H - Radiology Flashcards
What are the 3 basic principles of exposures that must be considered?
Justified - Is there sufficient benefit to offset any detriment caused by x-ray exposure.
Optimised - ALARP (as low as reasonably practical)
Limited - Radiation dose limits applied
How do you combat undesirable magnification in paralleling technique?
use long fds (focus to skin distance)
What is binding energy?
The amount of energy required to remove an electron from its shell.
If you double the distance from x-ray source? what happens to the dosage levels?
Double distance - 1/4 dose
What is attenuation?
Reduction in the number of photons
Occurs due to scattering and absorption
partial attenuation grey
complete attenuation white
What kV is chosen for intra-orals and why?
60-70kV
Compromises between patient dose and image quality
What is an absorbed and equivalent dose?
Absorbed - energy deposited by radiation (Gy)
Equivalent dose - Absorbed dose x weighting factor (depending on type of radiation) X-ray WF = 1
Format of digital x-ray images?
DICOM
5 steps in film processing?
DWFWD Developing washing fixing washing Drying
How to affect film speed?
Number and size of halide crystals
Faster image with larger crystals
However, poorer image quality
What are some developing issues and fixing issues?
Developing
- image too light or too dark
Fixing
- brown or yellow image
How to use the parallax technique?
Take 2 radiographs to determine if a superimposed structure is lingual or buccal to the reference point.
PAL - if it’s palatal or lingual it will follow the beam of the radiograph.
For example, if you are looking for ectopic canines and take OPT and maxillary occlusal. If the canine is now closer to the apex in the MO then it’s palatally placed.
Image quality grades?
Grade 1 - excellent
Grade 2 - diagnostically acceptable
Grade 3 - unacceptable
Electrical requirements for an x-ray unit?
Direct current
2 voltages - one high one low
Two transformers - set up across tube and step down across the filament
Limitations of OPT?
Patients occlusions e.g class lll
Long exposure time, needs to cope
movement
broad shoulder
head position for oblique occlusal? mand + maxilla
Maxilla - horizontal to the ala-tragus line
Mandible - Corner of the mouth-tragus line horizontal
The angle of beam for OPT?
8 degrees upwards
Head positioning for true occlusal?
Head tipped back as far as possible
Beam at 90 degrees to the image receptor(i.e approaching from under the chin)
What is the ideal projection geometry?
Image receptor + Object parallel + in contact
X-ray perpendicular to both
What is the bisecting angle technique?
This is used in oblique occlusal to prevent magnification
image + receptor partly in contact, not parallel
X-ray beam at 90 degrees to the line bisecting the angle between tooth and receptor
Indications for an occlusal radiograph?
Peri-apical not possible
Pathology too large for peri-apical
Trauma
What are IRR17 and IRMER17?
Ionising radiation regulations which deal with occupational exposures and the general public
Ionising radiation medical exposure regulations
- deals with patient exposures
Size of films for intra-orals? Anterior PA Posterior PA Bitewing Occlusal
Anterior PA - size 0
Posterior PA - size 2
Bitewing - size 2
Occlusal - size 4
What are the two occlusal plane curves?
The curve of spee - A-P
The curve of mansion - B-L
Cause of horizontal magnification of anterior teeth in an OPT?
Patient too far back in the machine
To close to the X-ray source
What does KvP stand for?
Peak kilovoltage
