[wwwv]

Completely transparent as far as a luggage X-ray is concerned, they're very high power because they don't have to care about exposure limits like medical imaging would. They usually have multiple attenuators and detectors so they can do neat tricks like detection of organic materials.

[londons_explore]

But they usually can't detect an organic material behind a varying thickness metal plate.

And metal in laptops is often varying thickness (either due to the design, or poor manufacturing tolerance), hence the concerns that organic explosives can be hidden in their "xray shadow".