factors affecting image quality Flashcards
what is radiographic contrast?
CONTRAST (RADIOGRAPHIC) -
The range of densities (dark and light areas) visualized on a radiographic image.
what is definition?
DEFINITION (SHARPNESS) -
The ability of an imaging system to define the edges of structures or objects.
what is resolution?
RESOLUTION (DETAIL) -
The ability of an imaging system to demonstrate small structures.
for radiation beam factors what does exposure time change?
- Must be matched to patient and/or procedure
- “Standard” or “recommended” exposure times on dental x ray machines will work for the majority of patients, but some patients may need more or reduced exposure time to obtain an acceptable image
- Certain procedures will require longer exposure time (e.g. occlusals – adult patients)
- Overexposure may cause image to be too dark, underexposure will may cause image to be too light, sometimes to the point of the radiograph being unacceptable (THE GOOD NEWS – digital images can be “adjusted” after exposure, and images that initially appear “unacceptable” might be salvaged. Regardless, we should always strive to avoid using too high an exposure time to minimize patient dose)
what does the milliamperage do?
- Milliamperage (tube current – number of electrons flowing from cathode to anode per unit of time)
• Has the same effect as exposure time; higher mA results in darker image; lower mA results in lighter image
• mA is fixed in most dental x ray machines
what does kV change? what is it?
- kV (potential difference between cathode and anode at time of exposure)
• A higher kV will result in a darker image; higher potential difference results in higher kinetic energy of electrons flowing from cathode to anode; therefore, more electrons will enter into Bremsstrahlung interactions that will give rise to photons energetic enough to be x rays
• A higher kV will result in a more energetic, more penetrating x ray beam; more x rays will penetrate tissues/structures; the contrast
between radiopaque and radiolucent areas will be reduced
- effect on contrast - overall reduction in radiographic contrast
- Note that with the majority of dental x ray machines used for periapicals and bitewings, kV and mA are fixed, and we can only control the exposure time. It is important to understand, however, how kV and mA affect the final image, as your practice might use more specialized x ray equipment for which these factors can be adjusted.
what’s the difference between high contrast and low contrast?
High contrast – very obvious difference in the darkest and lightest areas on the image – easier to differentiate radiopaque and radiolucent areas (lower kV)
Low contrast – differences in the darkest and lightest areas on the image are not as great – not as easy to differentiate radiopaque and radiolucent areas (higher kV)
other radiation beam factors are x-ray source, focal spot size, collimation and filtration. What do these do?
- X ray source – image receptor distance
- Focal spot size
• These two factors influence the manner in which the “shadow” is cast onto the film or image receptor (see below – “Rule for Accurate Shadow Casting”) - Collimation
• The main purpose of more tightly collimating the x ray beam is to reduce patient dose (less tissue exposed), but greater collimation will also reduce the amount of scatter radiation reaching the image receptor and will result in better contrast. However, this improvement in contrast is not always visually visually apparent. - Filtration
• Removes the lower energy photons that do not contribute to image production, thereby reducing patient dose
• A filtered beam will have fewer x rays than a comparable non-filtered beam, but the photons in a filtered beam will have greater average energy, and the beam will be more penetrating; filtration reduces radiographic contrast slightly, but not enough to affect radiographic quality significantly
what are the three absorbing material factors?
- Thickness
- Density
- Atomic number
The greater any of these factors are, the more the x ray beam will be attenuated.
The greater these qualities of the tissue or object/structure being imaged, the more x rays will be attenuated, and the more “radiopaque’ the tissue/object/structure will appear on the final image
• Thickness of tissue – even soft tissue, if thick enough, can appear faintly radiopaque appearance compared to, for example, air; the thicker a tissue, the more x rays it will attenuate
• Density – soft tissue is denser than air, soft tissue is denser than fat; these differences may be noticeable on images conventional radiographic projections, but more so on images produced using computed tomography (CT)
what are the three image receptor factors for xray film?
a. “Speed” (senstivity to radiation)
• “Faster” films generally have less resolution and detail
• If a certain film is “faster” or has a higher “speed”, it means that it is more sensitive to x rays
• “Faster” films usually have greater silver bromide grain size, and this will result in lower definition and resolution
b. Storage
• Improper storage may cause increase in fog and reduced contrast
• If x ray film is stored in areas of high heat or humidity, or if it is subjected to stray radiation, fog (unwanted darkening) may result, and the contrast of the final image will be reduced
c. Age (e.g. outdated film)
• May have same effect as improper storage
• “Age fog” may be a problem if old or expired film is used; therefore, do not use expired film
*Proper storage of film (e.g. in refrigerator or freezer, away from possible x ray exposure) will prevent problems related to b) and c).
what are image receptor factors for film-screen systems?
Result in better radiographic contrast compared to “direct exposure” film
Not a major considerations now as film is used much less frequently
• Use of “film-screen” combinations (film enclosed in light-tight cassettes with intensifying screens) when producing film-based radiographs (i.e. panoramic radiographs, other “extraoral” radiographs) will yield images with greater contrast than what is capable with intraoral radiographic (direct exposure) film (i.e.. periapicals, bitewings, occlusals)
• Film-screen systems are rarely used now due to increased use of digital imaging
What are image receptors for digital image receptors?
3. Digital image receptor (phosphor plate or CMOS sensor)
a. resolution and sharpness not quite as good as film, but quite adequate for clinical needs
b. physical damage to plate/sensor will reduce quality of image
c. if plate improperly or incompletely erased, the “residual” image may reduce quality of next image that is made
(Either b. or c. can reduce quality of image from a digital system - see lecture on Digital Imaging)
how is resolution measured?
Resolution – measured by no. of line pairs/mm that can be observed
The more line pairs/mm. that can be detected, the better the resolution of the imaging system being used – more discussion in lecture on Digital Imaging…
What is processing factor for image quality?
For film-based imaging, a SIGNIFICANT factor in image quality
• Improper chemical processing of film will reduce radiographic quality - see lecture 6
what are 3 geometric problems which are always present on a projected image?
- “Unsharpness”
- Magnification
- Distortion
These geometric problems are always present because:
The focal spot is never a point
X rays diverge from the focal spot
Structures being imaged are three-dimensional; the image receptor is two-
dimensional
To some extent, these problems can be reduced for periapical radiography by using the paralleling technique and a longer distance between the x ray source and the object or the image receptor, usually by employing a longer PID on the x ray machine.
