The basic principles of Dual Energy X-ray Absorptiometry have been discussed in this presentation. DEXA is a instrumental technique used to measure bone mineral density (BMD), which is the widely accepted indicator of bone strength. DEXA scanner is the most widely used modern electronic machine to diagnose the disease osteoporosis, the thinning of bones. Human body being a heterogeneous system, use of a dual energy, rather than single energy, X-ray source is necessary for scanning. The interaction of the sample with the X-ray beams results in a reduction or attenuation of the energy of the X-ray beam. The extent to which the photon energy is attenuated is a function of the initial energy of the X-ray photon, the mass per unit area (M) of the absorber material and the mass attenuation coefficient (U) of the absorber. For a given absorber material, U (which is a measure of the degree of attenuation) is a constant at any given photon energy.
U increases with the density of the absorber material and decreases with the energy of the X-ray beam. U can be used to calculate the Mass per unit area (M) of a homogenous absorber irradiated at a specific incident X-ray energy. The mass of bone and soft tissue 'below' this square would represent the mass per unit area of the absorber, viz., leg. For instance, if there are 100 grams of bone and soft tissue below this square, the mass per unit area (M) would be 100 g/cm2. Knowledge of M of the human body components, especially of bone, is important in determining the possibility of osteoporosis. The calculations of M of the various components of the body are discussed in detail.
From knowledge of mass attenuation coefficient (U) of the absorber and the energy of the incident X-ray beam (Eo) and of the emerging beam (E), we can calculate M of a homogeneous absorber from the following relationship connecting these properties.
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