Diagnostic X-Rays
Physics: AQA A Level, topic 10.5.1
Diagnostic X-Rays
Each type of atom (or element) has its own characteristic electromagnetic spectrum and its own X-ray output. We can examine these to identify a material.
The electromagnetic spectrum
- X rays lie at the high-frequency end of an atom’s spectrum and are characteristic of each atom.
- We are able to look at these characteristics and use them in medical imaging.
X-ray production
- Electrons are emitted from a hot filament and then accelerated with a high voltage, this gives them a large kinetic energy.
- The electrons are then fired at the anode.
- There are two processes by which x rays are produced.
- The deceleration of electrons produces x rays, and these x rays are called bremsstrahlung, or braking radiation.
- The atomic structure produces characteristic x rays from electrons moving shells.
X-ray curve
- An X-ray spectrum is obtained when energetic electrons strike a material.
- The smooth part of the spectrum is called bremsstrahlung radiation.
- The peaks are characteristic of the anode material.
- A different anode material would have characteristic X-ray peaks at different frequencies.
Rotating Anode X-Ray Tube
X-rays to be used on patients can be created using a rotating anode X-ray tube.
Rotating anode
- The anode is a tungsten disc, rotated at high speed (3000 rpm).
- The rotation ensures that the target area is spread, permitting larger tube currents and voltages to be used, and hence more powerful X-ray beams to be produced, without overheating the target.
Beam intensity
- The beam intensity increases with an increase in the applied voltage across the tube and with an increase in the tube current.
- The X-ray photon energy is a continuous spectrum with a maximum that depends upon the voltage between anode and cathode.
Characteristic X-rays
- The 'characteristic' X-ray photon energy depends upon the changes in electron energy level that produce the photon.
- These depend upon the structure of the target material but are fixed for that material.
- Common anode materials are tungsten, molybdenum and rhodium, which are used for specific purposes because of the different characteristic photon energies.
Image produced
- The contrast between parts of the X-ray scan depends upon the proton numbers of the materials being scanned.
- Image sharpness depends upon the size of the anode and the physical dimensions of the X-ray set-up, as well as blurring caused by movement of the subject.
Patient safety
- The patient dose must be kept to a minimum, which is aided by using image intensifiers and improved detection devices.
Test yourself on Diagnostic X-Rays
Which type of electron production creates the characteristic lines?
Which of these statements are true about X-ray imaging?
Why do anode X-ray tubes rotate?
Which of these is NOT commonly used as the anode in X-ray tubes?