"Scanner" is a pretty commonly used term in science fiction stories. It's one of those terms, like "antigravity" or "blaster," that an author can use without any explanation. In 1977, its meaning was sufficiently well established that Philip K. Dick could spin it as wordplay in the title of A Scanner Darkly. It was also used in Star Wars without any comment.
However, it is not necessary to go back that much further, to find a time when the meaning did not seem to be so fully ingrained in the SF culture. Cordwainer Smith wrote "Scanners Live in Vain" in 1950, with a very different kind of "scanner"; the discrepancy in meaning was a bit weird when I first read the story.
So, I presume, that sometime between the 1950s and 1970s (not coincidentally, I suspect, an acme of science fiction writing), the term "scanner" came to have its now-standardized meaning. According to the Oxford English Dictionary, the meaning of "any device for scanning or systematically examining all parts of something" can be dated back to 1927, but the earliest citations are basically compositional and refer to real-world, not SF, technologies. So at what point did the meaning become basically standardized in the genre?