneous enhancement after IV contrast medium administration (Jee et al. 1996).

Osteomas are circumscribed sclerotic lesions that resemble mature cortical bone and arise juxtacortically (Table 10.6). The lesions have low signal intensity on T1- and T2-weighted images (Fig. 10.9a–c) and do not enhance after IV administration of contrast medium (Greenspan 1993).

An osteoid osteoma consists of a highly vascularized nidus, which is typically found in the cortex. Medullary or subperiosteal osteoid osteoma is less common. The nidus is less than 1.5 cm in size. A larger variant, known as osteoblastoma, commonly involves the spine. The lesion is rounded or spindle-shaped, and the nidus is surrounded by sclerotic bone (Greenspan 1993) (Table 10.7).

The nidus is isointense to muscle on T1-weighted images with intermediate to high signal intensity on T2-weighted images (Fig. 10.10a, b). It enhances strongly after IV contrast medium administration. The sclerotic rim is dark on all pulse sequences. Perifocal bone marrow and soft tissue edema may be present and is best seen on fat-suppressed T2-weighted or STIR images or on contrast-enhanced T1-weighted images (Assoun et al. 1994).

Conversion of red-to-yellow marrow refers to the shift in bone marrow composition from highly cellular, hematopoietically active marrow (red) to inactive fatty marrow (yellow). This conversion begins in the distal phalanges of the hands and feet and then progresses proximally. Conversion is best appreciated on unenhanced T1-weighted images. In the long bones, fatty degeneration starts in the apo- and epiphyses, progressing first to the diaphyses and then to the metaphyses. In the vertebral bodies, fatty degeneration begins near the central venous plexus and slowly progresses into the periphery. In adults, hematopoietic marrow is still present in the proximal metaphyses of the humerus and femur, the pelvis, the vertebrae, the ribs and sternum, the scapula, the calcaneus, and flat bones of the skull. With age, the physiologic process of red-to-yellow marrow conversion slowly leads to a progressive increase in the proportion of fatty marrow. The adult pattern is characterized by a balanced distribution of red and yellow marrow but varies from person to person and is affected by a variety of factors (Vogler and Murphy 1988; Ricci et al. 1990).

Reconversion is the repopulation of yellow marrow by hematopoietic cells. This process follows the pattern of red-to-yellow marrow conversion in reverse, meaning that it progresses from the center toward the periphery. In the long bones, reconversion begins symmetrically, progressing from the metaphyses into the diaphyses. From here it may even continue into the epiphyses if the demand for red cells is strong enough. Bone marrow reconversion is triggered by conditions associated with an increased demand for red cells. This includes anemia or blood loss, competitive sports, time spent at high altitudes, heavy smoking, cyanotic heart disease, and chemotherapy. Reconverted bone marrow may have very high signal intensity on STIR images (Fig. 10.11a) and very low signal intensity on T1-weighted images (Fig.10.11b), making it difficult to differentiate reconversion from diffuse infiltrative neoplastic conditions.

STIR sequences are very useful for detecting bone marrow pathology (Mehta et al. 1995; Mahnken et al. 2005; Schmidt et al. 2007). On STIR images, normal yellow marrow appears black, while hematopoietic marrow and lesions are of high signal intensity. STIR sequences have high sensitivity but poor specificity. Bone marrow pathology may be associated with decreased or increased cellularity (Fig. 10.12). When cellularity is decreased, yellow marrow predominates, making for a dark appearance on STIR images and a bright appearance on T1-weighted images. Conversely, pathology associated with increased cellularity is bright on STIR images and dark on T1-weighted images. A notable exception is storage disorders with excessive deposition of iron or calcium. These deposits produce a loss of signal intensity on both STIR and T1-weighted images. Two conditions associated with malignant cell infiltration of the bone marrow, metastasis and multiple myeloma, can commonly be detected incidentally on screening MRI (Delorme and Baur-Melnyk 2011; Kwee et al. 2011; Collins 2010). The other bone marrow pathologies listed in Fig. 10.12 are encountered less frequently.

The bone marrow is the third most common site of metastatic disease. At autopsy, bone metastases have been found in 30–70 % of patients with a malignant tumor (Papac 1994). The skeleton may be affected by single or multiple osteolytic or osteoblastic metastases of variable size. Bone metastases are common in patients with breast, lung, prostate, thyroid, and renal cancer. Normally, malignant cells spread hematogenously to sites of residual hematopoietic bone marrow in the spine, ribs, pelvis, skull, humerus, and femur. Metastatic growth destroys the cortical layer, resulting in invasion of soft tissues. Vertebral metastases tend to destroy the pedicles or endplates. Some primary malignancies, such as cancer of the prostate or urinary bladder (Fig. 10.13), produce factors that stimulate bone-forming cells. These give rise to osteoblastic metastases, which are characterized by the formation of new bone. Other tumors, including renal cell cancer or thyroid cancer, secrete factors, such as TGF or PDGF, that stimulate osteoclast activity, which leads to osteolytic destruction of the bone matrix. In breast cancer, both osteolytic and osteoblastic bone metastases can occur (Fig. 10.14). The MR signal intensities of osteoblastic and osteolytic metastases on different pulse sequences are summarized in Table 10.8. With its high sensitivity and diagnostic accuracy, the coronal STIR sequence appears to become the first-line tool to search for bone metastases – preferred over other imaging modalities such as skeletal scintigraphy or PET-CT (Mehta et al. 1995; Mahnken et al. 2005; Schmidt et al. 2007; Papac 1994; Chiewvit et al. 2009).

Multiple myeloma, or plasmacytoma, is the most common primary malignant bone tumor, occurring predominantly in men between 40 and 80 years of age. It is a hematologic disorder characterized by an uncontrolled proliferation of abnormal plasma cells. The neoplastic plasma cells invade the bone marrow, and by secreting nonfunctional monoclonal immunoglobulins, they suppress benign polyclonal plasma cells. The simultaneous release of osteoclast-stimulating factors and inhibition of osteoblast activity result in osteolytic demineralization and destruction of normal bone. Bone involvement may be diffuse, or multiple sharply demarcated osteolytic lesions of variable size may be present. Multiple myeloma affects bones containing red marrow, most commonly the spine, ribs, pelvis, skull, humerus, and femur. Unlike vertebral metastases, myeloma lesions generally only involve the posterior elements of the vertebral bodies in advanced disease. On MRI, the osteolytic lesions have high signal intensity on STIR and T2-weighted images (Fig. 10.15b, c) and intermediate to low signal intensity on T1-weighted images (Fig. 10.15a) (Baur-Melnyk et al. 2005; Delorme and Baur-Melnyk 2011).