Measurement of Radioactivity in Body Organs

This paper reviews methods for the measurement of radioactivity in body organs in which corrections are applied for the depth of the organ under study in the body tissues on the basis of observations of the counting rates due to two photons of different energies which are differently attenuated in the tissues. The photons in question may be derived from a single radioisotope or from two radioisotopes of the same element administered simultaneously in the same chemical form. Calibration is usually effected with an external standard. The mathematical basis of these methods is described and they are shown to have wide applicability. Other methods involving the use of coincidence counting techniques to observe the pairs of photons emitted in cascade by certain radioisotopes are similarly reviewed. These methods are absolute in nature and do not require calibration of the measurements. Their applicability is strictly limited to measurements on point sources of radiation. Nevertheless, they offer promise for the measurement of ¹²⁵I in the thyroid gland. (author)