RTBC X-ray Tube and Components Practice Test

The key idea is how the line focus (anode angle) changes the apparent size of the focal spot on the image. The actual focal spot is fixed, but because the anode surface is tilted, the focal spot is projected onto the image plane and appears shorter along the beam direction. This apparent or effective focal spot size is smaller when the anode angle is smaller, which reduces geometric unsharpness and increases spatial resolution. In this sense, the smaller the angle, the finer the detail you can resolve, because the image blur from the focal spot is reduced.

So, with the same actual focal spot, the smallest angle will give the smallest effective focal spot and therefore the highest spatial resolution. Among the options, the smallest angle is 4°, which would yield the sharpest image in terms of the direction of the beam (though in practice you’d balance this with heel effect and heat considerations).