7. what is an x-ray image Flashcards
LOs
- Describe the shadows that make up a conventional x-ray image
- Explain the various factors that can affect a conventional x-ray image
- Describe the different types of x-ray images that are available including film-captured images, tomographs and digital images
- Explain the limitations of these various different x-ray images
how is a radiographic image produced
- by x-rays passing through an object and interacting with the silver halide emulsion on the film or with a digital image receptor
- interaction results in blackening of image once processed
- final image = 2-D, superimposed
what determines the exten if blackening in the images
- n.o x-rays reaching the image receptor (this depends on density of object)
radiodensity of shadows/ colours in radiograph?
why this colour?
Black = radiolucent – no absorption
White = radiopaque – total absorption
Grey = mixed – varying absorption
what to consider when understanding radiographs?
Understanding factors affecting the shadows/ colours in the radiograph
* Object Material
* Object thickness/density
* Object Shape
* X-ray beam intensity (exposure factors)
* Position of object, image receptor and X-ray beam
- knowledge of anatomy hard tissues
- Appreciation of limitations of a 2D picture effect of superimposition
- knowledge of diff types if images and their limitations
what is burnout?
why may it occur?
clinical impact?
- as exposure increases, there is increased blackening (this is known as burnout in which small, fine structures of patient does not create a
radiographic shower) - using an intensity too high
- may prevent from seeing fine small structures
clinically why does bone produce a varied appearance radiographically?
- varies in thickness + density
EGs (AZ NOTES) (DON’T NEED TO LEARN, JUST UNDERSTAND)
Various factors affect these shadows:
* Object Material
- metal objects will appear completely white
- Object thickness/density
- larger thicker cylinders absorb more x-ray photons and appear white
- Object Shape
- the 2D image doesn’t always represent the full picture as there is the 3 rd dimension to consider
- X-ray beam intensity (exposure factors) - VARIES
- Position of object, image receptor and X-ray beam
- distortion of image may occur if positioning wrong
Ideal position of object, image receptor and X-ray beam for imaging?
effect of changing position of object, image receptor and X-ray beam?
- image foreshortened
- image elongated
- image distorted
Surface knowledge of anatomy for X-rays
Mandible/maxilla
Hard tissue of tooth structure
How roots are positioned
Anatomical cavities in jaws
- e.g maxillary air sinus
Internal anatomy of maxilla/mandible
- trabecular pattern of mandible
- cortical desne bone forms lower border of mandible
- limitations imposed by 2-D images?
- how these may be overcome?
1.
X-rays are a single 2d view
Can’t accurately tell what the 3D structure would look like
2.
Maybe take multiple radiographs with 2 views
E.g parallel + perpendicular
- maybe not reliable due to superimposition of overlying structures
3 main types of images
Conventional film captured images
Tomographs (sectional radiographs)
Digital images
What are the limitations of film captured images ?
= standard X-rays used in most clinics
2D representation of 3D objects
Affected by changing exposure factors
- kV, mA and time
Affected by geometrical distortion
Affected by chemical processing faults
Effects of exposure factors on film captured images
KV
- determines energy of photons/ quality of beam
- determines penetrating power of photons
- affects film contrast
- kilovoltage increased, image = greyer with many more shades of grey;
- KV decreased, image = black and white with fewer shades of grey
mA and TIME (secs)
- determine quantity of x-ray photos
- affect degree of blackening of film
- overexposed = too black
- underexposed = too pale
What are simple linear tomographs ?
Produced by moving the film + X-ray tube head during exposure
Centre section of patient = linear focal plane
- only this section of patient is imaged
- other structures in front + behind on centre section are blurred out due to movement
- thickness of centre section determined by movement taken place
- large movement produces a thin slice
- small movement produces a thick slice
- Blurring is common due the movement of equipment
What are complex multidirectional tomographs?
Uses complex spiral motion between X-ray tube head and film (instead of linear)
Still has a centre of rotation - corresponds to slice that will be I’m focus
Designed for use in maxillofacial region
- cross-sectional slices of jaws
why do you need to understand about tomographs?
How do panoramic machines work ?
X-ray tube head and film move around patients head
Tube head starts on RHS of patient
Tomographic layer starts on LHS
- curved section
- corresponds to shape of dental arch
- so teeth and supporting structures will be depicted in focus on final image
X-ray beam has to pass through structures on left and right before hitting film
- RHS structures (don’t want to see) can cause a shadow on LHS structures (want to see)
Limitations of panoramic radiography
Malpositioning of teeth can cause them to be blurred + out of focus
- wont fall inside width of sliced section
Operator can affect panoramic image by mispositioning the patient
X-ray beam has to pass through both sides of jaws
- creates ghost shadows
- appears at a slightly higher level than real shadow as X-ray beam angled upwards at 8 degrees
What are the real hard tissue shadows?
Nasal septum
Hard palate
Zygomaticus / cheekbone
Maxilla
Mandible
Hyoid bone
What are the equipment shadows ?
Bite peg
Plastic head support
What are the ghost shadows?
L shaped mandible
Cervical spine
Airspace between tongue and hard palate
