1. X RAYS

2. What are x rays?
Electromagnetic waves with a wavelength of between 10-8 and 10-13m
Found between UV and gamma radiation

3. How are X-rays produced?
X-rays are produced when fast moving electrons are rapidly decelerated as a result of striking a heavy metal target.

4. A very high voltage (100 kV) is placed across electrodes in an evacuated tube
Electrons are produced at the cathode which is heated by an electric current passing through the filament.
The high p.d. accelerates the electrons across to the anode.
Most of the EK of the incident electrons is converted to thermal energy of the tungsten atoms, about 1% is turned into X-rays

5. The target gets very hot and must be cooled by passing oil or water through it.
The tube has to contain a vacuum
to reduce the energy loss of the electrons colliding with gas molecules

6. Recall from quantum mechanics
E = hf
E = hc λ
If the accelerating p.d increases then the E of the X-rays increases.
If E increases, f increases and lambda decreases.

7. Recall from quantum mechanics.
E = eV
fmax = eV/h

8. Characteristic X-rays
These are produced when an incident electron knocks an inner shell electron out of an (tungsten) atom. Other electrons move levels to fill the gap. These transitions emit X-rays
These X-rays have definite energies.

9. Bremsstrahlung Radiation
As the incident electrons decelerate when they hit the tungsten target energy is released as X-rays
Bremsen "to brake" and Strahlung "radiation"
Bremsstrahlung = braking radiation
Bremsstrahlung radiation produces continuous spectrum X-rays

11. Calculate the maximum frequency and minimum wavelength for X-rays produced from an X-ray tube in which the accelerating voltage is
A) 80 kV
B) 120 kV

12. How do X-rays interact with matter?
X-rays can be considered as particles or waves
If you pass a beam of X-rays through matter its intensity will be reduced. This is because the X-rays are absorbed or scattered.
The intensity of the beam is attenuated.
There are three mechanisms

13. 1. The photoelectric effect
Incident X-ray photon ejects one of the orbital electrons.
An electron from a higher energy level may drop to fill the vacancy, emitting a characteristic x-ray photon

14. 2. Compton Scattering This occurs for higher incident X-ray energies
The incident photon is scattered by an orbital electron in an atom. Some of the photons energy is given to the electron which then goes off in a different direction from that of the scattered photon. The ejected electron may have an energy from 0 to 2/3 of the incident photon energy.
The greater the deflection of the photon, the greater its energy loss.

16. 3.Pair-pair Annihilation
A high energy X-ray photon interacts with the nucleus of an atom of the absorbing material.
An electron-positron pair is produced and when the positron is annihilated by an electron, two identical low energy X-rays photons are emitted.

18. Intensity W
Wm-2
m-2

19. If the X-ray beam is spreading out from a point source the inverse square law applies

20. In a collimated beam intensity is constant
Collimation
In a collimated beam intensity is constant

21. When a collimated beam passes through a substance its intensity decreases
The amount of absorption varies considerably with the frequency of the X-rays
The absorption of low frequency X-rays is mainly due to the photoelectric effect.
For higher frequency X-rays Compton scattering is the dominant absorption mechanism and for very high frequency X-rays it is pair production.

22. 2 In an X-ray tube, electrons are accelerated from rest through a pd of 72.4 kV before they hit the target anode.
2 (a) Calculate the kinetic energy of an electron as it reaches the anode. Give your answer to an appropriate number of significant figures.
(2 marks)
2 (b) Assuming that the electron gives up all this energy to form an X-ray photon, calculate the wavelength of the photon.

23. Attenuation coefficient = μ (units m-1)
The intensity of the x–ray beam decreases exponentially with the thickness (x) of the substance.
Write down the equation of intensity as the beam passes through a material.
𝐼= 𝐼 0 𝑒 −𝜇𝑥
Attenuation coefficient = μ (units m-1)
Typical values for μ are:
For a vacuum: 0
Flesh: 100 m-1
Bone: 300 m-1
Lead: 600 m-1

24. Half-Value Thickness
This is the distance through a material through which the X-rays must pass that halves the intensity.

25. Using the Equation
Calculate the percentage of the intensity of X-rays not absorbed after passing through 1 cm of flesh, bone and lead.
Calculate the half value thickness of bone.

27. Describe the use of X-rays in imaging internal body structures including the use of image intensifiers and of contrast media
Photographic Film: Requires considerable exposure, produces only a still image.
Quality can be improved using a film that is more sensitive to X-rays
Put a fluorescent plate behind the film.
Use an X-ray absorbing substance as a contrast medium.
Giving patients barium meal (barium sulphate) improves contrast.
Use an image intensifier
Using digital methods instead of film. This includes dots that respond to X-rays.
Can be recorded to give a moving image, or printed.

28. X-ray detectors
Black and white photographic film; requires a beam of high-intensity.
Intensifying screen, which contains a material that absorbs energy from the X-rays and re-emits it as light by fluorescence. This light then produces an image.
Fluoroscopic image intensifier. X-rays cause electrons to be emitted from a photocathode. They produce a bright image on a fluorescent screen. This is used when a moving image is needed.

29. Describe the use of X-rays in imaging internal body structures
Describe the use of X-rays in imaging internal body structures. 2D X-rays
Traditional X-rays show a shadow of the part of the body being imaged. This can be used to show bone breakages.
The shadow will be reasonably sharp provided the X- rays come from a point.
An extended light source will give a fuzzy image.
Draw a diagram to show the difference!

30. 2D imaging can be difficult in certain situations!
Tibia blocking the Fibula
Ulna blocking the radius
X-ray of the chest cavity
Radiographer not positioning person correctly
These can cause overlap and block out the desired area.

31. Angiograms
These are obtained by a method called subtraction technique.
X-ray is taken and then digitised.
A contrast medium is then injected, and another X-ray is taken and digitised.
The first image is “taken away” from the second, so that only differences are shown.
Eliminates all the detail that is not required.
Computer can be programmed to get rid of patient movement.

32. Computerised Axial Tomography
Computers along with X-rays can be used to give 3D images of the body.
Images must be taken from different viewpoints.
Final image is generally a slice of the body taken horizontally.

33. CT Scan The X-ray source is shielded.
X-ray beam emerges from a point and spreads out through the patient.
Fan shaped, very thin beam.
Irradiates only very small sections at a time.
X-rays are detected by a ring of up to detectors.
Once it completes one revloution, it has moved up 1 cm.
This can take slices, or compose a 3D image of an organ.

34. CT Scans
There is a dose of radiation, but this is less than it used to be because of increased sensitivity of the sensors.
Advantage: Can be taken quickly, so can have many in one day.