Osteoporosis is a widespread medical condition among older people. It causes the bones to weaken and become more likely to break. Osteoporosis and bone fracture risk are currently evaluated by looking at clinical risk factors and measuring bone mineral density (BMD). The lower the BMD is, the higher the risk of osteoporotic fractures in the future. An X-ray device called DXA is the main tool used to diagnose osteoporosis and fracture risk clinically. DXA measures two-dimensional BMD in the hip and spine. However, DXA devices are often not readily available at the point of care. The POROUS ultrasound device offers a different approach by measuring various properties of the outer layer of the bone in the lower leg. It has several advantages over DXA: (1) higher and three-dimensional image resolution; (2) the ability to measure bone properties without radiation; (3) portability, it is a mobile medical device; 4) lower operational costs. For this clinical study, we will recruit men and women between the ages of 21 and 55. Most of these study participants will not have evident clinical risks for osteoporosis. The goal for including this age group is to estimate the range of BMD values for younger people before BMD declines with age. In a separate clinical study, we are recruiting older participants. The study is anticipated to last one year. Our major research questions are: * Can the POROUS ultrasound device estimate BMD? * How does its performance compare to DXA? * What is the safety of the new device? The participants will: * answer questions about their medical history. * be measured for height and weight. * be examined with the two devices, DXA and POROUS.