| Dual
Energy X-ray Absorptiometry |
Definition
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/cm 2 . 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 (E0) and of the
emerging beam (E), we can calculate M of a homogeneous absorber from the following
relationship connecting these properties.
Dual
Energy X-ray Absorptiometry (DEXA) is an instrumental technique used to measure
bone mineral density (BMD) that includes the hip and spine, compared to SXA (Single
Energy X-ray Absorptiometry) that measures only the wrist or heel bone. BMD is
the widely accepted indicator of bone strength. DEXA (the whole body scanner)
uses low dose x-rays to give us information on bone content and density. It is
currently the most widely used machine in the clinical setting to diagnose the
disease osteoporosis, the thinning of bones.
DEXA
is the most commonly used modern technique to determine the bone density and hence
the bone strength. The DEXA results help to predict the patient's risk factors
for osteoporosis. It is a fast, accurate, and less expensive technique. It exposes
the patient to fewer amounts of radiations. So the risk is reduced to a great
extend. Studies using DEXA scanning have shown that women with osteoporosis have
substantially lower bone density measurements than normal, age-matched women.
Bone mineral density is widely accepted as a good indicator of bone strength.
Thus low values can be compared against standard bone density measurements and
help predict a patient's risk for fracture based upon the DEXA scan measurements.
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