DXA: state of the art
Introduction
Dual x-ray absorpiometry (DXA) was introduced in 1987 and, to date, it still represents the most widely used technique for the diagnosis of osteoporosis in the clinical practice. The World Health Organization (WHO) has defined the threshold levels for the diagnosis of osteopenia and osteoporosis using DXA technique.
DXA machines consist of a mobile source of X-rays, a patient couch, a detection system on which fall the radiations emerged from the bones under examination. The x-ray source is under the couch and moves together with the detection system which is located opposite to it over the body of the patient. The main characteristic of DXA lies in the fact that it uses an x- ray beam composed of two different photon energies. The method used to produce the two photon energies may be either constant or pulsed. In the first case it is produced a beam which is constant in intensity and voltage and is filtered by a filter made of cerium or samarium (rare earth) in order to obtain two distinct beams with different energies. The pulsed method produces an alternant emission of beams with different energies with a frequency of 60 Hz/sec. The energy used (70-140 kV Hologic; 40-70 kV Lunar; 45-80 kV Norland) is conveniently chosen to compensate the different attenuation coefficients of mineralized bone and soft tissue. Practically, the intensity of high energy photons and low energy photons is analyzed separately after they have passed through bones and soft tissues.
Using a particular computing algorithm, the attenuations of soft tissues are subtracted, obtaining in this way the attenuation values of bone.
The attenuation of the skeleton is related to its bone mineral density by comparing the values obtained with standard values determined on phantoms of known density (higher attenuation- higher density).
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Attenuation values of low energy photons are also used to produce the image, as they are computed pixel by pixel (indirect matrix) and transformed into a grey-scale map.
