Amyloidosis is characterized by deposition of amyloid protein, which is a glycoprotein not normally present in that tissue or organ. Systemic amyloidosis may occur on its own as a primary event or secondary to chronic inflammation, tuberculosis, multiple myeloma, rheumatoid arthritis, or connective tissue disease. Focal involvement or localized amyloidosis includes gingival and tongue deposition leading to functional disability. In addition, beta-2-microglobulin amyloidosis can occur in patients undergoing long-term hemodialysis. The diagnostic histological test shows an amorphous eosinophilic appearance on hematoxylin and eosin stain, while Congo red and polarized light (green) can be used for confirmation. Amyloidosis can involve various regions such as the shoulder, hip, and knee joints and may cause back pain with limitation of spinal motion. Deposition of amyloid in the soft tissue can cause compression symptoms such as carpal tunnel syndrome.

Imaging findings of amyloidosis include soft tissue mass from deposition in the bone with well-marginated bony erosions, subchondral bone cysts, or decreased bone density and deposition in the synovial membrane leading to widening of the joint space, joint contracture, and atrophy of muscle.

Joint changes may be rather similar to the findings of rheumatoid arthritis (Katoh et al. 2008), but amyloid arthropathy can be distinguished by the presence of a large soft tissue mass, the lack of joint space narrowing, and the presence of well-marginated subchondral bone cysts and erosions. A differential diagnosis includes gouty arthritis, xanthomatosis, and pigmented villonodular synovitis. Deposition of amyloid around the blood vessels can cause osteonecrosis of the epiphyseal regions. Amyloid deposition on MRI is seen as soft tissue with signal intensity between that of muscle and fibrocartilage (Fukusa and Yamamoto 2001) with additional features such as thickening of the synovium, paratendinous deposition, joint effusion, and bone erosion (Fig. 35.7).

Leukemia is a malignancy of myelogenous or lymphocytic white cells which can occur in both acute and chronic forms. Leukemia is one of the most common cancers of children; 75 % of leukemias are acute lymphoblastic leukemia (ALL), 15–20 % are acute myeloid leukemia (AML), and 5 % are chronic myeloid leukemia (CML). Clinical manifestations include focal or diffuse bone pain or joint pain, joint effusion, fever, anemia, and recurrent infection. Radiographic features are normal or decreased bone density with a permeative pattern or vertebral compression fracture secondary to leukemic infiltration (Fig. 35.8). A transverse radiolucent line through the metaphyses termed a leukemic line can be found around the knee joint, the ends of the humerus, or even in the vertebral body. Leukemic line is due to impaired bone formation rather than direct infiltration of leukemia cells and can be also be found in children under 2 years of age with severe malnutrition or systemic disease. These transverse lines transform into sclerotic lines after treatment. In some cases, there are multiple well-marginated lytic lesions and punched-out lesions, but osteoblastic lesions are extremely rare. Occasionally a laminar or sunburst type of periosteal reaction can be present. MRI (Olson et al. 1986) is more sensitive at detecting bone marrow lesions of leukemia patients. Most patients have a diffuse bone marrow infiltration pattern on MRI (Fig. 35.9), although various heterogeneous patterns may be seen depending on the degree of marrow cellularity. Histological examination is necessary to confirm the diagnosis and to subtype the leukemic process, while repeated bone marrow biopsy is needed to evaluate treatment response. The role of MRI of the bone marrow has been studied as an alternative to invasive bone biopsy. An increased fat ratio is indicative of a good treatment response (Vande Berg et al. 1996). However, elimination of leukemic cells and regeneration of bone marrow occur independently, and changes in T1 relaxation time are not a sensitive indicator of treatment response. Dynamic contrast-enhanced MRI is expected to provide practical information on treatment response since the time and degree of contrast enhancement vary with the cellularity and perfusion of the bone marrow.