One of the things I’ve been working on lately is collect images to fill out my image artifact gallery. Today while acceptance testing a new C-arm, I had a chance to demonstrate what happens when large hunks of metal get moved past or near the image intensifier.
All conventional vacuum bottle based image intensifiers are prone to distortions caused by magnetic fields or changes in the ambient magnetic field (which is what large metal objects do), changing the path of the electrons as they travel from the input photocathode to the output window. Normally, image intensifiers are shielded from any stray magnetic fields by mu-metal. It usually works pretty well, but mu-metal can only do so much. IIs will still be prone to distortion when large enough hunks of metal get moved by.
Below is an image of a linear grid phantom (click on the images to see a larger version in the teaching file gallery). All the lines should be pretty straight and perpendicular.
Below is the same phantom, but now a small stretcher has been placed next to the image intensifier. If you look carefully, you will see that now the lines look slightly S-shaped.
Here a much larger stretcher with more metal in it has been placed next to the II. Now the lines are much more distorted and more S-shaped.
If the large metal object is moved past the II during fluoroscopy, you see this cool twisting distortion effect going on as the object gets closer, and then returning back to normal as the object moves away. If you’ve ever been working on a CRT monitor when a large metal object moves by fairly closely, you will notice a similar phenomenon.
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