Electronics Seminar - DEXA Dual Energy X-ray Absorptiometry

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 (E0) and of the emerging beam (E), we can calculate M of a homogeneous absorber from the following relationship connecting these properties.

Seminar Introduction

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) use s 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.

Advantages of DEXA SCANNING

Dual Energy X-ray Absortiometry, or DEXA scanning, is currently the most widely used method to measure bone mineral density. For the test, a patient lies down on an examining table, and the scanner rapidly directs x-ray energy from two different sources towards the bone being examined. The X-ray source and the detector move in a coordinated rectilinear pattern over the patient. The mineral density of the patient’s bone weakens, or prolongs the transmission of these two sources of x-ray energy through a filter onto a counter in a degree related to the amount of bone mass present. The greater the bone mineral density, the greater the signal picked up by the photon counter. The use of the two different x-ray energy sources rather than more traditional radioisotope studies (such that would be used for a bone scan) greatly improves the precision and accuracy of the measurements.

DEXA scanning has become the most widely used method for measuring bone mineral density for several reasons. When compared with radiographic absortiometry or single energy x-ray absortiometry, DEXA scanning more precisely documents small changes in bone mass and is also more flexible since it can be used to examine both the spine and the extremities. A scan of the spine, hip or the total body requiresonly one, two or four minutes respectively. Qualitative computed tomography (QCT) is the only technique that can directly measure bone density and volume but can distinguish trabecular from cortical bone. DEXA scanning is less expensive, exposes the patient to less radiation and is more sensitive and accurate at measuring subtle changes in bone density over time or in response to drug therapy than is QCT.

Seminar Conclusions

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.