DXA scans of the forearm are very low, typically less than 1 μSv irrespective of the type of scanner and protocol or mode. Lumbar spine, hip or whole body scans each result in an effective dose of about 1 μSv for a pencil beam unit and up to about 10 μSv for a fan beam unit. Some earlier fan beam DXA units initially delivered effective doses as high as 80 μSv. There is not much data for doses from cone beam units, but doses appear to be similar to those for fan beam units.

Personnel radiation protection Operators that perform the DXA procedure may also receive a radiation dose due to scattered radiation from the patient. This scattered dose is much less than the dose in the primary beam. While the occupational dose limit prescribed in the BSS is 20 000 μSv/a averaged over five consecutive years with a limit of 5 000 μSv in any single year. The ALARA principles must be applied The operator’s desk should be positioned at least 1 m away from a pencil beam system, and at least 2 m away from a fan beam system. Some older, less common fan beam models require a distance of 3.5 m.

Limitations of DXA DXA provides only two-dimensional measurements of BMC and BA for the three dimensional bone. Thus, BMD is not a measure of volumetric density (g/cm3) because it provides no information about bone depth. The weight limitation of the scanning bed (typically 300 lb) The width of the scanning area (usually ∼60 cm) Degenerative changes, aortic calcifications and fractured vertebrae are difficult to visualize and can cause significant bias to the BMD results When an individual's body dimensions exceed these limits, the accuracy of the measurement is typically compromised.

DXA Measurements DXA machine used to measure the following Bone mineral content (BMC) BMC is the mineral mass component of bone in the form of hydroxyapatite, Ca10(PO4)6(OH)2. BMC is typically measured in grams. BMC does not include the mass of any of the organic components of bone (marrow, collagen, etc.). Thus, accuracy can only be assessed against as hed bone samples.

2. Bone area (BA). BA is the projected area of the bone onto the image plane, typically in cm2. The accuracy of the BA is questionable, especially in whole body scans where bold assumptions need to be made, particularly in the upper torso. 3. Areal bone mineral density (aBMD). aBMD is the mineral mass of bone per unit image area in g/cm2.

The following measures are only from whole body scans. Fat mass (FM) Fat free mass (FFM) Soft tissue mass Total body mass (TBM) Percent fat mass (PCTFM)

T-scores and Z-scores Both T-scores and Z-scores are derived by comparison to a reference population on a standard deviation scale. The recommended reference group for the T-score is a young gender-matched population at peak bone mass The Z-score should be derived from an age-matched reference population.

Currently T-scores and Z-scores are used in the: Assessment of fracture risk Diagnosis of low bone mass or osteoporosis Criteria for patient selection for clinical trials ofinterventions which affect bone. Management guidelines for the evaluation and treat-ment of osteoporosis.