1725 Distortion

Distortion Definition

• A misrepresentation of the size and or shape of the anatomical part being imaged

Distortion can be classified as either

• Size

• Shape

Factors Affecting Size Distortion

• OID

• SID

Factors Affecting Shape Distortion

• Alignment

  • Central Ray

  • Part

  • IR

• Angulation

  • Direction

  • Degree

Size Distortion (Magnification)

• An increase in the image size of an object compared with its true or actual size

• Radiographic images of objects are always magnified in terms of the true object size

  • SID and OID are important factors

Magnification

• Reducing magnification - Reduces size distortion, therefore increasing spatial resolution

• The objective in radiography is to minimize magnification as much as possible

  • Magnification radiography is the exception to this rule

Magnification Radiography

• Used in Interventional Radiology and Mammography

• Enhances the visualization of small structures

  • Conventional radiography strives to minimize magnification

  • Magnification Radiography deliberately increased OID to cause magnification

How to calculate the Magnification Factor

MF = Image Size/Object Size

How is magnification assessed

By calculation of the magnification factor = degree of magnification

How to calculate size distortion

M = SID/SOD

What does the magnification factor permit calculation of

The actual size of an object that is projected as an image

Formula for actual size

• O = I/M

• O = Object Size

• I = Image Size

• M = Magnification Factor

Large SID

Use as large a source to image receptor distance as possible to reduce magnification

Small OID

Place the object as close to the image receptor as possible (To reduce magnification)

Source to Image Receptor

• A major effect on magnification

• The greater the SID, the smaller the magnification

• This is because as SID increases, the percentage of total distance that makes up OID decreases

• Magnification size distortion is minimized by increasing SID

Object to image receptor

• Very critical when discussing magnification and resolution

• Short OID, Large SOD - Low Entrance skin exposure

• Large OID, Small SOD, - High entrance skin exposure

Shape Distortion

Unequal magnification of different portions of the same object

Shape Distortion depends on

• Object thickness

• Object position

• Object shape

Displaces the projected image of an object from its actual position and can be describes as either:

• Elongation

• Foreshortening

Elongation

Projects the object so it appears to be longer than it really is

Foreshortening

Projects the object so it appears shorter than it really is

Any misalignment of the CR among which 3 factors alters the shape of the part recorded in the image

• Tube

• Part

• Or Image receptor

Factors Affecting Shape Distortion

• Alignment

  • Central Ray

  • Anatomical Part

  • Image Receptor

• Angulation

  • Degree

  • Direction

Alignment

• Shape distortion can be caused or avoided by alignment of the central ray with the anatomical part and central part

• Proper position is achieved when the central ray is at right angles to the anatomical part and the image receptor

  • This means the part and image receptor must be parallel

Creative Alignment

• When the position of the body part or object within body do not permit alignment, this is utilized

• Examples:

  • 25 degree cephalic angulation of the pelvis to demonstrate sigmoid colon

  • 10 degree caudal angulation of the coccyx

    • These angles help minimize distortion

Incorrect Centering

Alignment adjustments involve bringing the tube central ray, the part, and image receptor back to their correct relationship

Central Ray

• Ideally, the central ray is intended to be projected perpendicular to both the anatomical part and the image receptor

• Whenever the central ray is not perpendicular, some degree of distortion will occur

• This occurs in every image because the central ray is not truly perpendicular

Any structure which is not positioned at the central ray

Will be distorted because of the divergence of the beam

Farthest from the central ray

Greater distortion

The long axis of the anatomical part is intended to be positioned

Perpendicular to the central ray and parallel to the image receptor

The image receptor is intended to be positioned

• Perpendicular to the to the central ray and parallel to the anatomical part

• If IR is not parallel to object or if central ray is not centered to part, serious distortion can take place

Normal Relationship between part and IR

Foreshortening and magnification due to part alignment

Elongation and magnification due to changes in part/IR and Central Ray/IR alignment

What does angulation refer to

The direction and the degree the tube is moved from its normal position perpendicular to the image receptor

What is angulation used to avoid

Superimposition of parts

Angulation in tube results in

Some form of distortion

Angulation does what to SID

Increases SID which will decrease exposure to IR

Most common tube angle

Longitudinal which are often termed cephalic or caudal

Angled Transverly

“Roll”

What is direction of tube angle dependent on

Patient position Ex: 25 degree cephalic AP projection is identical to 25 degree caudal PA projection

Evaluating Shape Distortion

• Shape distortion is more subjective than size

• Relies on radiographer’s knowledge of normal anatomy and the normal projected images for each position

What is size distortion generally

Magnification

All magnification involves

A degree of loss of resolution