The DEXA exam is the most accurate way to measure Bone Mineral Density. It is fast and uses very low doses of radiation and can measure as little as two percent of bone loss per year. It uses two different X-ray beams to estimate bone density in your spine and hip. The area scanned is the lumbar spine (lower back), hips and sometimes forearm. Since strong, dense bones allow less of the X-ray beam to pass through them, the DEXA test compares the percentage of each X-ray beam blocked by bone and soft tissue.
Qualitative ultrasound is generally used to look for problems. Ultrasound uses sound waves to measure BMD (some machines pass the sound waves through air and some through water.) The area scanned is usually the heel. Ultrasound is quick, painless, and does not use potentially harmful radiation like X-rays. One disadvantage of ultrasound is it cannot measure the density of bones most prone to fracture from osteoporosis (e.g., the hip and spine). Nor is it used to keep track of how well medicine used to treat osteoporosis is working. If an ultrasound test finds low bone density, DEXA is recommended to confirm the results.