What device is used to measure the thickness of body parts for exposure factor selection?

The device used to measure the thickness of body parts for exposure factor selection is the caliper. Calipers are specifically designed to provide accurate measurements of the dimensions of an object, making them ideal for assessing the thickness of various body parts. This measurement is crucial in radiographic imaging as it helps determine the appropriate exposure factors, such as kVp and mAs, to ensure optimal image quality while minimizing patient exposure to radiation.

Other devices mentioned do not fulfill the same function. For instance, a fulcrum is typically associated with the pivot point for levers and does not measure thickness directly. A densitometer is used to measure the density of film or image receptors but does not provide measurements of body part thickness. A ruler, while it can measure length, is less precise than a caliper when it comes to measuring the thickness of irregular body shapes, which is often crucial in medical imaging procedures.