Answers to revision questions

Chapter 4: The X-Ray Tube

1. Describe the structure of the cathode.

The cathode is made of a tungsten wire in a spiral. This is surrounded by a focussing cup, which ensures the electrons generated move in a tight group towards the anode.

2. How is broad and fine focus determined?

Broad and fine focus is determined by the length of the tungsten wire in the cathode. A short wire will give fine focus, a longer wire will give abroad focus. This is fixed in some x-ray machines, whilst in others it can be manually selected. In small animal practice a fine focus is required but broad focus is ideal for large animal chest imaging.

3. What function does the focussing cup have?

The focussing cup prevents the divergence of the electron cloud between the cathode and the anode, ensuring the smallest focal spot size possible.

4. How are the electrons necessary for the production of x-rays produced?

Electrons are produced in the filament wire. This is heated and the heat releases electrons from the wire. The higher the mA selected, the more electrons produced.

5. What is the difference between a stationary and rotating anode?

A stationary anode does not move. It is made of a copper rod with a tungsten target area. It is only suitable for lower energy examinations. A rotating anode consists of a tungsten target mounted on a molybdenum support. This rotates, ensuring that no one spot is exposed continually to the electrons. Continual bombardment on the same spot would cause damage to the target and overheating within the tube.

6. What is the focal spot?

The focal spot is the point at which x-rays are made.

7. What is the effective focal spot?

This is the size of the focal spot used in the production of the radiographic image.

8. Give three parts of the tube housing.

The tube housing consists of an aluminium outer layer which deflects heat. An inner layer of lead which prevents the escape of x-rays and a layerof oil which dissipates heat.

9. Describe three ways that the x-ray tube can be protected.

The x-ray tube can be protected by the use of set protocols, warming the tube up before use each day and the proper regular maintenance.

10. What is the tube rating?

The tube rating describes the amount of electrical energy (load) that can safely pass through the tube during each exposure. This will look at the heating of the tube with each exposure or multiple exposures. It is measured in heat units.