math unit 2

Image quality separates into

Sharpness and visibility

Sharpness

a measureable geometric property of radiographic quality

• measured in lp/mm

• 5 lp/mm average visual acuity

• deals with image resolution or the number of structural lines recorded

Sharpness divides into

Spatial resolution and distortion

Other names for spatial resolution

• detail

• Sharpness

• Recorded detail

• definition

What is spatial resolution

formation of structural lines of an image

What effects spatial resolution?

• motion

• IR

• geometry

What is the biggest influence on spatial resolution?

Motion

How does the IR effect spatial resolution?

Image noise

Two types of motion

Voluntary and involuntary

How do you control involuntary motion?

Changing techniques

Ways to control voluntary motion

• communication (#1)

• Immobilization

• Technique

Types of immobilization

• whole body

• Partial

• Sponges

Types of whole body immobilization

• Piggostat

• Brat board

• Mummy wraps

• Compression bands

Types of partial immobilization

• tape

• Sandbags

• Ace bandages

Types of sponge immobilization

• position maintainers

• Position achievers

Factors that affect geometric unsharpness

• SID

• OID (#1)

• FSS

Two types of distortion

Size and shape

Size distortion

Object appears larger/smaller than it actually is

As SID increases, magnification..

Decreases

As OID increases, magnification…

Increases

Shape distortion

Causes elongation or foreshortening of the image

• “Fun house mirror”

Factors that effect shape distortion

• part-IR alignment

• CR direction

• rotation/oblique

Ways to change technique

• mA/time

• Kvp/mAs

• SID

percent of magnification formula

(OID/SID) x100

geometric unsharpness formula

GU = (FSS x OID) / SOD

what does an increase in SID do to detail

increase

what does an increase in OID do to detail

decrease

what does an increase in focal spot size do to detail

decrease

what to strive for in terms of sharpness

• high SID

• low OID

• small FSS

what is quantum mottle

a lack of sufficient incoming data to process an image

• caused by nit enough mAs

what is imaging noise

total noise that the image receptor receives

• includes system noise, ambient noise, and quantum noise

what is penumbra

unsharpness in the recorded edges and lines within an image

what is the difference in focal spot sizes

small focal is more detailed than a large focal

• tube will not allow you to use small focal on all images

image receptor exposure

the amount of blackness in an image

• has to do with the quantity of radiation that reaches the IR

contrast

differences in density in the image

• how the xray beam goes through each part of your body

  • thin vs. thickness of body parts

  • the thicker, the whiter

visibility

what you can see

what does visibility divide into

IR exposure and contrast

IR exposure

• amount of radiation received by IR

• overall blackness of image

what factors affect IR exposure

• patient factors

• mAs

• additional factors

what patient factors affect IR exposure

tissue thickness and opacity

how does mAs affect IR exposure

when mAs increases, quantity of radiation increases

what are the additional factors that affect IR exposure

• SID

• kvp

• anode-heel effect

• casts

• grid

• collimation

• image receptor

• filter

how does SID affect IR exposure

increased SID decreases intensity

how do you see the anode-heel affect

open collimation with low SID

how do grids affect IR exposure

a high grid ratio lowers IR exposure

how does collimation affect IR exposure

an open collimation decreases IR exposure

how do casts affect Ir exposure

you have to up your technique to get through them

• dry plaster: 2x mAs

• wet plaster: 2x mAs and increase kvp 10%

• fiberglass: increase but not as much as plaster

do grids reduce dose

no

inherent filtration

thin sheet of copper or aluminum in the xray tube

• used to reduce patient dose

• filters out useless xray beams

compensating filters

produces a uniform IR exposure

• wedge or trough shaped

• slides into spot on the outside of tube

what is the primary factor that effects IR exposure

mAs

contrast

difference between two or more receptor exposures (densities)

• must be adequate IR exposure to see contrast

• very subjective

divides into subject contrast and IR contrast

two factors of contrast

quantity - # of useful receptor exposures recorded

quality - discriminate the recorded densities from one another

subject contrast is affected by

• tissue thickness

• tissue opacity

• tissue density

what affects tissue thickness

• age

• sex

• level of development

• condition (sickness)

what effects tissue opacity

• cellular composition

• tissue structure

• surrounding structures

tissue densities (list least to most dense)

1. gas/air

2. fat

3. cartilage

4. hollow organs

5. muscle

6. solid organs

7. filled organs

8. bone

9. tooth enamel

IR contrast separates into…

digital IR contrast and film contrast

what is fog

any unuseful densities on an image

what are the two factors that influence scale of contrast

kvp and fog levels

scale of contrast

the number of useful densities or shades of grays recorded on an image

• same as latitude

film contrast

range of exposures that will produce densities in the acceptable disagnostic range

• acceptable ra ge is .25-2.5

what kind of contrast does short exposure latitude have

high contrast

what kind of exposure latitude does low contrast have

long exposure latitude