Osteoporosis is a skeletal disorder characterized by compromised bone strength predisposing to an increased risk of fracture. Bone strength reflects the integration of two main features: bone density and bone quality . The National Osteoporosis Foundation states that 10 million people in the United States have osteoporosis and there are 1.5 million fractures due to osteoporosis per year. This disease is a major drain on our health care dollars with both direct and indirect costs of as much as $17 billion dollars per year.
Low bone mass is a key characteristic used by the World Health Organization (WHO) in the consensus definition of osteoporosis . Although the definition also incorporates architectural deterioration, the WHO’s diagnostic categories for osteoporosis are based on bone mass measurements . Furthermore, the indications for drugs approved for osteoporosis prevention and treatment include a low bone mass measurement, obtained through any of a variety of techniques. Therefore, bone densitometry has come to play a major role in both the diagnosis and follow-up of patients with osteoporosis.
Several different modalities have been developed to measure bone density utilizing very different technological concepts. The two most common are dual-energy X-ray-absorptiometry (DXA) and quantitative computed tomography (QCT), which can be used to measure both central (spine and hip) as well as peripheral (forearm and heel) sites.
QCT was developed in the late 1970s. Most systems utilize solid phantoms that compare bone to a series of K2HPO4 standards for which bone density equivalence has been established. In comparison to DXA, QCT is a true volumetric measurement of bone and is measured in milligrams per centimeter cubed (mg/cm 3 ). It measures trabecular bone density separately from cortical bone .
Trabecular bone is metabolically more active than cortical bone and is the most sensitive indicator of early bone loss in vertebral fracture risk. There is a strong association between vertebral fracture and spinal trabecular (bone mineral density [BMD]) as measured by QCT. This modality has been shown to have the strongest ability to discriminate between healthy postmenopausal women and those with vertebral fractures . Spinal trabecular BMD also correlates with trochanteric fracture risk .
In a two-dimensional (2-D) QCT scan, the calibration phantom is placed under the patient’s back while the body is scanned. A computed radiographic localizer view is obtained to determine the levels of L1 to L3 and each vertebral body is imaged with a 1.0-cm section thickness. BMD is then calculated by comparing the spine scan results with the calibrated standards. While accurate, the reproducibility (precision) can be diminished by variability in sampling . With the advent of spiral CT scanners and three-dimensional (3-D) software that acquires true volumetric imaging, reproducibility has been significantly improved. Patient positioning errors, artifacts from bowel gas, or increased image noise from gantry angulation are eliminated. Precision has been measured at 0.9% CV on an average in normal and osteoporotic bone (range 0.7%–1.1%) .
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