Osteoporosis is the end result of bone loss. Bone loss is a natural, age related process. Bone mass peaks by age 30 and then starts a gradual decline. When enough bone mineral is lost so that the remaining bone will not support normal loads, the bone collapses. Low bone density as measured by SXA, DEXA, QCT, or ultrasound is a strong risk predictor for the occurrence of the non-traumatic bone fractures, which are characteristic of osteoporosis.
Postmenopausal women over 60 who have had limited or no estrogen replacement therapy at menopause are the major high-risk group. Asthmatics or other lung patients, or rheumatoid arthritis patients treated with high-dose corticosteroids, lose trabecular bone and experience fractures, as do patients with Cushing’s Syndrome. Other disorders including renal failure and certain types of cancer cause bone loss, along with chronic use of drugs such as anticonvulsants, anticoagulants, excess alcohol, and too much thyroid medication. Young women who experience amenorrhea due to athletic activity, weight loss, stress, or the nutritional deficiency of bulimia or anorexia nervosa lose bone; so do young women who have an early natural or surgical menopause and are not given estrogen replacement therapy. Not all of the patients in all of these groups will develop osteoporosis; however, they will all lose bone more rapidly than otherwise and be at increased risk for the development of osteoporosis over the long run.
Over seventy percent of people at risk for development of osteoporosis and it’s associated fractures have a low bone density as measured quantitatively using SXA, DEXA, QCT, or ultrasound. The other factors contributing to bone strength are varied, including the internal structure of bone, level of physical activity, neuromuscular coordination, and lifestyle factors that are difficult to quantify. Because of this, bone density is the single most useful measurement in estimating an individual patient’s relative risk for osteoporosis and associated fractures. Identifying patients with low bone density early, before fractures develop enables your physician to prescribe treatments that will stop and reverse the process of bone loss.
All commercially available methods for bone density measurement pass a low-intensity beam of X-rays or gamma-rays through a patient, and a radiation detector on the other side measures how much of the beam is absorbed. Part of the beam is absorbed by the bone and part by the surrounding soft tissue, and each technique measures these differently. Quantitative Computed Tomography (QCT) provides a cross-sectional or 3-dimensional image from which the bone is measured directly, independent of the surrounding soft tissue. Dual energy X-ray absorptiometry (DEXA) measures the bone by computing the difference in absorption of low-energy photons and high-energy photons by the mixture of soft tissue and bone in the path of the beam, and can generate a 2-dimensional image for localization of the bone. Single energy X-ray absorptiometry (SXA) computes bone mineral from the increased absorption of the beam as it passes from a constant thickness of soft tissue or water bag into the bone. Localization for SXA is normally done using external landmarks without an image. Radiographic absorptiometry (RA) measures bone density in the fingers relative to an aluminium calibration wedge on the film. Non-absorptiometric methods such as ultrasound of bone do not measure bone density directly, but give alternative information about properties of bone such as the speed of sound that are related to bone density and structure.
QCT refers to a class of techniques in which the CT numbers, or X-ray attenuation of a tissue is properly referenced to a calibration standard and then used to quantify some property of the tissue. Techniques were developed and published from 1978 to 1982 for bone mineral density, lung nodule calcification, liver and brain tumor volumes, body fat measurement, muscle mass, liver iron measurement, kidney stone composition, and tissue blood flow. Of these, bone mineral density, lung nodule calcification, and tissue blood flow have been commercialized.
QCT is the only commercially available technique that is 3-dimensional, meaning it can be used to measure 100% isolated trabecular bone. All other techniques measure a mixture of both trabecular bone and the overlying compact bone. There are two bone compartments; trabecular bone is located deeper inside the bone is the more metabolically active bone compartment. Compact bone is the bone near the surface of the bone. In the spine, trabecular bone makes up 30 to 35% of the total, in the ultradistal radius (wrist) it is 35 to 50%. Bone mineral loss affects the trabecular bone first and that is why the trabecular bone in the spine is considered the most reliable indicator of overall bone health. By focusing only on the trabecular bone compartment, 3-D QCT takes the best picture of bone health.
It is also important to consider the precision of any measurement of bone density. QCT is the most accurate tool to measure bone density. In terms of the ability to pick up bone disease, 3-D QCT is 2-3 times more sensitive than DEXA and 5 times more sensitive than SXA for detecting bone disease.
QCT, like any bone density measurement, is used to compare an individual’s bone to an absolute reference value, and to measure any change in the individual’s bone density over time. Medical researchers have established a fracture threshold level for all bone density methods; people whose bone density is above this threshold level rarely develop osteoporotic fractures, while below it the prevalence of fractures rises. This level is about 100-110 mg/cm3 for QCT. As the value decreases below this the fracture prevalence increases, so that below 50 mg/cm3 most people already have spinal fractures. Your QCT value, when added to other diagnostic information, is used by your physician to determine you treatment. Serial QCT measurements can establish the rate of change of bone mineral density in both treated and untreated patients. In most cases, a decline of 8-10 mg/cm3 is considered significant and several serial measurements all changing the same direction improve our confidence in the result. Women within 1-3 years after menopause average 7 mg/cm3/yr loss, so yearly measurements can be helpful. Bone loss may be slower in older individuals. The frequency for each patient will depend on other diagnostic and treatment factors, and it is important that your doctor interpret your bone density results within the context of your clinical status.