Distortion

1. Distortion Distortion

2. Distortion Distortion
Unequal magnification of different portions Unequal magnification of different portions
of the same object.of the same object.
Two types of distortion:Two types of distortion:
1.1.Size distortionSize distortion
2.2.Shape distortionShape distortion

3. Size DistortionSize Distortion
Refers mispresentation of actual size of the structure as Refers mispresentation of actual size of the structure as
recorded on the film. Another term for size distortion is recorded on the film. Another term for size distortion is
magnification.magnification.
Size distortion influenced by :Size distortion influenced by :
a)a)OFDOFD
b)b)FFDFFD
Formula to calculate size distortionFormula to calculate size distortion
 image width FFDimage width FFD
 ---------------- = ------------------------ = --------
 Object width FOD Object width FOD

4. Example of calculation of the amount Example of calculation of the amount
of magnification of an image width, of magnification of an image width,
object width.object width.

5. Magnification resulting from different Magnification resulting from different
OFDOFD

6. Magnification resulting from different Magnification resulting from different
FFD.FFD.

7. Minimal Distortion RadiographyMinimal Distortion Radiography
Size Distortion:Size Distortion:
Always place the structure to be examined Always place the structure to be examined
as close to the film as possible.as close to the film as possible.
Use a standardize FFD for all radiographic Use a standardize FFD for all radiographic
procedures. This should be the greatest procedures. This should be the greatest
distance consistent with the x-ray tube distance consistent with the x-ray tube
output and demands of the procedure with output and demands of the procedure with
regard to motion control. regard to motion control.

8. Size Distortion Used to AdvantageSize Distortion Used to Advantage
In angiography to visualize small blood In angiography to visualize small blood
vessel. vessel.
In macroradiography to visualize early bone In macroradiography to visualize early bone
destruction .destruction .

9. How is magnification achieved?How is magnification achieved?
By increasing the distance between the By increasing the distance between the
anatomy to be radiographed and the image anatomy to be radiographed and the image
receptor (OID)receptor (OID)

10. Magnification RadiographyMagnification Radiography
Can be intentional or unintentionalCan be intentional or unintentional
Unintentional:Unintentional:
–Occurs when the film cannot be placed near the body part being Occurs when the film cannot be placed near the body part being
radiographedradiographed
Intentional UsesIntentional Uses
–Vascular imagingVascular imaging
–NeuroradiologyNeuroradiology
–OrthopedicsOrthopedics
–MammographyMammography

11. The Magnification FactorThe Magnification Factor
The means of determining how much the The means of determining how much the
image will be magnifiedimage will be magnified
Magnification = SID/SOD or SID/SID - OIDMagnification = SID/SOD or SID/SID - OID

12. Technical Requirements for Technical Requirements for
MagnificationMagnification
A small focal spot must be usedA small focal spot must be used
–App. 0.3mm or lessApp. 0.3mm or less
Grids are not necessary as the increased Grids are not necessary as the increased
OID utilizes the air gap techniqueOID utilizes the air gap technique
Low mA must be usedLow mA must be used

13. Disadvantages of Magnification Disadvantages of Magnification
RadiographyRadiography
Patient dose is increasedPatient dose is increased
To obtain a magnification factor of 2, the To obtain a magnification factor of 2, the
patient must be placed halfway between the patient must be placed halfway between the
film and tubefilm and tube
A magnification factor of 2 increases patient A magnification factor of 2 increases patient
dose by a factor of 4dose by a factor of 4

14. Macroradiograph of hand-wrist phantom: magnification factor of 2; angle of 10°;
long axis of trabecular pattern at 90° to cathode–anode axis (fingers pointing at
90° to cathode–anode axis); measured positional-dependent effective focal spot
size of
0.14 mm.

15. Macroradiograph of hand-wrist phantom: magnification factor of 2; angle of 0°;
measured positional-dependent effective focal spot size of 0.76 mm.

16. Macroradiograph of hand-wrist phantom: magnification factor of 2;
angle of -10°; long axis of trabecular pattern at 90° to cathode–anode
axis (fingers pointing at 90° to cathode–anode axis); measured
positional-dependent effective focal spot size of 0.14 mm.

17. Shape DistortionShape Distortion
Often referred to a true distortion, caused Often referred to a true distortion, caused
by elongation or foreshortening in by elongation or foreshortening in
radiographic image.radiographic image.
Shape distortion influenced by:Shape distortion influenced by:
(a) Part- film relationship.(a) Part- film relationship.
(b) Central ray- part-film alignment.(b) Central ray- part-film alignment.
(c) Central ray direction. (c) Central ray direction.

18. Examples of incorrect (non-parallel) and correct Examples of incorrect (non-parallel) and correct
(parallel) relationship of structure/ plane of the (parallel) relationship of structure/ plane of the
interest to the of the film.interest to the of the film.

19. Examples of good (centered) and poor (off-center) Examples of good (centered) and poor (off-center)
alignment of the central ray, the body part, and the alignment of the central ray, the body part, and the
film. film.

20. Examples of proper (left) and improper (right) Examples of proper (left) and improper (right)
direction of the central ray alignment to the structure/ direction of the central ray alignment to the structure/
plane of interest. plane of interest.

21. Scapular images (a) free from distortion, (b) Scapular images (a) free from distortion, (b)
with elongation, (c) with shorten with elongation, (c) with shorten
(a) (b) (c)

22. Shape Distortion Used to advantageShape Distortion Used to advantage
To avoid superimposition.To avoid superimposition.
To demonstrate anatomy.To demonstrate anatomy.

23. Examples of arrangement of central ray-part-film relationship Examples of arrangement of central ray-part-film relationship
to produce distortion that (a) avoid superimposition and (b) to produce distortion that (a) avoid superimposition and (b)
demonstrates anatomy. demonstrates anatomy.

24. Minimal Shape Distortion Minimal Shape Distortion
Place the body part to be examined in a Place the body part to be examined in a
parallel relationship with the film surface parallel relationship with the film surface
whenever possible. whenever possible.
The central ray should passed through the The central ray should passed through the
midpoint of structure to be examined.midpoint of structure to be examined.
The central ray must be directed at right The central ray must be directed at right
angles (perpendicular) to the long axis of angles (perpendicular) to the long axis of
the structure. the structure.

25. THANKSTHANKS