The DXA machine consists of X-ray detector It detects the x- rays that have not been absorbed by the patient body X-ray generator Emits a beam of low dose x-rays which will pass through the patient body Monitor At which the scan will appear on it Foam cube Used to put the patient leg in a comfortable position to keep the spine flat Laser Positioning. The Laser-On Lamp is an amber light above the Laser switch on the Instrument Control Panel.

Types of DXA scanner Pencil beam system Fan beam systems

Pencil beam system The scanners used a highly collimated beam of x-rays in conjunction with sequential detectors or a single detector that moved in a raster pattern (i.e., in a series of thin parallel lines) across the patient. This pencil beam system produces the most geometrically correct information, with little or no magnification of the area being scanned.

Fan beam systems Fan beam systems use a slit collimator to generate a beam that diverges in two directions in conjunction with a linear array of solid-state detectors, so bone measurements can be made with a single sweep of the x-ray arm. The fan beam systems use higher energy photon intensities and a greater photon flux, thus producing a better-resolution image considerably faster than the older pencil beam machines. The lumbar spine can be scanned in 30 seconds with the fan beam, as compared with the 3–10 min required for the pencil beam system.

The most recent advance has been the introduction of the narrow fan beam bone densitometer. This machine uses a narrow fan beam x-ray source in conjunction with semiconductor detectors. It scans in a rectilinear raster fashion, much like the original pencil beam machines. However, because the beam is wider than the original pencil beam machine, it can cover the body in a much faster time, typically 30 s. Recent cross- calibration studies demonstrated no detectable magnification effect between the old generation pencil beam scanner and the new narrow fan beam machine

Effect of DXA Radiation Exposure The amount of radiation exposure in DXA is extremely low compared to many other x-ray imaging techniques. It has been difficult to directly estimate the degree of risk associated with these very low levels of radiation except by extrapolation from studies that involved distinctly higher levels of radiation exposure.

Presently, studies have not been able to establish a link between health risk and the low levels of radiation exposure that are typical of DXA. According to the Health Physics Society, the risks of health effects for exposures less than 5–10 rem (Roentgen Equivalent Man) “are either too small to be observed or are nonexistent”. Health effects of radiation have been demonstrated at doses above 5– 10 rem (greater than 50,000–100,000 PSv). The principal risk due to radiation is random x-ray interactions with the body, which can result in carcinogenic or genetic effects. Typically, carcinogenic effects will not manifest in an individual for several decades following an exposure. This is an important consideration when scanning children because they have a longer amount of time for expression of an effect than adults.

Typical patient doses from DXA Patient effective doses in DXA depend on the type of unit Pencil beam Fan beam Cone beam) The protocol or mode used for the scan (scan area, tube current, scan speed) and the body region being scanned.