Image Formation & Differential Absorption (RAD 1102)

What are the two primary parts of the X-ray tube?

cathode and anode

What is the cathode composed of?

filament and focusing cup

What is the filament made of?

tungsten

What is the focusing cup made of?

molybdenum

What charge is the cathode?

negative charge

What charge is the anode?

positive charge

The anode is made up of what?

target

The target in the anode is made of what?

tungsten

The anode type that is used in everyday x-ray is known as....

rotating

thermionic emission

when filament is heated and electrons are boiled off

Heating of filament is controlled by what?

mA (milliamperes)

When mA is increased, what happens to the filament, the number of electrons boiled off, and the number of radiation?

gets hotter; increase; increase

Electrons are driven over to the anode at high speed by the ________ ________ of the negative charge placed on the ______ and a positive charge on the _________

*potential difference*; cathode; anode

When KV is increased, what happens to the energy of the x-ray, penetration, and the amount of radiation?

all will increase

quantity or exposure (intensity or output)

amount of radiation passing per second through a unit area of the surface perpendicular to the direction of the beam at the designated area

mA (milliamperes) controls what of the X-ray beam?

quantity or exposure (intensity or output)

quality

ability of the X-ray beam to penetrate matter

KV (kilovolts) control what of the X-ray beam?

quality of the X-ray beam

Direct transmission

radiation gets to the IR without interacting with tissue

Radiolucient - areas with low brightness

photoelectric interaction

*total absorption* of radiation by the body and is responsible for subject contrast

image receptor (IR)

devices that receives remaining radiation leaving the patient

differential absorption

image formation is result of this as x-ray beam passes through the body as variations of absorption and transmission of the exiting X-ray beam structurally represent the anatomic area of interest

The combination of ________ and ________ of the X-ray beam provides an image that represents the anatomic part

absorption and transmission

brightness

amount of light seen on the monitor

direct transmission

radiation gets to IR with *no interaction* of any tissues and passes through anatomic part unaffected (known as radiolucent material) which shows area with low brightness

radiolucent material

tissues that have low tissue density/opacity allowing X-ray to pass through it/easy to penetrate (i.e. air) and is shown as black/low brightness in a radiographic image

attenuation

total reduction in number of x-ray photon intensity as the photon passes through material (i.e. patient body) and occurs through absorption and scatter radiation

absorption

radiation is absorbed by the tissue through photoelectric effect and somewhat by Compton's effect

photoelectric effect

represents area with high X-ray absorption or is radio-opaque showing an increase in brightness

radio-opaque

tissues that have high density/opacity making it hard to penetrate or blocks radiation (i.e. heart and bone) and is shown as white in a radiographic image

What factors determine the probability of a photoelectric (PE) interaction?

energy of the radiation and the atomic number of the tissue

Compton interaction/scatter

*partial absorption* of radiation and gives fog on image (aka indirect transmission)

scatter radiation

some incoming photons are not totally absorbed but lose energy during the interaction and changes its original direction

The probability of scatter radiation depends on the energy of the _________ __________

incoming photon

What direction can scatter radiation occur?

any direction

Technologists experience the most occupational exposure because of...

scatter radiation

Intensity of scatter is affected by 3 things. What are those things?

- KV

- collimation (control of field size)

- patient factors such as thickness and density of tissue (i.e. fat creates more scatter than bone)

What are things that can control scatter radiation?

- grids

- beam restriction

- air gap

- compensating filters

- compression

Scatter photon ________ the quality of an image

decreases

The _______ & ______ of radiation will control the interactions that occur in the various tissues of the body

quantity (mA) & quality (KV)

What are factors that affects differential absorption?

- energy of beam

- tissue thickness

- atomic number of structure

- mass density or compactness

If there is a change in thickness of the body part, the technologist should change what?

mAS (milliamperes)

If there is a change in the atomic number or mass density (opacity) of a structure, the technologist should change what?

KVs (kilovolts)

i.e. using barium for upper GI

Tissue that has high tissue opacity will appear _____ on the radiographic image

light

Tissue that has low tissue opacity will appear _____ on the radiographic image

dark

What is the least dense tissue?

gas

What is the most dense tissue?

tooth enamel

primary photons

radiation produced in X-ray tube but occurs before the patient

Reduction of the X-ray beam occurs through which interactions?

photoelectric and Compton's/scatter

exit or image radiation

radiation which transmitted through or scattered in the body

low optical density or radiographic density

the overall degree of light or clear areas (like heart) on an X-ray image because the body's tissue blocked (attenuated) the X-rays

high optical density or radiographic density

the overall degree of black or dark areas (like lungs) on an X-ray image because a high amount of X-ray radiation passed through the body part to reach that area

high tissue density or opacity

tissue in which the cells are packed very close together (or are dense) like bone

low tissue density or opacity

tissue in which the cells are not packed close together (or are not dense) like air

Since the heart or cortex bone has high tissue density/opacity, it will have a ______ optical/radiographic density on a radiographic image

low

Since the lungs or cancellous bone has low tissue density/opacity, it will have a _______ optical/radiographic density on a radiographic image

high

Give examples of structures with a high tissue opacity and low optical/radiographic density. How does this show up on an image?

bones or heart; white

Give examples of structures with a low tissue opacity and high optical/radiographic density. How does this show up on an image?

air; black/dark

How does an image look when structures attenuate (absorb) more x-rays?

white/light

How does an image look when structures are easily penetrated?

black/dark

If tissue thickness increases, absorption will...

increase

If tissue thickness increases, attenuation will...

increase

If tissue thickness increases, transmission will...

decrease

If photoelectric effect increases, absorption will...

increase

If atomic number of tissue increases, absorption will...

increase

If atomic number of tissue increases, transmission will...

decrease

If tissue density increases, attenuation will...

increase

If mA increases, heat of filament will...

increase

If mA increases, number of electrons will...

increase

If mA increases, amount of radiation will...

increase

If kV increases, potential difference will...

increase

If kV increases, speed of electrons will...

increase

If kV increases, energy of x-rays will...

increase

If kV increases, penetration of x-rays will...

increase

If kV increases, transmission of radiation will...

increase

What is the effective atomic number of bone?

14

What is the tissue density (in g/cm³) of bone?

1.87 g/cm³

What interaction does bone have with X-rays?

photoelectric (total absorption)

What does bone look like on a radiograph?

white

What is the effective atomic number of muscle?

7.46

What is the tissue density (in g/cm³) of muscle?

1.0 g/cm³

What interactions does muscle have with X-rays?

photoelectric and Compton scatter

What does muscle look like on a radiograph?

grey

What is the effective atomic number of fat?

5.92

What is the tissue density (in g/cm³) of fat?

0.91 g/cm³

What interaction does fat have with X-rays?

Compton scatter

What does fat look like on a radiograph?

dark grey as it is porous

What is the effective atomic number of air?

7.64

Why is air's atomic number so high?

Because it is a gas

What is the tissue density (in g/cm³) of air?

0.00129 g/cm³

What interaction does air have with X-rays?

transmission (goes straight through)

What does air look like on a radiograph?

very dark/black

Alterations of Tissue Density

additive pathology

disease made harder to penetrate

destructive pathology

easier to penetrate i.e. osteoporosis

positive contrast media

barium or iodine added to see better structure better (i.e. during upper GIs)

negative contrast media

air

medical prosthesis

harder to penetrate but does not change technique i.e. hip replacements

Does additive pathology have a higher or lower atomic number/tissue density?

higher

Does additive pathology have an increase or decrease in transmission of x-rays?

decrease