Lecture 4 GEOMETRICAL FACTORS

Photons travel in straight, divergent lines

Since photons move in straight but different directions (divergence)

magnification and distortion are seen on CT and portal images

Divergence of an x-ray beam

• Because the beam widens as it gets the farther from the source:

• More tissue is exposed as the beam exits the patient than at the anterior skin surface

• This also accounts for magnification

Magnification

• When an image appears larger than it actually is

  • All images on a CT, portal image or LCD

    monitor are larger or smaller than the

    structures actual size

Magnification may depend on:

• Geometrical arrangement of the target

• Patient size

• The medium on which the image is displayed

Sometimes determining the size of internal anatomy is not

possible.

To determine the size of an object inside a patient, there are 2

ways to measure the magnification factor.

Magnification Factor

is directly proportional to the distance of the object from the target and is dependent on the distance of the object from the image receptor

2 formulas to measure Magnification Factor

To determine the size of an object inside a patient, magnification can be calculated by using the ratio of TID (target to image distance) and TOD (target to object distance)

Magnification- Measuring Devices In Radiographic Imaging

• Devices that are used for comparing objects of a predetermined size with objects of an undetermined size

  • Fiducial plate / graticle

  • Rulers / Calipers

  • Rings

  • Coins

  • Digital Scaling Software

Distortion

a change in the size, shape or appearance of the structures being examined

Magnification

is a type of size distortion

Size distortion

Shape distortion

• is the misrepresentation of the actual shape of the structure that is examined

• This happens when the object plane or part being imaged is not parallel with the image plane

Accessing Radiographic Distortion (2 types)

• Radiographic misrepresentation of either the size or shape of the anatomic part is called distortion.

• 2 types

  • Size

  • Shape

When angling the beam, shape  distortion will occur on the image,

If the 2 planes are parallel:

The only size distortion that occurs is Magnification

OID

OID has a direct relationship with size distortion

• As OID increases = size distortion/magnification increases

• As OID decreases = size distortion/magnification decreases

Close the Object is to the film (OID), the less distortion

When looking at the obturator foramina, why do they do look of unequal size?

Answer: the patient is rotated (patient also has a Ewing's sarcoma of the pelvis)

Gantry angle examples

3 rule for accurate image formation to reduce distortion:

• Decrease OID = decrease in distortion and magnification

• Have central ray perpendicular to film

• Have plane of object parallel to film