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Annual Occupational Dose Limits - Relative to Stochastic Effects
50mSv and 100 mSv in 5 yr cumulative
**Stochastic = probability of occurrence of the change, rather than severity, is dose dependent
Annual Occupational Dose Limits - Relative to Deterministic Effects
- 150mSV to lens of eye
- 500 mSv to skin and extremities
** Deterministic = severity of response is proportional to dose
Annual Nonoccupational Dose Limits - Relative to Stochastic Effects
and, if higher, not to exceed 1mSv annual average in 5 yrs
- **Stochastic = probability of occurrence of the change, rather than
- severity, is dose dependent
Annual Occupational Dose Limits - Relative to Deterministic Effects
- 15 mSV to lens of eye
- 50 mSv to skin and extremities**
- Deterministic = severity of response is proportional to dose
Transcranial imaging of the TMJ
- sagittal view of the lateral aspects of the condyle and temporal component
- ** Taken in open and closed mouth positions **
Transpharyngeal (Parma) imaging of the TMJ
sagittal view of medial pole of condyle
Transorbital imaging of the TMJ
- view entire mediolateral dimension of articular eminence, condylar head and neck
- *especially good for condylar neck fractures
multiple slices make it superior to the transcranial for depicting condylar position and osseous changes
- indicated when information is needed about 3D sharp and internal structure of osseous components
- **cannot produce accurate images of disk
- Indirect image of disk obtained by injecting radiopaque contrast agent into one or both joint spaces.
- ** Used to determine disk position, function, morphology in integrity
- Best technique to see TMJ soft tissues, especially good for articular disk
- -- Does not have morbidity associated with arthrograms
- Used to examine skull for disease, trauma, and developmental abnormalities.
- Provides good view of frontal and ethmoid sinuses, nasal fossae, and orbits
- -- Frankfort horizontal is parallel to floor, pt is facing film
- Used to evaluate Maxillary sinuses
- Shows frontal and ethmoid sinuses, orbits, zygomaticofrontal suture, nasal cavity, and coronoid process
- Used to examine suspected fracture of condylar neck
- also shows posterolateral wall of maxillary antrum
- Especially good for revealing a medially displaced condyle
Used to demonstrate base of skull, condylar position and orientation, sphenoid sinus, curvature of mandible, lateral wall of maxillary sinuses, & displacement of fractured zygomatic arch
Oblique Lateral Projection
- Used to demonstrate the premolar-molar region and inferior border of the mandible.
- Oblique ramus projection - cassette places over ramus and condyles - shows ramus from the angle to the condyle (useful for 3rd molars)
Lateral Skull Projection
- Used to survey the skull and facial bones for evidence of disease, trauma, and developmental abnormalities.
- Used by orthodontist to assess growth and treatment effects
What is the usual source-to-patient distance?
- 5 feet
- -- this minimized magnification due to different patient-to-film distances
What is the usual patient-to-film distance?
- 15 cm
- -- To minimize variation in magnification from patient to patient and to obtain consistent measurement on the same patient over time
For a ceph, which side of the patient is closest to the x-ray source and which side is closest to the film?
- The patient's right side is closest to the X-ray source
- The patient's left side is closest to the filmThus, when evaluating bilateral structures below and ahead of the ear rods, the right side structures will be further below and further ahead of their left side counterparts
Target Object Distance
- 2 X distance = 1/4 Intensity (less exposure)
- -- Inverse square law
How do you increase exposure? (roentogen)
- Increase mA
- Increase kVP
- Increase time
- Decrease filtration
- Decrease distance
What happens if you increase kVp?
- Increase energy per photon
- Increase penetration
- Decrease tissue absorption
- Decrease wavelength
What happens if you increase time or mA?
Increase photons and exposure time
What is the purpose of Intensify Screens?
- Increase sensitivity of the film (decrease dose) for extraoral films
- -- decrease exposure by 50%
Why use higher speed film?
Using E speed vs D speed decreases exposure by 50%
How does variation in mA and time effect density and contrast?
- Only affects density, has no effect on contrast
- Doubling mA will allow exposure time to be halved
How does variation in kVp affect density and contrast?
- -- The higher the kVp, the greater the film density and the lower the visual contrast (many shades of gray)
- -- The lower the kVP, the higher the contrast (mostly distinct blacks and whites)
- To penetrate bony structures of the skull, settings below 70kVp should not be used
How do you increase sharpness?
- Increase object distance
- Decrease film distance
- Faster film
What does colimation do?
- Concentrates the beam
- Decreases volume of the irradiated tissues
- Decreases scatter
- Increases film quality
What is attenuation?
- Uses rare earth filters
- --reduces exposure and improves contrast
What is the greatest single factor in reducing the diagnostic quality of the cephalometric radiograph?
- Scattered radiation
- Any x-ray photon whose initial direction or path is scattered while exiting the source tubehead or is deflected by the patient's hard and soft tissues creates noise or radiographic scatter in the x-ray image
What is the purpose of the X-ray grid?
To reduce the amount of scattered radiation that reaches the film
An x-ray grid consists of lead strips either parallel to each other or in a converging pattern with radiolucent spacers in between. The grid is placed between the patient and the film cassette, as close as possible to the film cassette. X-ray photons not traveling in the same direction as the primary beam strike the lead strips and are absorbed
What type of grid is more desirable?
- A Focused Grid -- the strips are at increasing angles toward the source of the x-ray beam from the center of the beam outward
- -- Parallel grids are undesirable because they absorb greater proportions of energy in the outer regions of the beam, producing a film with gradually decreasing density from the center of the film outwards
What determines a grid's effectiveness?
- the ratio of the length of the strips to the size of the space between the grid.
- The higher the ratio, the greater the amount of scatter absorbed, resulting in image contrast
What is the most commonly used grid ratio in cephalometrics?
8 - with 80-100 line-pairs or spaces per inch
What are the disadvantages of grids?
- 1. A faint radiopaque pattern of the grid on the film image
- 2. Increased exposure settings (exposure enrgy must be doubled or tripled to produce a radiograph with the same density as one made without the grid)
What are the 2 types of intensifying screens used?
- 1. Convential or blue emitting - coated with calcium tungstate, film speed 200
- 2. Rare earth screens - coated with gadolinium and lanthanum, emit green light, only require 1/2 the xray energy - therefore film speed 400