6: Explaining the radiographic image Flashcards
How is a radiograph created (6 steps)
1) Beam of xray photons generated by the xray tube is directed at a person
2) A detector is placed behind the object + receives the transmitted xray photons
3) The transmitted xray photons not attenuated form the image
(The xray photon beam is attenuated as it passes through the object, some are absorbed while others may be deflected or scattered- this is dependent on the objects density, thickness and structural composition as well as the energy level of the photon beam)
4) Computer algorithm constructs the image and displays it
Transmission
Photons pass straight through body with no interactions with the body atoms
Photoelectric effect
Photons collide with the body’s atoms
Entire energy of the incident photon is absorbed
This energy is then transferred to a single inner shell electron
Which is then removed from the electron cloud
Does not contribute to the resultant image
(Low energy phenomenon)
Coherent/ Rayleigh scatter
X-ray photon comes in, interacts with electron cloud and goes out scattered however still maintains the same energy
Compton scattering
Photons collide with the body’s atoms
The incident photon transfers only a part of its energy to an outer shell weakly bound electron
The interacted photons are scattered by the electrons
Factors affecting beam attenuation
Thickness of the examined region
Tissue type (higher Z = more absorption)
Tissue density
Radiopacity / radiopaque
Bright regions on the radiograph, structures which have attenuated the xray photons
Radiolucent
Dark regions, structures easily penetrated by xray, very little attenuation
Optimum xray image quality (6)
Accurately displaying the ROI
Minimal distortion or magnification
Sharp clear image, no blurring
Balance between brightness and contrast
No avoidable artefacts present
Suitable for diagnosis
Image density
Amount of image blackening
Low contrast structure
Show little difference in density between structures
High contrast image
Show larger difference in density between structures
Radiation contrast
The difference in xray intensities
Radiography contrast
The difference in image density
FFD/ SID
Film focus distance/ Source image distance
