GEOMETRIC FACTORS Flashcards
GEOMETRIC FACTORS
-MAGNIFICATION
-DISTORTION
-FOCAL SPOT BLUR
Quantitatively, magnification is expressed by the
magnification factor (MF)
For most radiographs taken at a source to-image receptor
distance (SID) of:
• 100 cm the MF is approximately 1.1
• 180 cm the MF is approximately 1.05
The SID is standard in most radiology departments:
-180cm (72’’) for chest imaging
– 100cm (40”) for routine examinations
–90cm (35”) some special studies, such as mobile radiography and
trauma radiography.
–50 to 70 cm for dedicated mammography imaging systems
Three conditions contribute to image distortion:
- Object thickness
- Object position
- Object shape
– Thick objects are more distorted than
thin objects.
–With a thick object, the OID changes
measurably across the object.
Consider, for instance, two
rectangular structures of different
thicknesses
Object Thickness
– If the object plane and the image
plane are not parallel, distortion
occurs.
Object Position
– If the object plane and the image
plane are not parallel, distortion
occurs.
Object Position
- 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
occurs because the focal spot is not a
point.
Focal-spot blur
is the most important factor for determining
spatial resolution.
Focal-spot blur
Described as varying radiation intensity
across the x-ray field in the anode–cathode
direction caused by attenuation of x-rays
HEEL EFFECT
The ___ is small on the anode side and
large on the cathode side of the image.
FSB
