Bone density (or bone mineral density) is a medical term normally referring to the amount of mineral matter per square centimeter of bones. Bone density (or BMD) is used in clinical medicine as an indirect indicator of osteoporosis and fracture risk.
This medical bone density is not the true physical "density" of the bone, which would be computed as mass per volume. It is measured by a procedure called densitometry, often performed in the radiology or nuclear medicine departments of hospitals or clinics. The measurement is painless and non-invasive and involves low radiation exposure. Measurements are most commonly made over the lumbar spine and over the upper part of the hip. The forearm may be scanned if the hip and lumbar spine are not accessible. Average density is around 1500 kg/m3 or 1.5 kg/L .
There is a statistical association between poor bone density and higher probability of fracture. Fractures of the legs and pelvis due to falls are a significant public health problem, especially in elderly women, leading to much medical cost, inability to live independently, and even risk of death. Bone density measurements are used to screen women for osteoporosis risk and to identify those who might benefit from measures to improve bone strength.
- Dual-energy X-ray absorptiometry (DXA, previously DEXA is a means of measuring bone mineral density (BMD). Two X-ray beams with different energy levels (e.g. a low kev and high kev) are aimed at the patient's bones. When soft tissue absorption is subtracted out, the BMD can be determined from the absorption of each beam by bone. Dual-energy X-ray absorption is the most widely used and most thoroughly studied bone density measurement technology.
The DXA scan is typically the most widely used to diagnose and follow osteoporosis, as contrasted to the nuclear bone scan, which is sensitive to certain metabolic diseases of bones in which bones are attempting to heal from infections, fractures, or tumors.
X-ray detectors vary in shape and function depending on their purpose. Imaging detectors such as those used for radiography were originally based on photographic plates and later photographic film but are now mostly replaced by various digital detector types such as image plates or flat panel detectors. For radiation protection direct exposure hazard is often evaluated using ionization chambers, while dosimeters are used to measure the radiation dose a person has been exposed to. A bone density scanner uses an overhead x-ray detector since the xray source (x-ray tube) in under the table.
Pencil Beam DXA uses a singular beam of X-Ray to produce an image and is the first and original method. This method uses a single detector as the X-Ray tube scans across the measurement region. Although the pencil beam is considered the gold standard for precision, the scan time is increased due to travel time for the single beam to cover the body. The pencil beam is an excellent choice for practices that are scanning fewer than 15 patients per day. The GE Lunar DPX Duo and GE Lunar DPX Bravo are great examples of pencil beam DXA systems.
Fan Beam DXA (wide angle fan beam) was introduced in the early 2000’s as an alternative to pencil beam. The X-Ray tube emissions are received by multiple detectors which are swept across the measurement region. While the downside is minimal image distortion due to magnification of the tissue, fan beam systems provide for shorter scan times. The Hologic Discovery Series all use fan beam technology.
Narrow Angle Fan Beam
The final method, often referred to as third generation DXA is known as Narrow Angle Fan Beam (or Micro Angle Fan Beam). This technology was introduced exclusively by GE Healthcare, beginning with their Lunar Prodigy line in 2004. Although still a fan beam, the narrow-angle uses multiple passes to acquire multiple images. Image reconstruction compiles these images to mesh the precision of Pencil Beam and the speed of a Fan Beam system. The GE Lunar Prodigy Primo and Advance are third generation systems that are recommended for practices scanning more than 15 patients per day. 
- ↑ FDA. "Medical Imaging." 06/05/2014. http://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProductsandProcedures/MedicalImaging/ucm2005914.htm
- ↑ Shelly Middleton . Pencil vs Fan Beam Bone Densitometer: How to Decide Which One is Right for You. Block Imaging. http://info.blockimaging.com/bid/65934/Pencil-vs-Fan-Beam-Bone-Densitometer-How-to-Decide-Which-One-is-Right-for-You