Bone density (also called bone mineral density: BMD) is the amount of bone matter in a certain volume of bone. It can be measured using a special X-ray called a quantitative computed tomogram. Osteoporosis (or porous bone) is a disease in which bones become weak and are more likely to break, and heal slowly. Without prevention or treatment, osteoporosis can progress without pain or symptoms until a bone fractures. Part of screening for osteoporosis involves bone density tests. Bone density measurement procedures are painless and non-invasive. Measurements are most commonly made over the lumbar spine (lower back), hip, and/or wrist, which are the most common sites of fractures due to osteoporosis.