Geometric Properties - Detail and Distortion

Geometric properties are focused on ___

recognizability (sharpness)

What are the factors that affect geometric quality?

• sharpness of recorded detail

  • resolution

  • spatial resolution

• size distortion

  • magnification (this is the only type of size distortion)

• shape distortion

  • elongation

  • foreshortening

What are some other terms for recorded detail?

resolution and definition

What is recorded detail?

• the ability to accurately image an object: measured by the ability to see pairs of lines

• abruptness with which the edges of a particular image stop

• the ability to image 2 separate objects and visually distinguish from one another

What is spatial resolution? How is it measured?

• the ability to distinguish between two adjacent structures (degree of geometric sharpness)

• measured in line pairs per mm (lp/mm)

  • the greater the number of lp/mm, the better the resolution

What is the lp/mm measurement of film, CR, and DR?

detailed film/screen: 15 lp/mm (THE BEST SPATIAL RESOLUTION)

CR: 5 lp/mm

DR: 10 lp/mm

What is contrast resolution?

ability to distinguish between differences in shades of gray

Does film, CR, or DR offer the best contrast resolution? Explain

DR; more shades of gray available (one of the greatest benefits of digital)

What is umbra?

sharp area of a shadow

What is penumbra?

• geometric unsharpness at periphery of image

• shadow surrounding the umbra

What is edge gradient?

the width of unsharpness (penumbra)

What is another term for unsharpness?

focal spot blur

What is the equation to calculate geometric unsharpness?

geometric unsharpness = ^{(focal spot size x OID)}/_SOD

What are 4 things you can do to improve resolution (IN ORDER)?

1. eliminate or minimize patient motion

2. minimize OID

3. increase SID

4. reduce focal spot size

How can you reduce patient motion?

• use short exposure time

• give good instructions

• use positioning devices

• make patient comfortable

• try to avoid holding patient

Does FSS, SID, or OID have the greatest effect on penumbra?

OID

What is the relationship between SID and penumbra?

increased SID decreases penumbra (central portion of beam contains straighter rays)

What can you do if OID cannot be decreased? Why?

increase SID (to use straighter portion of beam and decrease penumbra)

Explain how small focal spot size can improve resolution

• decreased area over which x-rays are produced

• reduces penumbra

• small focal spot → sharp detail

  • harder on tube

  • decreased heat dissipation

Can we ever have 0 OID?

NO (at least some part of the body part will not be touching the IR)

Focal spot size only affects ___

detail

Explain the line focus principle

when the target angle is less than 45^o, the effective FS is smaller than the actual FS

The steeper the anode angle, the ___ the focal spot

smaller

What is distortion?

misrepresentation of size or shape of structure

What are the 2 types of distortion?

size and shape

What is the only type of size distortion possible in general radiography?

magnification

Size distortion/magnification is controlled by ___ and ___

OID and SID

What effect does magnification have on resolution?

magnification degrades resolution

As OID increases, magnification ___

increases

As magnification increases, detail ___

decreases

Because we have 3 dimensional anatomy, ___

some structures will be farther from image receptor (position area of interest closest to IR)

Short SID has ___ beam divergence

more

Increased SID has ___ magnification

decreased

For every inch of OID, increase SID by ___

7 inches

What are the equations for magnification factor(M)?

M = ^SID/_SOD

M = ^{image size}/_{object size}

SID - OID = ___

SOD

What is the equation for calculating object/image size?

O = ^I/_M

O is object size

I is image size

M is magnification factor

What is the equation to calculate any unknown factor?

^SID/_SOD = ^{image length}/_{object length}

What is the equation to calculate % magnification?

% magnification = (^{image size - object size}/_{object size)} x 100

Shape distortion is controlled by ___

beam/part/IR alignment

Explain elongation vs foreshortening

elongation

• makes object appear longer than it really is

• tube and IR misaligned

foreshortening

• makes object appear shorter than it really is

• body part misaligned

What are 2 things you can do to minimize distortion?

1. position part parallel to IR

2. position CR perpendicular to IR

How can superimposed structures be visualized?

tube angulation (may cause distortion)

(SUMMARY) How can you minimize distortion?

• minimize OID

• maximize SID

• tube and IR aligned

• CR perpendicular to IR

• long axis of anatomical part perpendicular to CR and parallel to IR

• CR through anatomy of interest

How does IR construction affect resolution?

• film/screen

  • size of crystals

  • thickness of emulsion

• digital

  • matrix and pixel size

  • sampling frequency

  • crystal size (scintillator)