Distortion Distortion

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

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) OFDOFDb)b) FFDFFD

Formula to calculate size distortionFormula to calculate size distortion image width FFDimage width FFD ---------------- = ------------------------ = -------- Object width FOD Object width FOD

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.

Magnification resulting from different Magnification resulting from different OFDOFD

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

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.

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 .

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)

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

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

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

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

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 of0.14 mm.

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

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.

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.

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.

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.

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.

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)

Shape Distortion Used to advantageShape Distortion Used to advantage

To avoid superimposition.To avoid superimposition.

To demonstrate anatomy.To demonstrate anatomy.

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.

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 the angles (perpendicular) to the long axis of the structure. structure.

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