What is it?
A bone density scan is a method to assess your bone’s condition. When osteoporosis is suspected this test can help your surgeon to predict the risk of fractures. The test is performed with X-ray and the density of the bones is compared with the average bone density of a thirty years old person, at the peak of bone density.  
Some contraindication due to the use of radiation must be evaluated in some particular circumstances (i.e. pregnancy).

How can I prepare for the densitometry test?
The exam is similar to a normal X-ray examination. You will be asked to undress and lie on an X-ray table, while an X-ray tube will pass over your body taking the images.
A radiologist will then interpret the result of the test.