A Rotational Technique for Assessing Quantity and Distribution of Body Radioactivity

Abstract

A new whole-body counter is described that provides a method for the determination of the amount and distribution of radionuclides in the human body. The quantitative measurements are relatively independent of differences in the distribution of the radioactivity in the body or the anatomical differences of the individuals who received the whole-body counts. The measurement is taken with the human subject rotating on a power-driven turntable while the scintillation detectors move down the longitudinal axis of the rotating body. The result of rotating the body with respect to the detector is the elimination of effects caused by changes in location of the radioactivity and the differences in the absorption; essentially the effects of these factors are averaged. Because sources of activity that are displaced from the axis of rotation in a revolving sample or human body produce oscillations in counting rate, the approximate location of the radionuclide can be determined at the time of the quantitative measurement. The position of the detector with respect to the different parts of the body can be defined as a point along an imaginary helix if the measurement is taken while the detector is moving down the longitudinal axis of the rotating body. The location of the radioactive source is indicated by increased counting rate at points along this helix which surrounds the body. Counting rate, linear distance of the detector down the body from a reference point, and body orientation with respect to the detector are the three variables for defining the position of activity. This technique of counting is especially applicable for the in vivo determination of radionuclides in the human subjects because both the body radioactivity and approximate distribution can be determined simultaneously. In addition to whole-body counting, the combined rotation and scan technique is also a method for the determination of activity in biological samples in which large volumes are required to obtain adequate sensitivity. The procedure can be used for any type of solid samples that contain an unknown distribution of activity. The density of the matrix material in which the activity is conctained and the distribution of the activity in the sample are discussed as factors affecting absolute counting. Applicability of the method to in vivo whole-body counting has been confirmed by counting 131I and 54Mn at various locations inside a human volunteer. The maximum variation of counting rate for different positionings of the sources in the body was 400 and 19 per cent for the static counting method and the new rotation method, respectively. The counting sensitivity is comparable for the two methods if the volumes of scintillation detectors are equal.