geometric properties distortion

Radiographic DistortionRadiographic Distortion

Principles of Radiographic Imaging

ObjectivesObjectives• Define Radiographic Distortion• Differentiate between Size and Shape Distortion• Summarize and analyze the relationship of factors that

control and affect distortion.• Perform calculations to determine image magnification

and percent magnification.• Explain the effects SID and OID have on image distortion• Discuss various methods of minimizing distortion• Explain elongation and foreshortening• Describe the routine relationships between central ray,

anatomical part, and image receptor•

Geometric Properties

Sharpness

Recorded Detail Distortion

Size ShapeReceptor

Unsharpness

Geometric Unsharpness

Motion Unsharpness

Assessing Radiographic DistortionAssessing Radiographic Distortion• Radiographic misrepresentation of either

the size or shape of the anatomic part is called distortion

• 2 Types of DistortionDISTORTION

SIZE SHAPE

Factors Affecting Distortion

Size Distortion - Magnification• An increase in the object’s image size

compared with its true, or actual size• There is a percentage of magnificationmagnification in

every radiographic image• As the percentage of magnification magnification

increasesincreases, the level of geometric unsharpnessunsharpness also increasesincreases

Influences of Size DistortionInfluences of Size Distortion

• Size distortion is influenced by• OID and SIDOID and SID

– Object-to-Image DistanceObject-to-Image Distance– Source-to-Image DistanceSource-to-Image Distance

OIDOID

Shadow

OID has a OID has a direct relationship direct relationship with size distortionwith size distortionAs OID increases, size distortion/magnification increasesAs OID decreases, size distortion/magnification decreases

A

B

• SID has an SID has an inverse relationship inverse relationship with with size distortionsize distortion

• A long SID produces less size distortion

• A short SID produces more size distortion

With a long SID the beam is more perpendicular creating less of a divergence

SIDSID

Lab DemonstrationsLab Demonstrations

Interactive ReviewInteractive Review

• Interactive Online Reviews– Plickers– Quizstar– Qzzr

• Group/Team Work

Calculating Size Distortion• Magnification Factor

– indicates how much size distortion is demonstrated on a radiograph

• MF = SID / SOD Source-to-Image Distance---------------------------------------- Source-to-Object Distance

• If SOD is unknown

–SID - OID

M

SID

SOD

Calculating MF

• Examples

Calculating Size Distortion

•MF = I / O•If Object Size is unknown

•Object Size = Image size / MF

O

I

M

Calculating MF

• Examples

If….Then…M

SID

SODO

I

M• If…

• MF = SID / SODMF = SID / SOD• And…

• MF = Image Size / Object SizeMF = Image Size / Object Size• Then…

• SID / SOD = IS / OSSID / SOD = IS / OS

Calculating MF

• Examples

Assessing Radiographic DistortionAssessing Radiographic Distortion• 2 Types of Distortion

DISTORTION

SIZE SHAPE

Shape Distortion• True Distortion• Produces a true perversion of the structures

of interest that can make the recorded image unrecognizable from the original structures of interest

Shape Distortion• Two Types:

– Elongation Elongation – ForeshorteningForeshortening

• Due to:– AlignmentAlignment

• Central rayCentral ray• Anatomical partAnatomical part• Image receptorImage receptor

– AngulationAngulation• DegreeDegree• DirectionDirection

Shape Distortion• ElongationElongation refers to

images of objects that appear longer than the true object

• ForeshorteningForeshortening refers to images that appear shorter than the true objects

ElongationElongationShape distortion resulting from improper alignment of the Image Receptor

ElongationElongationShape distortion resulting from improper alignment of the x-ray tube (CR)

ForeshorteningForeshorteningShape distortion resulting from improper alignment of the part

The Affects of DistortionTake a close look at this illustration. Do you see how the bone ends at the fracture site look a little different in each radiograph? These different views result from differences in the alignment of the CR, bone, and IR.

A simple fracture may go undetected due to shape distortion

Lab DemonstrationsLab Demonstrations

Minimizing Shape Distortion• X-ray tube alignment (CR)

– Perpendicular to Part and IR– *Elongation

• Image Receptor alignment– Parallel to Part– *If the IR is the piece un-aligned– (CR and part still perpendicular)

– *Elongation • Part alignment

– Parallel to IR – *If the Part is the piece un-aligned– (CR and IR still perpendicular)

– *Foreshortening

Interactive ReviewInteractive Review

• Interactive Online Reviews– Plickers– Quizstar– Qzzr

• Group/Team Work

SUMMARY• Size Distortion

– OID• Shortest OID

– SID• Longest SID

• Shape Distortion– Alignment and Angulation

• Elongation– CR, IR, and Part Perpendicular

• Foreshortening – Part and IR Parallel