Quantification of tissue self-absorption of weak beta-radiation in lyophilized whole-body sections of rats

Whole-body autoradiography has been widely used in the investigation of the distribution of radiolabeled compounds in animals. The newly introduced radioluminography offers a reliable way of quantifying the radioactivity distribution within whole-body sections. Since the radioactivity is distributed over the entire depth of the section, self-absorption of beta-radiation in tissues is supposed to relevantly affect the detection of radioactivity at the section surface. The self-absorption of radiation energy ((14)C) was investigated in 28 organs/tissues of routinely produced lyophilized rat sections. Nonradioactive whole-body sections with different thickness between 20 and 120 microm were placed between a homogeneous (14)C source and the imaging plates to detect the transmitted radioactivity. The self-absorption was expressed in terms of percentage of transmission of the radioactivity through the sections. Transmission decreased with increasing section thickness, e.g., from 44% (20 microm) to 28% (120 microm) for blood. Comparison of three complete sets of data disclosed intertissue variations of up to about 30% (i.e., +/-15%) disregarding bone. A defined bandwidth of +/-15% around the blood transmission would cover the transmission of almost all tissues. Thus, for most organs radioactivity can be quantified by direct comparison with radioactive blood calibration samples.