A 65-year-old postmenopausal woman presents for prescription refill.
IMAGING MODALITY OF CHOICE
Dual-energy x-ray absorptiometry (DXA)
Bone is made up of a variety of cells (eg, osteoblasts, osteoclasts, and osteocytes), an organic matrix (predominantly type I collagen), and an inorganic matrix (predominantly calcium hydroxyapatite). Osteoporosis is the most common metabolic disease of bone. It is due to disturbances in bone remodeling that result in less bone tissue (matrix and mineral) per unit volume. Bone mineral density (BMD), the average concentration of mineral in a defined section of bone, is used to diagnose osteoporosis, estimate the risk of insufficiency fractures, and select and monitor treatment. BMD is estimated by a DXA scanner, a low-dose x-ray tube with two energies. The low-energy and high-energy photons are attenuated differently in bone and soft tissue, allowing BMD to be estimated.
DXA devices are used to calculate BMD in g/cm2 and to compare the values to a reference database. The comparison is quantified as a T-score, which represents the standard deviation by which the BMD of the patient differs from the mean BMD of a young adult reference population of the same ethnicity and sex, and a Z-score, which represents the standard deviation by which the BMD of the patient differs from the mean BMD of an age-matched reference population of the same ethnicity and sex.
According to the World Health Organization, osteoporosis is “a systemic skeletal disease characterized by low bone density and microarchitectural deterioration of bone tissue with a consequent increase in bone fragility” and corresponds to a T-score of −2.5 or lower. Patients with osteopenia (see Box C1.1), defined as T-scores between −1 and −2.5, have less bone density loss than do those with osteoporosis. A T-score higher than −1 is considered normal.
Box C1.1 – TERMINOLOGY
The WHO definition of osteopenia conflicts with the way the word has been used by radiologists. Radiologists have used osteopenia as a nonspecific and descriptive term for increased lucency of bone as seen on radiographs. This rarefaction could be due to a variety of causes, such as osteoporosis or osteomalacia. Other words, such as demineralization, undermineralization, and deossification, were used incorrectly as synonyms for osteoporosis and osteopenia and added to the confusion.
Typically, the lumbar spine and proximal femur are scanned (Fig. C1.1A,B). The forearm (Fig. C1.1C) can be scanned as an alternative to the hip and lumbar spine when these parts cannot be scanned due to artifacts (read further) or when the patient is too heavy for the table. In patients with hyperparathyroidism, the ...