Date post: | 14-Apr-2017 |
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Healthcare |
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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