7: Exposure Techniques Flashcards
Position of what is most critical?
Source compared to specimen and film
Using screens in energy levels between ____ and ____ can reduce exposure from 1/3 to 1/2 then they would without.
150 keV and 2 MeV
What type of materials are particularly susceptible to scatter?
Denser metals
Describe Category 1 materials:
Parts of uniform thickness that have good contact with film, such as flat butt welds or flat plates
(Scatter mainly absorbed - measures don’t need to be taken)
Describe Category 2 materials:
May produce scatter at the edges but this subtle loss of sensitivity would be acceptable
Ex. Parts with large radius, tapered parts, chamfered parts and flat but slightly irregular castings
Describe Category 3 materials:
Can exhibit undercut scatter from one section of image to other, reducing image quality/clarity
Ex. Parts with complex geometry and heavy section thickness
often require masking/blocking/filters
What are the 3 main factors that govern exposure?
- mA for X-rays OR source for gamma
- Time
- Source-to-film distance
As a general rule, the mA required for a given exposure is directly proportional to the ___________
Square of focal film distance
M1/M2 = D^2,1/D^2,2
If the mA can’t be changed, what should you do?
Adjust exposure or time
T1/T2 = D^2,1/D^2,2
mA required for a given exposure is inversely proportional to what?
Time
What is referred to as the reciprocity law?
M1T1 = M2T2 = M3T3 = constant
Relationship between mA and time
When does the reciprocity law not apply?
When fluorescent screens are used
Ideally, direction of beam should be where compared to part?
Perpendicular to main axis of part
Medium tubular sections (20-40 mm) may require how many exposures?
Min. of 2 at 90 degrees to each other, but up to 4 may be necessary
Large tubular sections may require how many exposures?
3 exposures at 120 degrees or 4 exposures at 90 degrees
Double wall exposures are common for pipe diameters less than what?
90 mm
For pipe welds, a source placed perpendicular to weld plane is called what?
Double wall, double viewing
How many exposures are generally required for the superimposed method?
Typically 3, 60 degrees or 120 degrees to previous
When may it be acceptable for a source to be in contact with outside of pipe?
Diameters greater than 90 mm
This would be a double wall, contact exposure
What degrees are elliptical exposures offset by? And how many exposures?
Typically offset by 15-30 degrees, requiring 2 exposures set at 90 degrees
The location along a welded seam is usually indicated on a radiograph by means of what?
Lead numbers
In order to ensure good contact with the weld or casting being radiographed, what are preferred?
Flexible cassettes
Cassettes contains film are held in position for exposure by means of what 3 things?
- Wooden supports
- Magnetic strips or springs
- Tape
(Whatever works)
By using small diameter sources (0.3 mm), very small diameter pipes can be shot using the panoramic technique with the acceptable unsharpness. Although the source strength is low, exposure time is not too long because of what?
The source to film distance is small
