Modalities

The initial assessment of a bone lesion begins with plain film radiography using specific views to evaluate particular problems. A certain amount of image detail is necessary to see and diagnose the presence of a fracture, and therefore different screen-film combinations are used depending on the size and the depth of the bone to be examined. For small bones and in young children the best combination is single emulsion films (mammography screens ) with a resolution of more than 10 line pairs/mm. Digital radiography has been used to replace conventional film technique (Chapter 5), but it has a limited spatial resolution of 3 to

a	Figure 1. The significance of bone scintigraphy. a) Anteroposterior (AP) radiograph of a 36-year-old man with pain of three week's duration in the knee. In the absence of history of a previous fibular fracture, the cortical thickening of the fibula was considered significant. b) Bone scintigraphy shows increased uptake at the site of the head of the fibula (arrow) with a normal appearance at the site of the cortical thickening (the patient later recalled a previous ankle injury). (B = accumulation of isotope in the urinary bladder) c and d) Transaxial T1-weighted and T2-weighted (d) MR images of the proximal portion of the tibia and the fibula. The region of low (e) and high (d) signal intensity represents a stress fracture (arrows).
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a	Figure 2. Comparison between CT and MRI in osteosarcoma of the distal portion of the femur. a) Transaxial CT scan through the distal end of the right and left thigh. A large expanding lesion, representing an osteosarcoma of the femoral metaphysis, displaces the medial vastus of the quadriceps muscle (Vm) and contains fragments of bone with high density. Edema, of low density, surrounds the femur and the expanding lesion (arrows). b) T2-weighted transaxial MRI section at the same level as in a). The bone fragments within the soft tissue lesion that are shown in (a) are not visualized with MRI. The tumor within the bone marrow is seen clearly. The surrounding edema (arrows) has a high signal intensity on this T2-weighted image.
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5 lines/mm. Moreover, its diagnostic accuracy in the assessment of subtle fractures of small bones as well as bone quality still is uncertain.
Conventional tomography (Chapter 4) is used to evaluate structures or objects obscured by overlapping shadows on the plain radiograph and commonly is employed for routine radiography of the sacroiliac joint, the sternum, and the adjacent joints. It also is used to identify fragments of comminuted fractures.

Bone scintigraphy using Technetium-99m-methylene diphosphonate plays an important role in the identification and localization of bone lesions, particularly when the entire skeleton needs to be surveyed. Abnormal osseous uptake of the bone-seeking isotopes occurs as a result of increased osteoblast activity; therefore, certain osteolytic processes (e.g., osteolytic metastases and plasma cell myelomas) may not be detected. A normal bone scan generally excludes a serious condition when a sclerotic lesion is present on plain films (Fig. 1).

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Figure 3. Comparison of T1- and T2-weighted images of hip joint effusion. a) T1-weighted coronal section of the hip of a 12-year-old boy with osteogenesis imperfecta 3 days after trauma to the hip joint. A fracture of the acetabulum is present. The intracapsular hemorrhage has low signal intensity (arrowheads). b) T2-weighted MR image. The intracapsular effusion (arrowheads) has high signal intensity. f = fat.

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Computed tomography (CT) provides excellent visualization in the transverse, or transaxial, plane. Conventional tomography has, to a large extent, been replaced by CT. CT commonly is used as a routine supplement to plain film examination of injuries and lesions of the spine, pelvis, wrist, ankle, and foot as well as in the assessment of bone and soft tissue tumors (Fig. 2).

The widespread availability of magnetic resonance imaging (MRl) in recent years has had a great impact on musculoskeletal radiology. Although it cannot image cortical bone directly, MRI provides better definition of soft tissue and bone marrow than any other imaging technique currently available. Muscle, fat, fluid, tendons or ligaments, and cartilage can be distinguished from one another (Fig. 3). In the assessment of traumatic, tumor-like, and inflammatory lesions of the skeleton and the soft tissue, a fat suppression MRI sequence (e.g., short tau inversion recovery or STIR) used in conjunction with a T1-weighted sequence after the intravenous administration of a gadolinium compound is extremely useful (Fig. 4).

Ultrasonography typically is used to evaluate the soft tissue of the musculoskeletal system. This readily available, inexpensive, and rapidly

performed study also can be used in infants and children. The indications for sonographic examination are expanding and include the following:
- Assessment of joint instability (e.g., developmental dysplasia of the hip.)
- Joint effusion (Fig. 5).
- Injuries to joints, tendons, and ligaments.
- Detection of foreign bodies in the soft tissue, especially pieces of glass and wood not detectable on routine radiography
- Guidance in fine needle aspiration and biopsy

Arthrography today in most cases has been replaced by MRI, but several major indications for arthrography still exist, including the assessment of the shoulder, hip (Fig. 6), knee, and wrist. CT arthrography with double contrast technique (air and positive water soluble contrast material) (Fig. 7) remains useful in the detection of intraarticular bodies or labrum tears of the shoulder.

Niels Egund, Kjell Jonsson, Holger Pettersson and Donald Resnick