[Automatic setting of the contrast range of whole body scintigrams using a histogram method]

To get standardised results on the display of whole body scintigrams regarding an optimal contrast setting (using linear scaling), an algorithm was developed, which can be used for an automatic setting of an upper intensity truncation value.

From the integral of the gray level histogram of 30 randomly selected whole body images upper threshold values LA were defined, where a certain area percentage A (80% < or = A < or = 99.5%) of all pixels with intensity > 0 are contained in the intensity range. Independently an interactive upper level Lvis of the contrast range was determined by two observers to get an optimal visual appearance of the image.

For the threshold levels with an area percentage A of 92% ... 98% (L92 ... L98) a very close linear correlation (r > 0.9) was found to the visual levels. Levels resulting to area percentages A < 92% or A > 98% correlated worse to the interactive values. Based on our results we propose as a good definition for an upper threshold on whole body scintigrams with dual intensity ranges: Range 1 (high activities): Upper threshold = min (2.5 x L95, 100%), Range 2 (low activities): Upper threshold = min (1.25 x L95, 100%).

The procedure is well suited for an automatic definition of the contrast settings of whole body scintigrams. It can be implemented easily; its use gives a substantial contribution to the standardisation of the documentation of scintigrams.