Distortion - Ch 31

Parts with large inherent OID

lateral cervical vertebra chest

entrance skin exposure is greater for

lateral projections because the skin surface is closer to the source, making their SOD much less

what is an example of a study developed to ensure accurate CR centering

long bone studies - with a ruler, and spot exposures of hip, knee and ankle are made

most common direction of tube angle

longitudinal

Often during a lateral projection,

mAs must be increased in order to provide sufficient IR exposure - increases pt skin exposure/dose

size distortion

magnification

what is the only size distortion possible with film/screen systems

magnification

OID is the critical distance for

magnification and resolution

exception to the magnification rule of minimizing magnification as much as possible

magnification radiography - to increase the size of structures that are too small to be eaily visualized

objects further away from the IR will be

magnified

All size distortion is controlled by

maximizing SID and minimizing OID

shape distortion

misrepresentation by unequal magnification of the actual shape of the structure being examined

Distortion

misrepresentation of the size and shape of the structures exists even when it cannot be seen due to poor visibility (density and contrast)

smaller objects are perceived as

more distant

shape distortion is more

subjective evaluation than size - more difficult to assess - there is no effect that can be calculated

size distortion is

symmetrical

The least amount of distortion is seen when

the CR is perpendicular to the IR, and the part is parallel to the IR

what is a magnification factor

the degree of magnification of an object

when the pt position is reversed,

the direction of the tube must also be reversed

magnification formula assumes that

the focal spot is a point source

the farther from the CR,

the greater the distortion

M = 1 means

the image was perfectly imaged without magnification

the size and distance relationship in a radiographically projected image is

the opposite of that perceived visually

foreshortening can happen if

the part is angled or normal angulation within the body

When does elongation happen

when the tube or IR is improperly aligned

ribs are seen as

wider as they become more posterior - on a PA chest exam

object size calculation

MO = I or O = I/M

The standard distance is

40"

Some institutions use xx as the standard SID

48"

standard reference point is

90° from the patient's head

factors that affect shape distortion

CR alignment, anatomical part alignment, IR alignment, CR direction of angle, CR degree of angle

% of magnification calculation

I - O / O x 100

Is it possible to record ALL diagnostic information?

No, it is not possible - there is always some loss of information.

Factors that impact distortion

OID, SID, CR-part-IR alignment, and angulation of CR (direction and degree)

Angling the tube also changes the

SID - so it must be compensated for by using a new SID or it produces a decrease in IR exposure

factors that affect size distortion

SID, OID, pt thickness

SID =

SOD + OID

Examples of when CR angling is used to compensate for normal part angulation or to prevent superimposition

Sacrum - normal part angulation AP bladder - to prevent superimposition

Elongation is the result of

Tube or IR angulation

Both magnification and minification are possible with

digital systems - thru post-processing parameters

what is required to adjust distortion

a knowledge of normal radiographic anatomy minimizing distortion to provide accurate imaging

degree

a method of describing the exact amount of angulation - described as the angle b/w the CR and the IR plane from the standard reference pt of perpendicularity

Anything that affects distortion,

affects recorded detail, but not because of penumbra - it's because of the divergence of the beam.

shape distortion involves both elongation and foreshortening and is a serious

alteration in the projected image

roll

angled transversely - right and left of the pt

Centering away from the specified CR entrance point is equivalent to

angling the tube away from perpendicular because the entire perspective of the anatomical part is distorted.

Angulation can be used to advantage in these situation

angling to compensate for normal part placement angling to avoid superimposition of other parts

why is shape distortion common

because structures lie at different levels in the body and because of the divergence of the x-ray beam

transverse angulations are not possible during

bucky tray projections due to grid cutoff

objects smaller than the effective focal spot

cannot be demonstrated

adjustment of shape distortion requires

careful consideration of the beam-part IR geometry involved in the projection

Distortion increases at greater distances from the

central ray - because of divergent rays

longitudinal angulations

cephalad and caudad

If IR and part are parallel, the only result of off-centering of the IR is the

clipping of a portion of the area of interest

larger objects are perceived as

closer

The angulation of the tube is designed to cause a

controlled or expected amount of shape distortion to avoid superimposition

all magnification results in a

degree of loss of resolution, even when special systems are designed to minimize the loss

magnification factor

degree of magnification - calculated by: M = SID/SOD

When the most perpendicular portion of the beam is used,

distortion is decreased as well as penumbra

When the long axis of the part is not perpendicular to the CR

distortion may occur

changes in tube angle cause

elongation (but never foreshortening)

magnification is the only size distortion possible with

film/screen radiography - due to divergent property of xray photons.

The magnification factor is used to calculate

image size or object size during size distortion

When AP and lateral projections cannot be performed due to superimposing structures,

include 2 oblique projections at 90° to one another.

patient thickness impacts distortion because of

increased or decreased OID

Reducing magnification

increases resolution of recorded detail

A change in SID requires

new grids and x-ray unit design so tube-to-tabletop distances can be achieved, and low-table design so radiographers can reach the tube. The increased resolution is worth the expense and trouble

When the CR is angled, it is

not in a perpendicular position

When does foreshortening happen

only occurs when the part is improperly aligned

M will always be

over 1

Foreshortening is the result of

part angulation

Magnification increases

penumbra (blur), therefore decreases detail

The IR s/b positioned

perpendicular to the CR and parallel to the part

CR should be

perpendicular to the part and IR

foreshortening

projects the object so it appears shorter than it really is

elongation

projects the object so it appears to be longer than it really is

CR angulation can be useful to

provide a projection that would otherwise be impossible to differentiate from superimposing structures

angulation

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

the CR s/b at a

right angle to the part and IR - so for parts that are not parallel or perpendicular to the IR, a CR angle is used

If the IR plane is NOT parallel to the object or if the CR is not centered to the part

serious shape distortion will result exactly as if the object were not parallel

alignment helps with

shape distortion

the tangential os calcis is example of when

shape distortion can be used to advantage

The 2 types of distortion are

size and shape

magnification

size distortion

when the CR is not perpendicular to the part/IR

some distortion will result

The entire assessment of shape distortion lies with

the radiographer's knowledge of normal anatomy and the normal projected images for each position

The greater the SID

the smaller the magnification

the greater the SID,

the smaller the magnification

CR

the theoretical photon that exits from the exact center of the focal spot.

How is minification possible in a digitial system

through post processing algorithms

Some projections use angulation

to avoid superimposition of parts

shape distortion is

unequal in symmetry

A PA lumbar uses distortion to

use the divergence of the beam to open the lordotically curved intervertebral jts