RADT 1075: Ch 29 Distortion

What is distortion?

a misrepresentation of the size or shape of the structures (anatomical part) being examined

T/F: Distortion like detail, exists even when it cannot be seen due to poor visibility/when the IR exposure and/or contrast are poor.

True

Distortion is directly related to ________.

Positioning

What are the 2 types of distortion?

Size and shape

What are the 2 factors that affect size distortion?

OID and SID

What are the 2 factors that affect shape distortion?

Alignment and angulation

When it comes to alignment, what are the 3 factors that affect distortion?

1-central ray

2-part

3-image receptor

When it comes to angulation, what are the 2 factors that affect distortion?

Direction and degree

T/F: OID has more affect on size distortion than SID.

True

___________ is the only possible size distortion in radiography.

magnification

*due to the divergent property of X-ray photons

Can you resize the image in digital image receptor post-processing?

Yes

In all instances, reduced magnification size distortion _________ (increases/decreases) the spatial resolution.

Increases

Magnification size distortion is controlled by positioning the body part and tube to ______SID while _______ OID.

Maximize, minimizing

The ______ (greater/lesser) the SID, the smaller the magnification.

Greater

As SID increases, the percentage of the total distance that makes up the OID ______ (increases/decreases)

Decreases

As SID decreases, size distortion _____ (increases/decreases)

Increases

As SID increases, size distortion ______ (increases/decreases)

Decreases

As SID decreases, magnification _____ (increases/decreases)

Increases

As SID increases, magnification _____ (Increases/decreases)

Decreases

When objects within a structure are at different levels how will they be projected onto the image?

As different sizes

Smaller objects are perceived as more _____ and larger objects are perceived as _____

Distant, closer

T/F: When describing objects, it's important to remember that the size and distance relationship in a radiographically projected image is the opposite of that perceived visually

True

T/F: When AP and lateral projections cannot be performed because of superimposing structures, it's important to include two oblique projections at 90degrees to one another

True

*because the 2 90degree opposing images can be used to verify the positional relationship of structures

Why is OID important in dosimetry?

Because it establishes the source-to-entrance skin distance that is the benchmark maximum exposure to the patient

T/F: Because OID varies with the part size and position of the patient, it accounts for the decreased exposure that is part of many examinations.

False-it accounts for INCREASED exposure

Larger patients receive a ______ (lesser or greater) exposure simply because their entrance skin surface is closer to the source making their SOD much less

Greater

OID must be _______ to decrease magnification

Minimized

Magnification, or size distortion can be assessed by calculation of the ___________.

Magnification factor (MF)

What is the Magnification factor (MF)?

The degree of magnification that is demonstrated on a radiograph

What formula is used to calculate MF?

M= SID/SOD

The MF (magnification factor) permits calculation of the _______ size of an object that is projected as an image by using the formula.

Actual

If the image size and the object size are known, the percent of magnification can be determined by what formula?

I-O/O x 100= percent of magnification of the object

What are the 2 types of shape distortion?

Elongation= object appears longer than it really is

Foreshortening= object appears shorter than it really is

T/F: Elongation occurs when the tube or the IR is improperly aligned.

True

*changes in the tube angle cause elongation never foreshortening

T/F: foreshortening occurs only when the part is improperly aligned

True

T/F: Any misalignment of the Central Ray can cause shape distortion.

True

Anatomical part should be ______ to IR

Parallel

IR should be ______ to the anatomical part

Parallel

The Central ray should be ______ to the part and image receptor

Perpendicular

What is Angulation?

The direction and degree the tube is moved from its normal position perpendicular to the IR

The angulation of the tube is designed to cause a controlled or expected amount of shape distortion to avoid _________

Superimposition

T/F: All magnification (shape distortion) involves a degree of loss of resolution

True

When SID increases, size distortion ______ (increases/decreases)

Decreases

When SID decreases, size distortion _______(increases/decreases)

Increases

When OID increases, size distortion _______(increases/decreases)

Increases

When OID decreases, size distortion ____(increases/decreases)

Decreases

When patient thickness increases, size distortion _____(increases/decreases)

Increases

When patient thickness decreases, size distortion _____ (increases/decreases)

Decreases

When the central ray is aligned improperly, shape distortion ______ (increases/decreases)

Increases

When the anatomical part is aligned improperly, shape distortion ______(increases/decreases)

Increases

When the IR is aligned improperly, shape distortion ______(increases/decreases)

Increases