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It's called Backscatter Xray. Medical x-rays are made to penetrate an object but backscatter xray detects radiation that reflects from the object. It's weaker than normal xrays and can't penetrate things much.
The tires aren't "hot". They are reflecting Xrays. If this was an infrared device the tires might be "hot" but so would the ground leading up to the tires (braking turns forward momentum into energy that is pushed into the ground which becomes heat). The brakes themselves would also be very hot. Other parts of the truck such as the engine, exhaust and top or side of the panels might be hot as well if it was in the sunlight. If infrared could "see" through metal, the heat it would "see" would be too dispersed so those people would look "blurry", and not sharp as they appear in this image.
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