Photons interaction with matter

Transcription

1 ب س م هللا الر ح من الر حیم Photons interaction with matter

2 Ionization Ionization is the process of removing an electron from an electrically neutral atom to produce an ion pair. An ion is an atom or subatomic particle with a positive or negative charge. Ionization negative ion (electron) + + positive ion: atom with 3 protons, 2 electrons

3 Photons interaction with matter

4

5 Coherent scattering

6 Coherent Scattering primary electron x-ray vibrates strikes electron in outer shell scattered x-ray emitted with same energy as primary x-ray, different direction

7 Photoelectric Absorption ejected electron (negative ion) atom with net positive charge (positive ion) The primary x-ray strikes an inner-shell electron, knocking it out of its orbit (ionization). The x-ray loses all of its energy and disappears. There is no scatter.

8 The photoelectric effect

9 Compton Scattering primary x-ray ejected electron (negative ion) scattered x-ray atom with net positive charge is positive ion The primary x-ray strikes an outer-shell electron, knocking it out of its orbit (ionization). The primary x-ray loses some of its energy and continues in a different direction as a scattered x-ray.

10 The compton effect

11

12 The pair production effect

13 The photodisintegration effect

14 film body X-Ray source Dark Light

15

16 Filtration

17 Filtration Aluminum filter(s) Added Glass window of x-ray tube Inherent Oil/metal barrier Total 2.5 mm 70 kvp 1.5 mm The amount of filtration is regulated by the government (NCRP). Any machine capable of operating at a kvp of 70 or above must have 2.5 mm aluminum equivalent of total filtration. If the maximum kvp of the machine is 65, only 1.5 mm aluminum equivalent is needed. The manufacturer automatically provides this on x-ray machines.

18 Beam restricting devices

19 Beam restricting devices

20 Grid

21 The x-ray film

22 Gelatin Emulsion Substratum Base

23 Many different types image receptors are used in modern diagnostic radiology. They all have in common that they form an image by absorption of energy from the X-ray beam (after transmitting through the body). The main characteristics will be discussed on the example of the direct exposure film. Direct-Exposure Film Direct exposure film has a relatively low absorption efficiency for photons in the diagnostic range, however it is still used in many combinations of image receptor systems. Direct exposure film material has special design of two photographic emulsions with protective layers between to optimize the absorption efficiency. Correct exposure is important to produce a reliable image on the film. Over- or underexposure will result in loss of contrast and therefore possibly in loss of diagnostic information.

24 The correlation between the optical density D and the maximum number of sensitized grains results in a relation between the optical density D and the received dose X : This is displayed in the figure. The curve relating the optical density to the film exposure dose is called the characteristic curve of the film material. In the center part of the curve the relation between optical density and the logarithm of the dose is approximately linear:

25 Intensifying screen

26 Intensifying Screens Calcium Tungstate Barium Strontium Sulfate Rare Earth Lanex (used with T-Mat film) Ektavision Intensifying screens for panoramic or cephalometric imaging should be a type of rare earth screen (greenlight emitting). These screens require less radiation exposure (than blue light emitting) with no loss of image detail. Patient exposure is reduced.

27

28 Image intensifiers convert X-ray photons to electrons by photoelectric effect on a photocathode. The electrons are focused onto by electrical fields on a fluorescent screen where they form an intensified image which can be recorded on film or viewed with a TV camera.

29 The END - Wake up!!!