Geometric Factors Flashcards
The three principles of Geometric Factors
Magnification
Distortion
Focal Spot Blur
All images on the radiograph are larger than the objects they represent
Magnification
Quantitatively, magnification is expressed by the
Magnification Factor
For most radiographs taken at a source to image receptor distance (SID) of:
100 cm MF is 1.1
180 cm MF is 1.05
Ways to minimize magnification
Large SID
Small OID
Chest imaging
180 cm(72”)
for routine examinations
100 cm (40”)
Some special studies, such as mobile radiography and trauma radiography
90cm(35”)
For dedicated mammography imaging systems
50 to 70cm
Three conditions contribute to image distortion
Object thickness
Object position
Object shape
Unequal magnification of different portions of the same object is called
Shape distortion
Thick objects are more distorted than thin objects
Object thickness
Object Position:
If the object plane and the image plane are not parallel
Distortion occurs
“Reduction in image size”; related to the angle of inclination of the object
Foreshortening
Image appear longer than it really is
Elongation
“Is the misrepresentation” in the image of the actual spatial relationships among objects
Spatial distortion
-The most important factor for determining spatial resolution
-occurs because the focal spot is not a point
Focal spot blur
The focal spot blur is small on the_________ and large on the__________of the image
Anode side, Cathode side
To minimize focal spot blur:
Low Focal spot
Low OID
high SID
Describe as varying radiation intensity across the x-ray field in the anode-cathode direction caused by attenuation of x-rays in the heel of the cathode
Heel Effect
