View
128
Download
1
Category
Preview:
Citation preview
FUNDAMENTALS OF IMAGING
RAD 206
Geometric factors
15.1: Optimization of protection in radiography: technical aspects 3
IMAGE QUALITY CHARACTERISTICS
DensityContrastResolutionUnsharpnessNoiseDistortion & Magnification MTF
Distortion Bushong Ch 10 pg 173
Which car is the biggest
WHICH ONE IS AN EGG
To produce high quality radiographs, the technologist must maximize the geometric factors
Geometric Factors• Magnification• Distortion• Focal-Spot Blur
DISTORTIONMisrepresentation of the true SizeSize or ShapeShape or PositionPosition of an object
• size distortion is only Magnification • shape distortion is True Distortion
Why ?? Because Shape distortion is just unequal magnification of different proportion of the same object
MAGNIFICATION(SIZE DISTORTION)
TRUE DISTORTION(SHAPE DISTORTION)
Magnification is a type of distortionEvery pixel or area is equally
Magnified
True Distortion is unequal Magnification of areas
DISTORTION
MAGNIFICATIONWhy magnification happensAs the X-ray beam diverges, the image produced on the film is always slightly larger than the original image, and this effect is called magnification.
All images on the radiograph are Larger than the actual object
For almost ALL Radiographic Exams , minimizing magnification is required ..by:
Reducing OID Increasing SID
Minimizing Magnification also improves Detail
The position of the tube to IR (SID) will influence how the structures appear on the image
The farther away
the less magnified
SID Magnification
THE MAGNIFICATION FACTOR “M“is the degree or ratio of magnification , and is calculated by:
M = SID/SODM = SID/SOD
PRACTICE
If the SID is 100 cm and the SOD is 80 cm, what is the magnification factor?
If the SID is 115 cm and the SOD is 80cm, what is the magnification factor?
If the SID is 115 cm and the SOD is 95cm, what is the magnification factor?
If the SID is 100 cm and the SOD is 95 cm, what is the magnification factor?
If the SID is 115 cm and the SOD is 110cm, what is the magnification factor?
1.25
1.44
1.21
1.053
1.045
The magnification factor allows for the calculation of the actual size of an object from the image by using this formula:
O = I O = I / / MMO = object sizeI = image sizeM = magnification factor
the value of M can be substituted byM = SID/SOD
O = I / (SID/SOD)
PRACTICE QUESTIONSA child brought for a chest x-ray who said that he swallowed 100 fills coin, SID used was 115 cm and the estimated S0D is 105cm, how big the image would be?
2.6mm
A child brought for abdomen x-ray who said that he swallowed 100 fills, SID used was 100 cm and the estimated OID is 8cm, the image on x-ray shows a 2.4 mm, is he saying the truth ?
Yes, two 50 fills coins stuck together
PRACTICE QUESTIONSTwo erect chest x-ray were done using AP and PA techniques, Estimate distance between the heart and the back surface of the patient..
Note:Direct exposure on film, No Bucky was used, SID for erect CXR is 180cm
Recommended