Osteoporosis afflicts an estimated 10 million Americans, mostly elderly women. Some 189 million others have low bone mass. All are at risk for severe injury, such as breaking a hip, as well as chronic pain and stooped posture as bones in the spine and other areas fracture. However, tests for detecting this crippling disease earlier are getting easier and less expensive, thanks to a system developed by Hologic Inc. (Waltham, MA). Called Sahara, the system is said to be the first osteoporosis test that does not use x-rays. Instead, the device relies on ultrasound to assess a woman's bones by measuring the density of her heel. Slip the foot into a small box about the size of a laser printer and the sound waves painlessly penetrate for a mere 10 secs. Bone density is determined by how easily and quickly the sound waves move. The system automatically analyzes the results, and, a minute later, spits out a slip of paper with the bone measurement. The Sahara costs $30,000, and Hologic estimates that patients will pay about $40 for the test. In contrast, today's osteoporosis tests are performed by large, specialized x-ray machines that typically measure a patient's hip or spine. They cost $70,000 to $150,000, and patients are charged $127 for the leading x-ray test, according to Eric von Stetten, Hologic's director of ultrasound technologies. FAX (781) 890-8031.