From Imaging to Therapy: How Radioactive Decay Shapes Clinical Use

2026-09-08

☢️ Not all radionuclides play the same role in nuclear medicine.

Whether a radionuclide is used for imaging or therapy depends largely on what it emits during radioactive decay—and how that radiation interacts with tissue.

A simple way to understand it:

Radiation that can escape the body can be detected for imaging. Particles that deposit energy locally can be used for therapy.

🔍 γ rays — SPECT imaging Gamma photons can travel out of the body and be detected externally, making radionuclides such as ⁹⁹ᵐTc and ¹²³I suitable for SPECT imaging.

📡 β⁺ decay — PET imaging Positron emitters such as ¹⁸F, ⁶⁸Ga, ¹¹C and ¹³N produce positrons that eventually annihilate with electrons, generating photon pairs detected by PET scanners.

Together, these radionuclides allow us to visualize physiological and molecular processes inside the body.

🎯 β⁻ particles — therapeutic radiation Beta particles travel over a relatively longer range in tissue, allowing radiation to affect both targeted and nearby cells—the cross-fire effect. ¹⁷⁷Lu and ⁹⁰Y are representative β⁻ therapeutic radionuclides.

⚡ α particles — short range, high energy deposition Alpha particles travel only a very short distance in tissue but deposit energy densely along their path. Radionuclides such as ²²³Ra, ²²⁵Ac and ²¹¹At are used or being investigated for targeted alpha therapy.

🔄 Different emissions. Different clinical roles.

γ / β⁺ → Imaging 🔍 β⁻ / α → Therapy 🎯

But even radionuclides within the same category are not interchangeable. Their range, energy, half-life and decay characteristics all influence how they can be used.

Ultimately, radionuclide selection starts with one question:

What do we want the radiation to do inside the body?