Does the temporary manipulation of JPEG archives affects the image's optical density?

Abstract

In order to evaluate if temporary manipulation of digitized images acquired in the JPEG format affects its optic density, 26 periapical radiographies were digitized, stored in the JPEG 12 format and digitally reproduced three times. One of the sets did not suffer any alteration and was called JPEGd. The others had their image imported to the Adobe Photoshop program and were submitted to one or two alterations, which were undone and saved, constituting the JPEG1a and the JPEG2a groups, respectively. The program ImageLab was used for the digital subtraction of the following pairs of images: JPEG x JPEGd, JPEG x JPEG1a and JPEG x JPEG2a. The average density values were obtained, as well as the standard deviation of each image. In all of them the average value was 128, however some variability observed in the standard deviation. The average standard deviation (0.50, 1.47 and 1.50, for JPEGd, JPEG1a and JPEG2a, respectively) of each group were submitted to the Variance Analysis complemented by the Multiple Comparison Dunnet T3 Test. Significant differences were found in the standard deviation values among the JPEG x JPEGd and JPEG x JPEG1a/JPEG x JPEG2a. We conclude that the JPEG file manipulation does not interfere in the average value of optic density, however it takes to the increasing of the standard deviation, a fact that can influence studies of this nature.