What is the name of the isotope used in single photon absorptiometry (SPA)?

• A. Iodine-123
• B. Iodine-125
• C. Technetium-99
• D. Copper-64

Answer: (B)

The isotope used in single photon absorptiometry (SPA) is Iodine-125. This is primarily because Iodine-125 has suitable properties for measuring bone mineral density. It emits low-energy gamma photons that can be accurately detected in the presence of bone and soft tissue, making it effective for SPA techniques, which rely on measuring the amount of radiation that passes through bone to assess its mineral content.

Additionally, Iodine-125 has a half-life of approximately 60 days, making it practical for clinical use as it may remain stable enough for repeated measurements. Its energy levels are optimal for the soft tissue absorption characteristics and for achieving an effective signal-to-noise ratio in the measurements of bone density.

In SPA, the choice of isotope is crucial because it impacts the precision and reliability of the measurements. Therefore, Iodine-125's characteristics as a photon source make it the instrument of choice for this specific imaging technique.