chapter 29 -- principles of radiographic imaging

distortion

• one of geometric properties of image quality

• a misrepresentation of the structures

• two types:

  • size

  • shape

• can only be minimized

factors affecting size distortion

• radiographic distances

  • magnification only

    • due to the divergent property of x-ray beam

    • minimize magnification, increase recorded detail

  • controlled by SID and OID

    • maximum SID and minimum OID

    • PA vs AP chest

    • routine is 40’’ for most studies, getting longer along with advanced technolofy

• digital systems

  • magnification is a type of post-processing

SID

• large = smaller magnification

• small = large magnification

OID

• large = large magnification

• small = small or no magnification

calculating M/I/O

• M = I/O

  • I = image size (size of object image)

  • O = object size

  • M = magnification factor/ratio

• M = 1 ; no magnification

• M > 1; image is greater than object

calculating size distortion

• magnification factor:

  • M = SID/SOD

  • M = I/O

  • M = I/O = SID/SOD

optimizing image quality

• size of image(I) = size of object(O) x M

• size of image(I) = size of object(O) x SID/SOD

• minimize magnification

  • minimize OID

  • maximize SID

calculating object size

• size of image (I) = size of object (O) x SID/SOD

• (I - O)/O x 100% = percent of M of the object

factors affecting shape distortion

• the misrepresentation by unequal magnification of actual structures

• what causes it:

  • wrong relationship between CR, body part, and IR

  • incorrect centering — off-centering the tube, incorrect centering body part, off-centering the IR

• two types:

  • elongation

  • foreshortening

• alignment:

  • central ray, anatomical part, image receptor

• to minimize:

  • CR perpendicular to IR; part parallel to IR

  • sometimes, angle is necessary

  • CR: further away from center, more distortion

• evaluation is very subjective

elongation

tube/receptor is improperly aligned

foreshortening

only when part is improperly aligned

angulation

• away from perpendicular to the IR

  • degree

  • direction

  • controls how much distortion to avoid superimposition

  • changes the SID

    • more angle (thicker body part), the longer SID

      • will affect IR exposure if not compensated

      • to compensate, decrease SID (lower tube) or increase mAs

degree

• how much angulation — how much distortion

• the angle between CR and IR

direction

caudal or cephalic

effects on image appearance

• size

  • magnification

  • always involves a degree of loss of resolution

• shape

  • both elongation and foreshortening

  • can be utilized sometimes