Unit 7 Distortion

types of distortion

> size distortion > shape distortion

SOD

> source-to-object distance > the physical distance from the source of radium (target/focal spot) to the object > "object" is surface of body or the structure within the body

images distorted by size/or shape have

less recorded detail which could hinder the radiologist in making a diagnosis

the ideal OID

0"

for each 5° of the tube angle, decrease the SID by ________ to maintain IR exposure

1" (2.5 cm)

SID used for most exams

40" (100 cm)

some equipment detents to the bucky at

42" or 44" or more

sharpness of recorded detail is affected by 2 geometric properties

> Distortion > Spatial resolution

visibility of detail is affected by two photographic properties

> IR exposure > Contrast

factors that increase distortion

> OID (size only) > part thickness (size only) > tube angle (shape only)

factors that decrease distortion

> SID (size only) > CR/part/IR alignment (shape only)

factors that affect size distortion

> SID > OID

factors that affect shape distortion

> alignment > angulation

alignment

> central ray (CR) > part > image receptor (IR)

exams with a large OID that require a 72" SID (180 cm)

> chest > lateral C-spine > lateral sternum

because an x-ray beam diverges, only the _____ is truly perpendicular

CR

Ideal CR alignment

CR perpendicular to the anatomical part & IR

T/F: minification is possible

False

a thicker body part will increase the

OID

objects within the body at different levels will project on to the IR as different sizes due to variances in

OID

SOD + OID =

SID

both _____ & _____ control magnification

SID, OID

Image Quality

a combination of visibility of detail & sharpness of recorded detail

any body part not perpendicular to the CR or not parallel to the IR will have

a greater degree of shape distortion

ideal angulation

a zero (0) degree tube angle where the CR is perpendicular to the IR

advantages of shape distortion

allows for visualization of anatomic structures that might otherwise be hidden > positioning: obliques > beam angulation: degree & direction Example: L5/S1 spot angulation 5°-8° caudal angle

part

anatomical structure being imaged

reversing the patient position reverses the

angle direction

ideal anatomical part alignment

body part perpendicular to the CR & part parallel to the IR

angulation of the tube increases the SID, so unless compensated for, magnification IR exposure will be ______

decreased

as CR, part and/or IR alignment increases, shape distortion

decreases

as OID is decreased, size distortion (magnification)

decreases

as SID increased, size distortion (magnification)

decreases

as beam angulation decreases, shape distortion

decreases

as part thickness decreases, size distortion (magnification)

decreases

the relationship between OID & magnification is

direct

angulation

direction & degree

OID

due to the divergent property of x-ray photons, only magnification as a type of size distortion is possible

as CR, part and/or IR alignment decreases, shape distortion

increases

as OID is increased, size distortion (magnification)

increases

as SID is decreases, by size distortion (magnification)

increases

as beam angulation increases, shape distortion

increases

as part thickness increases, size distortion (magnification)

increases

the relationship between SID & magnification is

inverse

most common angulation

longitudinal angulations (cephalad & caudad) > direction can also be transverse aross part

Distortion

misrepresentation of the size or shape of an object being examined

elongation

object appears longer than its true size

foreshortening

object appears shorter than its true size

angulation & misalignment of the CR/part/IR can result in ________

shape distortion

angulation affects

shape distortion

any anatomic structure not directly under the CR will exhibit some degree of

shape distortion

knowing where the CR should enter/exit for a particular x-ray exam is needed to minimize

shape distortion

the further away the anatomy is from the CR, the greater the

shape distortion

magnification factor

the degree of magnification of an object on an image M= SID / SOD

angulation (degree)

the degree the x-ray tube is moved from its normal perpendicular position in relation to the IR

angulation (direction)

the direction the x-ray tube is angled in relation to the part

factors that have negligible or no effect on distortion

> kV >mAs > FS size > Filtration > beam restriction/collimation > grid ratio/frquency > tissue (mass) density & Z#

size distortion

> magnification > enlargement of an object on an image

shape distortion

> misrepresentation of an object on an image

T/F: SID has been increasing over the years

True

T/F: all Bucky images have increased magnification as compared to table-top due to increased OID

True

T/F: all x-ray images have some degree of geometric distortion, the goal is to minimize distortion through proper positioning & use of technical factors

True

SID

due to the divergent property of x-ray photons, only magnification as a type of size distortion is possible

angulation & misalignment of the CR/part/IR can result in shape distortion. If the OID is affected as well, the image will exhibit __________ and ___________

size and shape distortion

OID has a greater effect on

size distortion

a smaller object with greater OID may appear as _______ as a larger object with less OID

the same

CR

the theoretical center of the x-ray beam where photons exit the focal spot

best means of minimizing size distortion

use a far SID & close OID