Lecture 7

Question 1

What is meant by the term geometric unsharpness and what causes it?

Answer 1

• Caused as a result of enlargement of image relative to object (magnification) - increased object to film distance (increased magnification)
• OFD = object to film distance
• FFD = Focal (spot) - Film distance

Question 2

Why is magnification an important consideration?

Answer 2

• We want to place the area of interest as close to the film as possible

Question 3

What is meant by the term inherent unsharpness?

Answer 3

• The unsharpness inherent to film screen combination - slower film screen combinations will have better detail as well as single emulsion films

Question 4

What is an important consideration with central ray position?

Answer 4

The central ray is vertical. And the central ray needs to be projected straight through the radiolucent gap

Question 5

What is summation and what is the effect of it?

Answer 5

• Summation occurs when parts of patients in different planes are superimposed
• The film density = sum of all superimposed structures at that point (‘swiss cheese’ analogy)
• Often has the potential to be mistaken for pathology

Question 6

What is meant by border effacement?

Answer 6

• Structures of same opacity are in physical contact - results in loss of individual margins - structures merge + appear as one

Question 7

What is the mach effect?

Answer 7

• Occurs when 2 structures are superimposed creating an imaginary line at the division of the 2 structures
• For example 2 overlaping bones could be interpreted as a fracture