Evaluation and interpretation of grain density autoradiographs by reflectance photometry

The tritium content of nervous tissue sections is calculated from grain-density of autoradiograph without loss in anatomical information. The calculation is not based on determination of (expected) grain numbers but on photometric measurements (SKphot), interpreted as the result of the function of exposure time (T) and incorporated activity (beta): SKphot = f(T, beta). As experimental radiation standards homologous material (optic nerve preparations) with different activities determined by liquid scintillation counting has been used. Exposure times between 12 h and 28 weeks for these preparations with known radioactivities yielded a set of curves (nomogram) relating photometric values of exposure times. At any given exposure time the knowledge of photometric readings and related radioactivities within that set of curves enabled the development of a function covering all photometer readings within the range of the nomogram. Thus, any photometric reading could be correlated to the corresponding radioactivity. Such a function has been applied for the transformation of photometric values obtained from particular brain-section areas exposed for 16 weeks. Since the results are given in Bq, they may be related and, if necessary, corrected to results obtained by other tritium detection methods. Terminology used in the text: experimental radiation source = experimental standards prepared from optic nerve sections; standard radiation source = polymer reference source.