I took at least 10 seconds to spot it. This image was used in a study with radiologists, where they were asked if there is something wrong with the image. 83% of radiologist didn't see the gorilla. This doesn't mean that the radiologist don't pay attention to the images, but rather that they will not see what they aren't looking for... Please see the video that inspired this study in the links and understand better what this means.
Links:
http://www.theinvisiblegorilla.com/videos.html
http://life.nationalpost.com/2013/02/13/dancing-gorilla-image-on-ct-scan-passed-over-by-more-than-80-of-radiologists-study-finds/
