[wmkn]

For the EU what is a 'medical device' is pretty well described in 93/42/EEC which is the law describing the medical CE process (http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLE...)

There are quite a bit of edge cases though, so the definition of medical device actually has a guidance document that gives a bit more context (http://ec.europa.eu/DocsRoom/documents/10278/attachments/1/t...).

> I worked producing software that segmented images produced by a medical imaging device. The software would be built-in but it was the kind of thing one could have also have done exporting the images to Photoshop.

Segmentation software could certainly be considered a medical device if it has a medical purpose. For instance, if the segmentation is used to produce a diagnosis or a measurement. If you would like to sell it to medical professionals you would be required to have a medical CE for the software. Photoshop might have the same functionality, but it does not have a medical CE, so in principle it should not be used by medical professionals.