Final answer:
The part of the x-ray tube that controls the penumbra includes the focal spot size on the anode and the use of collimators to shape the x-ray beam, aiming to minimize the blurred edge of the image for greater clarity.
Step-by-step explanation:
The part of the x-ray tube that controls the penumbra involves the focal spot size on the anode and the various collimators or diaphragms used to shape the x-ray beam. The penumbra is the blurred or less distinct peripheral edge of the image created by x-rays, which ideally should be minimized for a sharper image. The smaller the focal spot size, the sharper the image and the lesser the penumbra. Collimators can also help control the penumbra by limiting the size and shape of the x-ray beam, reducing the spread of the beam as it exits the tube and interacts with the tissue being imaged.