Factors in Measurement of Dose by Gamma Scintigraphy

Abstract

ABSTRACT¹

Gamma camera scanning offers researchers the opportunity to measure both total and regional deposition of drugs in the lungs and other parts of the body. While a number of clinical studies have measured patterns of drug deposition, there are few data assessing the gamma camera's ability to quantify deposition of an inhaled drug. Like the geiger counter, the gamma camera provides a measure of radioactivity [e.g. counts per minute (CPM)] but not the absolute number of microcuries (μCi) of a given substance deposited in the body. To relate CPM to μCi, it is necessary to represent the geometry of the appropriate organ such as the lungs and its surrounding tissue by suitable correction factors [attenuation coefficients (AC)]. These coefficients convert the measured count rate into an absolute quantity of deposited isotope. Different techniques have been used to estimate AC; for example, "models" of the thorax (lung phantoms), direct measurement of thoracic attenuation (perfusion scanning) and independent measurements of lung deposition (e.g. via mass balance techniques) to allow calculation of an AC. The simultaneous use of independent techniques can be used to confirm gamma camera accuracy and precision. In addition to the complex geometry associated with the thorax itself, the regional distribution of deposited particles within an organ (e.g. central vs peripheral airways) further complicates estimates of deposition using global AC (ie a single AC for the whole lung).