Radiation doses from DXA
Several studies have reported doses to children and adults from DXA [27–34]. For the first-generation pencil-beam devices the effective dose was negligible i.e. about 0.001 mSv for a spine and femur DXA. However, the doses are considerably higher for the fan-beam devices, especially for children and adolescents. Figure 1 shows effective doses to adult and paediatric patients from a spine and a hip DXA reported in a recent paper [33]. Dose figures were estimated by using the default adult imaging length of 20 cm for the spine and 15 cm for the hip. In general, the effective dose from a spine and hip DXA examination performed on a 5-year-old child is two to three times higher than the adult dose. This may be attributed to the fact that the exposure parameters and the image size are optimised for standard-sized adults. Paediatric doses can be reduced by adjusting image lengths to the size of the child’s body. Table 1 shows typical organ and effective doses from fan-beam DXA and spine radiographs. DXA values are for Hologic DXA devices (Hologic Inc, Bedford, MA) examined by Blake et al. [33]. The patient dose will vary between DXA systems of different models and manufacturers depending on a number of variables, including differences in acquisition techniques and X-ray tube filtration. Doses for spine radiography were calculated by using the Monte Carlo N-particle code (MCNP, Los Alamos National Laboratory, Los Alamos, NM) and a mathematical phantom of human anatomy constructed with the BodyBuilder software package (BodyBuilder, White Rock Science, NM). Table 2 shows doses associated with various diagnostic X-ray examinations derived from the literature [35, 36]. Patient effective doses from peripheral DXA are lower than 0.01 mSv [37]. Whole-body DXA is an established procedure for the assessment of skeletal mineral status of the whole body and the measurement of body composition [38]. Effective doses for whole-body DXA examinations were found to be 0.0052, 0.0048, 0.0042 and 0.0042 mSv for a 5-, 10-, 15-year-old child and adult respectively for an examination performed on the Hologic Discovery A device. Corresponding values for the Hologic Discovery W were 0.0105, 0.0096, 0.0084 and 0.0084 mSv [33].
Fig. 1 Effective doses from a single DXA of the spine (a) or hip (b) as a function of patient age (adapted from Blake et al. [33]). Patient dose was estimated for Hologic DXA using three imaging modes: Array mode (60-s data acquisition time); Fast mode (30-s data acquisition time); and Express mode (10-s data acquisition time)
Table 1 Effective and organ doses for DXA and spine radiographs
Examination Effective dose (mSv) Organ dose (mGy) Relevant organs
Adult spine DXA 0.013 0.003 BM, ovaries
Adult hip DXA 0.009 0.005 LLI
Paediatric spine DXA 0.027 0.008 Ovaries
(5-year-old child, scan length 11.7 cm) 0.007 Stomach
Paediatric hip DXA 0.022 0.015 Testes
(5-year-old child, scan length 9.0 cm) 0.009 LLI
Paediatric spine DXA 0.021 0.006 Ovaries
(10-year-old child, scan length 14.5 cm) 0.005 Stomach
Paediatric hip DXA 0.018 0.010 Testes
(10-year-old child, scan length 12.4 cm) 0.008 LLI
Thoracic spine AP radiograph 0.4 0.8 Lungs
Thoracic spine LAT radiograph 0.3 1.2 Lungs
Lumbar spine AP radiograph 0.7 2.5 Stomach
Lumbar spine LAT radiograph 0.3 2.3 Liver
Paediatric doses are given for scans lengths adjusted to the size of the child’s body
AP anterior-posterior, LAT lateral, BM bone marrow, LLI lower large intestine
Table 2 Typical effective and organ doses for various diagnostic X-ray examinations
Examination Effective dose (mSv) Organ dose (mGy) Relevant organs
Dental radiography (intraoral) 0.005 0.005 Brain
Chest radiography (posterior-anterior) 0.02 0.01 Lung
X-ray mammography 0.4 3 Breast
Adult abdominal CT 8 10 Stomach
The patient radiation dose from a DXA examination depends on a number of parameters. The most important are the number of images, the size of the patient, the specific design of the device, beam filtration, the tube current (mA), the tube potential (kVp), the imaging speed and the imaging length and width. Most of these parameters cannot be controlled by the operator performing the DXA examination. However, it is important for the user to know that patient dose varies depending on the imaging mode for a specific examination (Fig. 1). Special attention to imaging protocols and radiation dose is needed when imaging children and adolescents. The use of a standardised and fixed clinical protocol designed for adults leads to unnecessary overexposure of children.
Although rare, DXA is occasionally performed on pregnant patients for the diagnosis or the differential diagnosis of pregnancy-associated osteoporosis. Pregnant patients receiving heparin may require BMDa estimation. The maximum conceptus dose during the first trimester associated with DXA performed using a pencil-beam device was found to be 0.0034 mGy related to the scan of the hip [39]. In this study, conceptus doses from DXA performed during late pregnancy were found to be up to 0.0049 mGy. The highest dose was recorded for spinal imaging carried out during the third trimester of gestation. According to the ICRP, a dose to the conceptus below 100 mGy should not be considered a reason for terminating a pregnancy [40]. Because radiation dose to the unborn child from DXA is always less than 100 mGy, termination of pregnancy based on radiation risk is not justified. Although radiation dose to the conceptus is very low, DXA examinations on pregnant patients should be performed only when the expected benefits clearly exceed the reasonably suspected risks. When DXA is considered justified, the patient should be counselled before imaging on the actual dose received by the conceptus and the radiation risks involved.