Which isotope is used in Single Photon Absorptiometry (SPA)?

• A. Technetium-99
• B. Radon-222
• C. Iodine-125
• D. Carbon-14

Answer: (C)

Single Photon Absorptiometry (SPA) utilizes Iodine-125 as its isotope for measurements. Iodine-125 is a gamma emitter that has properties suitable for assessing bone density. The energy emitted is effectively absorbed by bone and surrounding tissues, allowing for the calculation of bone mineral content based on the amount of attenuation of the photon beam. This makes it an ideal choice for SPA, as it provides accurate and reliable data necessary for evaluating bone health.

In SPA, the use of Iodine-125 enables the detection of changes in bone density, which is crucial for diagnosing conditions like osteoporosis. Its half-life and energy characteristics allow for sufficient penetration and interaction with the bone tissue while minimizing exposure to surrounding soft tissues. This makes Iodine-125 particularly advantageous for this specific application in bone densitometry.