Chapter 29: Distortion

Describe adjustments of SID and OID that will minimize magnification

The SID must me maximized to decrease magnification.The OID must be minimized to decrease magnification.

What is distortion?

a misrepresentation of the size or shape of the structures being examined

How can shape distortion be used to advantage?

can be used to view a part that is normally superimposed

Caudad

towards the tail

Explain why the proper terms used to describe angulation direction and degree

Angulation refers to the direction and degree the tube is moved from normal perpendicular to IR. Direction is referred to as caudad (towards feet) and cephalad (towards the head). transverse angles are termed right and left Degree is a method of describing how much angulation is used from the standard.

What is the difference between elongation and foreshortening?

Elongation projects the object longer than it really is, while foreshortening projects the object to be shorter than it really is.

How do the direction and degree of angulation affect shape distortion?

It is designed to cause a controlled amount of distortion so that parts are not superimposed.

Differentiate distorted images from routine projections

It is difficult to assess shape distortion unless a radiographer has a good understanding and knowledge of the patient's normal for each position or for a normal part

How does the alignment of the anatomical part affect shape distortion?

the less aligned, the more distortion

Define Size and Shape Distortion

the misrepresentation of the size and/or shape of the anatomical part being imaged.

If a projected image measures 5" and the magnification factor is 1.02, what is the size of the actual object?

O=I/M. O=5"/1.02 O=4.9"

What is the magnification factor formula?

M=SID/SOD

If the SID is 40" and the SOD is 30", what is the magnification factor?

M=SID/SOD 40"/30" = mag factor of 1.33

Discuss various methods of minimizing distortion through variation of SID and OID.

Reduce OID as much as possible and increase SID as much as possible. The greater the SID and the lower the OID, the less magnification, and therefore less distortion

Describe the routine relationships between central ray, anatomical part, and image receptor

Shape distortion can be caused or avoided by careful alignment of the central ray with the anatomical part and the image receptor. Proper positioning is achieved when the central ray is at right angles to the anatomical part and to the image receptor.

Describe adjustments of central ray, anatomical part, and image receptor that will minimize shape distortion

Shape distortion can be caused or avoided by careful alignment of the central ray with the anatomical part and the image receptor. Proper positioning is achieved when the central ray is at right angles to the anatomical part and to the image receptor.

Explain why elongation and foreshortening are relational definitions

Shape distortion is the misrepresentation by unequal magnification of the actual shape of the structure being examined. Shape distortion often results because structures lie normally at different levels within the body. Shape distortion also occurs because of the divergence of the x-ray beam. Elongation projects the object so it appears longer than it really is, and foreshortening makes it appear shorter than it really is.

What is the difference between size and shape distortion?

Size distortion is generally a matter of magnification from OID and SID. Shape distortion involves elongation and foreshortening and is a serious alteration in the projected image. ex.alignment (central ray, part, IR), and angulation (direction, degree)

Explain the effects of SID and OID on image distortion

The greater the SID, the smaller the magnification, because as SID increases, the percentage of the total distance that makes up OID increases. The OID must be minimized to decrease magnification.

How do the SID and the OID affect size distortion?

The greater the SID, the smaller the magnification. As OID increases,s size distortion increases.

Calculate the magnification factor when given SID and SOD

The magnification factor is the degree of magnification and is calculated by M=SID/SOD where M = magnification factor SID=Source to IR Distance SOD= Source to Object Distance

Calculate the actual size of an object when given the projected size, SID, and OID

The magnification factor permits calculation of the actual size of an object that is projected as an image by using the formula O=I/M O=object size I=image size M=Magnification factor

Cephalad

Towards the head

distortion

one of 2 geometric properties affecting radiographic image quality. A misrepresentation of the size or shape of the structures being examined

elongation

projected object that appears longer than it really is

foreshortening

projected so it appears shorter than it really is