Dual Energy X-ray Absorptiometry
Published on Mar 12, 2016
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.
Principal components of a DEXA system
A typical DEXA system is shown in the figure.
The DEXA scanner consists of the following basic components:
1. Source of X-rays
2. The sample space
3. The detector
A block diagram of the instrument is shown below as Figure 2. Two separate beams of X-rays of known energies (Eo,i and £0,2) produced at the source are passed through a desired absorber material (sample)-usually human body- positioned in the sample space (DEXA table). The sample interacts with the incident beams altering their energies to EI and £2. The detector determines the energies (Ei and £2) of the emerging beams of X-radiation. A data acquisition and control unit manipulates the data. The operation of this electronic machine is fully controlled by a computer system.
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