Species: Feline | Classification: Miscellaneous
Interaction of x-rays with tissues
- A photon of electromagnetic energy with energy just slightly greater than the electron binding energy ejects an electron from an inner shell of an atom.
- The photon gives all its energy to the electron in displacing it and is truly absorbed.
- The displaced electron can ionize other atoms.
- An electron from a higher orbiting shell drops into the vacant space releasing energy → characteristic radiation.
Features of photoelectric absorption
- Important at low energy.
- Proportional to cube of atomic number → small variation in atomic number ie in body tissue → large contrast in film exposure.
Photoelectric absorption is important at kV<100 and at high kV reduced film contrast occurs because photoelectric effect less important.
- An electromagnetic photon with energy greater than the binding energy of the electron interacts with an electron in one of the shells of an atom.
- The photon displaces the loosely bound electron which can ionize other atoms.
- The photon is diverted and continues in a different direction with a lower energy → scattered radiation.
Features of Compton absorption
- Predominates at higher beam energies than those susceptible to photoelectric effect.
- As energy increases more of 'scattered radiation' is directed in a forward direction ie more likely to reach x-ray film (effectively not 'scattered' therefore).
- Independent of atomic number of tissue, for a given beam energy.
Compton scatter is significant at kV >70.
Production of scatter
- Scatter can be produced when x-rays interact with matter.
- Lower energy than primary beam.
- Travel in any direction.
- Very important in large animal radiography.
- At high kV a higher percentage of the radiation is incident on the film. Since more of the scattered radiation is moving in a forward direction towards the film there is an increasing fogging of the film at high kV and contrast is reduced.
- Increases with increasing volume of tissue irradiated.
Effects of scatter
- Increases radiation exposure to personnel.
- Increases radiation dose to patient.
- Reduces film contrast (increases overall film density in a non-specific way).
Reduce scatter production
Collimate x-ray beam
- Reduces the radiation dose reaching the patient and therefore the volume of tissue being irradiated.
- Reduces volume of tissue irradiated (by displacing tissue mass outside primary beam).
- Can be achieved using Bucky band - a webbing strap which can be tightened around the body (particularly abdomen).
- Reduces the amount of scatter produced.
- Reduces the amount of scatter reaching the film.
Reduce scatter affecting film
- Placed between film and patient to absorb scatter.
- Most scatter is traveling in an oblique direction and therefore is unable to pass through grid.
- Results in increased exposure factors required.
- Grid lines can appear on film.
Alternative filtration devices
- Air gap between patient and film.
- Radiation traveling obliquely misses film.
- Important in large animal radiography where film is often some distance from object.
- Air gap increases magnification and reduces image sharpness Radiography: image quality so need to increase focal film distance to compensate.
- Filter between patient and film.
- Intensifying screens Radiography: cassette and intensifying screen act as a sort of filter.
Not practical since primary beam also significantly attenuated by filter.
Lead backing to film cassettes
- Absorbs radiation penetrating film and screen.
- Prevents back scatter.
Reduce effects of scatter on film
- Intensifying screens Radiography: cassette and intensifying screen (particularly rare earth) intensify primary photons more than scatter so that film contrast is enhanced and effect of scatter is reduced.
- Increase film contrast by reducing general fog of film by scatter.
- Parallel strips of lead separated by radiolucent interspacers (plastic with aluminum surround).
- With 30-60 strips per cm.
- Primary rays are traveling virtually parallel and pass through interspaces.
- Scattered radiation, traveling obliquely, cannot penetrate lead
- Lead strips are parallel and equal height across the grid.
- At edge of grid, effect of diverging beam is apparent and some primary rays are absorbed.
- Reduced exposure density at periphery of film.
- Lead strips are angled from center .
- This accommodates for the diverging pattern formed by the primary beam.
- center of x-ray beam must pass through center of grid.
- Must be used right way up!
- Pseudofocused gridshave shorter lead strips at edges than in middle .
- Two grids of low grid ratio at right angles to one another.
- Central axis of incident beam perpendicular to the grids.
- Blurs grid lines by moving grid across film during exposure.
- Grid must move 3-4 interspaces to be effective.
- Need special x-ray table and Bucky linked to machine so that movement of grid linked to exposure.
- Grid pulled at uniform speed across film.
- Moves about 2.5 cm in 0.2-15 seconds.
- Grid moves backwards and forwards during exposure.
- Moves 1.25 cm either side of center over 0.5 seconds.
- How much the exposure factors must be raised to compensate for use of grid.
- Depends on
- Number of lead strips per cm.
- Thickness of strips.
- Determined by taking x-ray and noting exposure → add grid and increase exposure to get back to same film density.
- Usually 2.5-3.
- Greater for parallel than for focused.
Contrast improvement ratio
- Measures the improvement in contrast created by using a grid.
- Calculated by dividing film contrast with a grid by that without.
- Usually 1.5-3.5.