What exactly causes it when a metal object is placed near the scanner?
Perhaps the x-rays are reflecting/scattering off of the metal?
See 'scatter" on page 7: http://www.glidewelldental.com/downloads/dentist/services/nobel-guide-glossary.pdf [Broken]
It is part of the math of the reconstruction. When you take an x-ray you get everything on top of each other in a single projection, so in a chest x-ray you see the spine and the ribs and the lungs all on top of each other. A CT works by taking a bunch of projection x-rays all around a circle and then mathematically untangling them so that the ribs are separate from the spine and the lung etc.
If you acquired an infinite number of projections, each with infinite resolution, then you could reconstruct the image exactly, but since it is finite you get what is called a point spread function. The point spread function has "streaks" in it which become visible for very bright objects, like metal.
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