Geometrical Qualities of the Radiographic Image LECTURE (copy)

What are the 3 components of Recognizability

1. Sharpness (spatial resolution)

2. Magnification (size distortion)

3. Shape Distortion

What are some other names for Sharpness

Spatial resolution

Detail

Should spatial resolution be maximized or minimized?

MAXIMIZED

What is the most important component of the recognizability of an image

Sharpness

What is the ARRT definition for spatial resolution

The sharpness of the structural edges recorded in the image

What is another definition for sharpness 

Abruptness with which the particular edges stop

What is Umbra

The sharp area of a radiographic image

How is Umbra casted 

cast from a point source of light 

(no blur; perfectly sharp edges) 

Umbra is referred to as

“pure”, “complete”, or “true” dark shadow

What is Penumbra

The unsharpness or blurring that surrounds the edge of a radiographic image 

How is Penumbra casted 

Cast from an area source of light (disk source)

What is the partial shadow cast around pure shadow called

Penumbra

What is motion penumbra 

Movement of source of light, object, or IR spreads penumbra

What is magnification

Difference between the size of the real object and the size of its projected image

What is Shape Distortion

Difference between the shape of the real object and the shape of its projected image

In a given axis of direction, (lengthwise or crosswise), shape distortion will consist of either __________________________ of the image or ________________________ of the image.

Elongation; Foreshortening

Is unsharpness a subjective or objective quantity

objective 

How is unsharpness an objective quantity (List 3)

• directly measured

• geometrically predicted

• mathematically calculated

Mathematical expressions of sharpness are always _________

derived and expressed as ________ numbers.

indirectly; relative 

What are the 3 controlling factors of unsharpness in projection geometry of an x ray beam

• FSS

• OID

• SOD

What is the formula for Geometrical Unsharpness

U = FS x OID/SOD

What is the formula used to calculate SID using OID and SOD

OID + SOD = SID

What is the spread of penumbra (precisely) with a

FSS = 0.6 mm

OID = 40 cm

SOD = 80 cm

0.6 mm x 40

•          80

40/80 is simplified to ½ 

Since we are already multiplying to a decimal (0.6), ½ can be converted to 0.5 

0.6 × 0.5 = 0.3

U = The penumbra spreads across an area of 0.3 mm 

1 inch = ______ cm

2.54cm

What is the inverting formula used for sharpness 

SOD/OID

Why is the inverting formula used for sharpness 

For a given particular focal spot size the SOD/OID ratio becomes an indicator of the relative sharpness that will be produced in an image

How will image sharpness be affected in the second exposure?

First exposure: the SOD is 45 cm and the OID of 2.5 cm.

Second exposure the SOD is changed to 90 cm and the OID is reduced to 1.25 cm

First Exposure: 45/2.5 = 18

Second Exposure: 90/1.25 = 72

Second Exposure/First Exposure: 72/18 = 4

Second Exposure is 4 times sharper than the First Exposure 

Is high contrast the same as high sharpness

NO

How is sharpness measured

Sharpness is measured by how quickly the transition from light to dark changes as image is visually scanned

Select 2 images that have high sharpness 

A and C 

Scatter radiation cannot affect sharpness

T/F

TRUE

Does scatter radiation increase penumbra?

YES

Can scatter radiation decrease the visibility of the edges of a radiograph

YES

What factors primarily control sharpness

SID

SOD

OID

FSS- biggest factor that affects sharpness

Motion

In a magnified image the objects length and width will measure ______ than the real object by _______ __________

larger; equal porportions

Magnification is based solidly on the

geometry of similar triangles

What is the formula for magnification

Image Size = SID

Object Size SOD

Magnification is the _________ between the size of the real image and the size of the real object

RATIO

An object measuring 20 cm in width is radiographed using an

SID of 40 inches and an SOD of 35 inches.

How wide is the projected image of the object?

1. Use the magnification formula

• Image Size/Object Size = SID/SOD

2. Input the information provided into the formula

• X/20 = 40/35

3. Cross multiply

• 40 × 20 = 800; 35x

4. Divide 35 from 800

• 800/ 35 = 22.86

The projected image of the object is 22.86 cm wide 

A radiograph is taken of an object 2 cm in width and the resulting image measures 8 cm in width. If a 125 cm SID was used, what was the SOD?

1. Use the magnification formula

• Image Size/Object Size = SID/SOD

2. Input the information provided into the formula

• 8/2 = 125/x

3. Cross Multiply

• 8x = 250

4. Divide 8 from 250

• 250/ 8 = 31.25 

SOD = 31.25cm 

What is the magnification factor

Represents the multiplication factor by which the length and width of the object in the image is increased

Factor of linear magnification

What is the formula for Magnification Factor

(SID/SOD - 1) x 100

An object is radiographed at 183 cm SID and 150 cm SOD.

What is the percentage magnification?

1. Use the magnification factor formula

• (SID/SOD - 1) x 100

2. Input the information provided into the formula

• ((183/150) - 1) x 100

3. Solve using PEMDAS

• 183/150 = 1.22

• 1.22 - 1 = 0.22

• 0.22 × 100 = 22

Magnification Factor: 22%

What is resolution

The ability to distinguish any two adjacent details in the image as being separate and distinct from each other

What are the 2 most important aspects contributing to resolution

• Sharpness (Recognizability factor)

• Contrast (Visibility factor)

Both poor ________ and poor _________ can degrade overall image resolution

sharpness; contrast

List some factors that could cause shape distortion

• Angle and alignment of the x-ray beam

• Angle and alignment of the part of interest

• Angle and alignment of the IR

What should you look for in sharpness

Abruptness; How abrupt in goes from one part to the next part

Why is a large FSS used for larger parts

The xray beam penetrates in a shorter amount of time than using a small FSS on a large body part

Prevent motion blur

What part of the Xray tube do you choose the FSS

Cathode

What is it called when the electrons in the anode hit the same part of the tube every time

PITTING

The line focus principle 

• By angling the face of the anode target, a large actual focal spot size can be maintained, and a small focal spot can be created

• relationship between actual and effective focal spot

Does FSS affect shape distortion 

Not a whole lot but yes 

Does SID or OID affect shape distortion

NO; affects size distortion

If there is an angle on the tube and the part of interest becomes 2 times bigger, what type of distortion takes place?

Size AND shape distortion

As penumbra increases, umbra _________

decreases

Forshortening is caused by magnification

T/F

FALSE; shape distortion

Will you see more sharpness on a long or short SID?

LONG; A longer SID reduces penumbra

What focal spot is recommended for high sharpness

short fss