Distortion

distortion

when accuracy is affected by the misrepresentation of the true shape and size of the anatomy being examined

always will exists but can be minimized

distortion with size (aka magnification)

causes image to appear larger than object being radiographed

magnification is the only possible size distortion

controlled by distance only (SID or OID)

digital post processing can resize

when size distortion decrease, resolution increase

SID with size distortion

when SID increase, size distortion decrease

makes up the OID

OID with size distortion

when OID increase, size distortion increase

AP vs PA: which is closer to the IR to reduce OID bw the IR and anatomy needed

OID and dosimetry

important bc establishes SOD

when OID is larger, skin is closer to the tube, ESE will increase

ex: AP less ESE than LAT bc OID is larger in a LAT than AP

large patients get more radiation bc they’re skin is closer to the xray tube naturally, causing more OID, smaller SOD, and more ESE

calculating magnification (formula)

M= SID/SOD

tells you how much longer and wider the image is compared to the original object

calculating actual object size (formula)

O= I/M

I: image size

M: magnification factor

O: object size

shape distortion

misrepresentation of the true shape of the object

either elongated (longer and thinner) or foreshorten (shorter and thicker)

elongation

occurs when tube or IR isn’t properly aligned

tube: when tube is angled and object parallel to IR

IR: when IR and object not parallel but CR is perpendicular

image appears stretched out

can be used as an advantage (open joint space, remove superimpose, show hidden anatomy, etc.)

foreshortening

occurs only when the object isn’t properly aligned when CR is perpendicular to the IR

image appears squashed together and produces unequal magnification

factors affecting shape distortion

structures at different levels/depths within the body (think OID within the body based on the part) causes beam divergence

lengths vary based on the angle bw the object and the diverging beam

Alignment

proper alignment of the CR, IR and object insures minimal distortion

achieved by CR being perpendicular to the parallel object and IR

when alignments are hard, creativity needed

When the CR is further

beam divergence increases and distortion increases

cases of creativity to avoid distortion

sometimes needs multiple projections (like casted elbow) to get entire elbow with less distortion

Angulation

direction and degree of the tube

used to avoid superimprovision

changes the SID so unless new SID used, IR exposure decreases and changes in magnification

almost always longitudinal (cephalad and caudad)

sometimes uses transverse angulation (roll)

Effects of changing factors on distortion chart