RTBC X-ray Tube and Components Practice Test

When the anode angle is larger, two geometric and thermal effects happen together. First, the apparent focal spot—the size of the heated region as seen by the x-ray beam—becomes smaller. This is because the projection of the actual focal spot onto the image plane is inversely related to the sine of the angle: increasing the angle tightens that projection, reducing the effective focal spot size and helping improve image sharpness.

Second, a larger angle exposes a greater area of the anode surface to electron bombardment, which means more surface area is available to absorb and dissipate heat. This increases the heat capacity of the anode, allowing it to sustain higher heat loads without damage.

So, a large anode angle yields a smaller focal spot and greater heat capacity. The combination described by this is decreased focal spot size and increased heat capacity.