To understand DEXA we need to understand a little about how X-rays work.
Traditional X-ray machines work by passing X-rays of a single energy through the patient’s body. The X-ray photons either pass through unaffected or they are ‘attenuated’ (absorbed or scattered) by the body.
The degree of attenuation (ie, how many photons pass through and how many are absorbed by the patient) depends on the thickness of the subject and, for a given thickness, on the density. Thicker and denser materials (like bones) attenuate X-rays more than thinner or less dense ones (like soft tissue).
For the simple identification of broken bones (when all that is needed is an image of the bone), single energy X-rays are all that is required.
To provide data about bone density, however, a problem arises because some of the X-ray attenuation is caused by the soft tissue surrounding the bone. An algorithm behind a single energy X-ray cannot be used to calculate the thickness of two unknown quantities (bone and soft tissue).
Fortunately, X-ray attenuation is also affected by the energy of the photon beam – the higher the energy, the lower the attenuation. By measuring attenuation based on both energy and subject thickness, the software algorithms behind dual-energy X-rays can use two simultaneous equations to calculate the thickness/density of both bone and soft tissue.
Further, since soft tissue is made up of fat and non-fat and if the density of fat is known (approximately 0.9 grams per cubic centimetre) then the mass of fat and fat-free (lean) tissue can be calculated, along with bone mass.
This essentially is the calculation behind DEXA (dual-energy X-ray absorptiometry) body composition measurement.
Accordingly, DEXA is referred to as a 3-compartment model, simply meaning that it measures the density of three components – bone, fat and fat-free soft tissue – to calculate their mass.