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