DXA: state of the art. Dosimetry
DXA examination has always involved low radiation dose both to patients and operators.
With modern machines the dose to patients varies from 20 to 100 micro Sv, according to different exam typology; at these exposures, operators distant more than 2 meters from the x-ray source do not need any protection.
The vast majority of machines are now equipped with an automatic device for the optimization of scanning parameters according to patient morphologic characteristics and exam typology.
Quality control and autocalibration
All updated machines need a daily quality control procedure to be performed on a phantom for the assessment of reproducibility of bone density (BMD) measurements. Such controls are very important for the verification of long term machine stability, so as to be able to accurately detect slight bone density variations.
Some newer machines are equipped with an autocalibration device which operates continuously during the examination interposing through the beam tissue-equivalent materials for both bone and soft tissue.
Exam results
DXA examination is a monoplanar bone density evaluation and the densitometric findings (BMD) are expressed as gram/cm2 (planar density).
BMD results are expressed as standard deviation (SD) by means of T-score and Z-score.
In details: T-score describes the difference between the BMD of the patient under examination and the BMD of a standard young adult population (20-30 years) and refers to the peak bone mass.
Figure 1 – Graphic representation of DXA findings.
The Z-score shows the patient’s results as the difference from the mean of age and sex-matched controls. Z-score is particularly important for patients aged 75 or more. The World Health Organization has defined osteopenia T- score values between -1 and -2,5 SD, osteoporosis T-score values equal or lower than -2,5 SD and severe osteoporosis T- score values lower than -2,5 SD associated with radiologic evidence of one or more fractures.
DXA results are reported both as numeric values of T-score and Z-score and by a graphic curve normalized for sex and age (Fig. 1).
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Lumbar spine and proximal femur are the sites most commonly examined; less common is the evaluation of the distal radius which is performed in particular conditions such as hyperparathyroidism or when the evaluation of other sites is impossible (dedicated machines).
To date, total body DXA examination is only indicated for the assessment of the relative body percentage of fat and muscle.
