Radionuclide Imaging and Radiotherapy

A tracer compound should reach the organ of interest and carry an isotope with a useful gamma energy and a half-life long enough for preparation and imaging, but short enough to limit persistent activity. Technetium-99m emits approximately 140 keV gamma photons and has a physical half-life near 6 h. Iodine-131 has an approximately 8-day half-life and emits beta particles as well as gamma photons; indium-111 has a half-life near 2.8 days and useful gamma lines near 171 and 245 keV. Chemical labelling and organ uptake matter as much as the numerical half-life.

A molybdenum-99/technetium-99m generator allows the daughter tracer to be separated locally from the longer-lived parent, helping hospitals obtain short-lived technetium. In a gamma camera a lead collimator selects photon directions; a scintillator converts absorbed gamma energy to light. Photomultiplier photocathodes release electrons, dynodes multiply them, and the signals locate scintillations. The collimator improves directional information at the cost of sensitivity.

A PET tracer emits a positron. After slowing down, it annihilates with an electron, usually producing two 511 keV photons travelling in nearly opposite directions. Coincidence detection identifies a line on which the event occurred; many events reconstruct tracer distribution. The emitted photons are not themselves positrons.

Physical decay and biological removal both reduce tracer activity in the organ. If they are independent exponential processes, their decay constants add, so effective half-life is shorter than either individual half-life.

External high-energy X-rays deposit energy in the target volume; beams from different angles and shaped fields concentrate dose while reducing unnecessary exposure of surrounding tissue. Fractionated treatment allows recovery between sessions. An internal beta-emitting implant deposits much of its energy over a short range near the source; positioning and removal or decay determine exposure.

For comparisons, gamma cameras and PET map function through tracer uptake; CT shows attenuation structure; MR and ultrasound use non-ionising mechanisms. Discuss resolution, convenience and radiation exposure in the actual context. A shorter half-life is not automatically better if too little activity remains by the imaging time.

Try each problem before opening the worked solution. State the physical model, show substitutions with consistent units, and explain what your answer means.