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
- effects of processing faults on film captured images?
- clinical impact?
1.
Over developed = too black
Under developed = too pale
Under fixed = brown
Solutions too hot
2.
chemical processing errors limit clinical usefulness of conventional film captured images
1.
What are tomographs ?
2.
benefits?
1.
Sectional radiographs
2.
‘Slice of an object’
Object is split in multiple sections (eg like loaf of bread analogy)
Avoids problems caused by superimposition
Different types of tomographic images of the maxillofacial skeleton
Simple linear tomographs
Complex multidirectional tomographs
Computed tomographs = CT scans
Cone-beam computed tomographs
Dental panoramic tomographs
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
How can blurring from simple linear tomographs be improved
- Reduced and resolution improved via use of complex multidirectional tomography
- Performed by scanora unit
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
What are computed tomographs / CT scan ?
Used in medical imaging
Complex equipment
Patient lies down and moved into central hole of CT unit
Ring of image receptors and X-ray unit spirals around patient
Designed to create axial tomographic slices
Can be placed together for a 3d image
What are cone beamed radiographs?
name if unit?
what it produces?
CT scan designed for dental use
Sirona-Galileous unit
- only images maxillofacial skeleton + part of cranial base
Produces cross sectional tomographic images of jaws in any plane
3d construction possible using software
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 real soft tissue shadows ?
Soft palate
Ear lobe
Dorsum of tongue
Nasal labial fold
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
Limitations of panoramic tomographic images ?
Only represents a sliced section of patient
- lacks an overview
Blurred
- movement of tube head + image receptor
Affected by patient shape + positioning of teeth
Complex
- overlayed by soft tissue shadows, equipment shadows, ghost shadows , air shadows
Affected by processing faults
Affected by exposure factors
Resolution relatively poor
- indirect action film screen combo
Digital images
limitation of digital images
(module 5 recap)
Digital Images:
- Made up of pixels
- 256 shades of grey
- Each pixel assigned shade of grey depending upon amount of radiation that reaches sensor
Limitations of Digital Images:
- 2D representations of 3D objects
- Affected by geometrical distortion
- Affected by software image processing
CONCLUSION
- x-ray images = complicated pictures
- not all x-ray images are the same
