Bone density tests work by giving a measure of a bone’s mineral content. The dual-energy x-ray absorptiometry (DXA) scan is the most common bone density test. It passes a beam of x-rays through a large bone, usually the spine or hip. The x-rays blocked by dense bone are different from those blocked by soft tissue. Based on the amount blocked by dense bone compared to soft tissue, you can figure out how dense your bones are.
Another common bone density test is the ultrasound. This method bounces sound waves over bone (typically, in your heel) to see if it bounces off. The only down side to this test is that you can’t use this method to assess the spine or hip, the most troubling spots for osteoporosis, or to monitor the effectiveness of osteoporosis treatments.