What is the role of single-photon absorptiometry (SPA) in the workup of osteoporosis?

Updated: Jan 19, 2021
  • Author: Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR; Chief Editor: Felix S Chew, MD, MBA, MEd  more...
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Single-photon absorptiometry (SPA) was established for the bone densitometric evaluation of the appendicular skeleton. SPA uses a single-energy source of gamma rays (iodine-125; photon energy, 27.3 keV) or americium-241 (60 keV) to produce a collimated pencil beam, which is tracked across the measurement site. The half-life of 125I is approximately 60 days; its useful life is around 6 months. The transmitted photons are counted by using a sodium iodide crystal/photomultiplier for each point along the track.

Because of the low photon flux and energy source, the technique is usually applied to a peripheral skeletal site, such as the forearm and, less commonly, the heel. The forearm chosen is that of the nondependent arm. To allow correction for soft tissues, the forearm must be placed in a water bath. The mean photon count through the water bath without the interposed limb is used as a baseline value. A reduction in the photon count below this baseline is assumed to be due to the bone. Muscles of the forearm have attenuation effect similar to that of water. The effects of a varying muscle mass are thus eliminated by the water bath.

Although both SPA and DPA were widely used, and although they provide valuable research data, the radionuclide source is a disadvantage. The energy source is subject to decay and must be replaced regularly. The low photon flux can cause the scanning times to be long (up to 40 min), and spatial resolution tends to be poor. SPA machines repeatedly scanned in a single line and were limited (because of the physics of their operating principle) to measuring bone sites that could be either immersed in water or embedded in material with absorption properties equivalent to soft tissue (to simulate homogenous overlying soft tissues).

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