This test, which is also called bone densitometry, uses specialized x-rays to measure the thickness and strength of your bones. Various scanners use different techniques. The one used most often is dual-energy x-ray absorptiometry (DXA). Quantitative ultrasound, which does not involve x-rays, may also be used. When bones are somewhat thin, the condition is called osteopenia. When bones become very thin, the condition is called osteoporosis. Bone density tests provide a precise measure of whether you have osteopenia or osteoporosis. Both sexes usually begin to lose bone thickness around age 50. However, women are much more likely than men to develop osteopenia and osteoporosis before age 70. Exercise and various treatments can help prevent and even restore bone loss. That's why diagnosing thin bones is important. Not only is a bone density test used to help detect osteopenia and osteoporosis. It's also helpful in monitoring your progress if you're taking bone-building medications.