Answer to Question #4650 Submitted to "Ask the Experts"

Imaging and localization studies in 10 CFR 35.200 are not part of uptake, dilution, and excretion studies in 10 CFR 35.100.

35.200 is for imaging and localization studies. Imaging requires a photon emitter with sufficient energy that can be imaged—usually with a gamma camera. Localization studies refer to studies where the localized radioactive material is detected by a radiation probe. The most commonly used isotope for imaging and localization studies is ^99mTc, but ¹³¹I and ²⁰¹Tl are also used. These materials can be injected, inhaled, or ingested. The same radionuclide (i.e., ^99mTc) can be attached to different molecules for selective absorption by different organs and tissues to produce images of bone, kidneys, lungs, liver, etc.

35.100 uptake, dilution, and excretion studies involve injection, inhalation, or ingestion of radioactive material into the patient and then removal of a bodily fluid such as blood or urine to test for uptake of the radionuclide by the blood or excretion of a radioactive component. Radiation-detection equipment is usually used to quantify the uptake, dilution, or excretion rates. These studies involve much lower levels of radionuclides that may also be used in 35.200 as well as low-energy photon emitters and beta emitters. The radioactivity measurements are made in laboratories with sensitive "bench top equipment" and not imaging cameras. Historically 35.100 studies have included ⁵¹Cr chromate for red cell survival and red cell mass studies, ⁵⁷Co vitamin B-12 absorption studies, ⁵⁹Fe citrate for iron absorption, iron plasma clearance and turnover, and iron red blood cell uptake studies, ¹⁴C urea, and tritiated water. New, more sensitive nonradioactive tests have replaced many of the historical 35.100 studies.

Cynthia Jones, PhD, ATE Editor
Donna-Beth Howe, PhD