X-ray imaging works by basically drawing an outline of where x-rays aren't in an image.
You have an emitter, a subject, and a sensor. The emitter shoots out x-rays, the subject absorbs/deflects some of the particles, and the sensor picks up what's left.
The Predator's helmet would act as the sensor, collecting x-rays from the environment to produce an image. The problem is, no natural source produces enough x-rays to show what we see, and most man-made sources don't either.
So either the Predator sees a highly enhanced x-ray image, or it could be some kind of spiffy augmented reality image based on information from a number of inputs giving a virtual view of something's insides.