Rad-103: Ch 4 Image Formation and Radiographic Quality

what is differential absorption

process where some amount of x-ray beam is absorbed through tissue and some pass through anatomy

what anatomy absorbs most x-ray photons

bone

what kind of image does a differential absorption of the primary beam create

that structurally represents anatomic areas of interest

what happens to photons as primary x-ray beam interacts with anatomy

absorbed, scattered, or transmitted

what happens to primary beam energy after it interacts with tissue

loses intensity/energy

what is attentuation

reduction in intensity or number of x-ray photons after interaction with anatomy

what are the two sub-types of attenuation

absorption

scattering

what happens during absorption

primary x-ray beam gets deposited within atoms of tissue

what allows for complete absorption of x-ray photons

a photon having enough energy to eject inner shell electron

what is the photoelectric effect

the total absorption of x-ray photons by transferring it’s energy to and ejecting an inner shell electron

what is the ejected electron called

photoelectron

what is ionization

the ability to remove an electron

what happens to the energy of the primary beam during absorption

is deposited within atoms of the tissue

photoelectric effect also creates a _____ x-ray photon

low energy secondary

how is the secondary photoelectric photon created

difference in binding energy of the electrons

what is the probability of photoelectric effect dependent on

energy of incoming x-ray photon

tissue atomic number

what is the compton effect

when incoming photons not absorbed ejects an outer shell electron by transferring it’s energy and changes energy as a result

what causes a compton/secondary electron

incoming photon ejecting outer shell electron from tissue electron

what happens to a lower energy photon during scattering

changes direction

leave anatomic to interact with IR

expose other near

what is compton interaction dependent on

energy of incoming photon, not atomic number of tissue

compton interactions occur at what energy range of primary beam

any

what allows more scatter to be produced

higher photon energy

what causes more absorption in tissue

lower photon energy

what decreases with higher kVp

percentage of photoelectric interactions

what increases with higher kVp

percentage of compton interactions

what is coherent scattering

incoming photon excites after interaction with the atom

what happens to x-ray photon once it excites

it changes direction but doesn’t lose energy

when does coherent scattering occur

low energy x-rays below diagnostic range

what factors affect beam attentuation

tissue thickness

type of tissue

tissue density

x-ray beam quality

how does tissue thickness affect attenuation

anatomic tissue increases attenuation by absorption or scattering

how are x-rays attenuated and reduced by

50% for each 4 - 5 cm of tissue thickness

how does type of tissue affect beam attenuation

tissues with higher effective atomic number increase attenuation

how does tissue density affect beam attenuation

more dense tissue have more compact atomic particles resulting in increased attenuation

how does x-ray beam quality affect attenuation

higher kVp increases energy of x-ray beam and decreases attenuation

what is remnant/exit radiation

radiation that has been attenuated by the tissue

what is remnant/exit radiation composed of

transmitted and scatter photons

what is fog

unwanted exposure in image receptor

how is radiopaque caused

by absorbed radiation

how is radiolucent caused

by transmitted radiation

what makes anatomic tissue visible in an image

various shades of gray

what is latent/invisible image

a visible image that cannot be seen on IR until it is processed

what happens when exit/remnant radiation interacts with digital IR

it’s converted into electronic signal values

what is a result of different levels of attenuation in the body

strength of signal value

difference in adjacent signal value

what is electronic data set

computer processed to produce a visible image displayed

what factors are responsible for visibility of anatomic structures

brightness

contrast

what factors are responsible for accuracy of structural lines (sharpness)

spatial resolution

distortion

what is brightness

amount of luminance of a display monitor

why does an IR need wide dynamic range

primary beam is polyenergetic

what does digital image compensate for

exposure errors

what does excessive or insufficient image brightness cause

lighter image

darker image

why is different brightness levels in an image important

creates contrast to differentiate between anatomic tissues

what is range of brightness levels a result of

tissues’ differential absorption of x-ray photons

what causes homogeneous object

image with sufficient brightness but no differences appear

factors that play a role in radiographic contrast

structure

radiation quality

IR capabilities

computer processing

display

what is subject contrast a result of

absorption characteristics of anatomic tissue radiographed

quality of x-ray beam

what factors contribute to subject contrast

tissue thickness

tissue density

effective atomic number

what causes low subject contrast

anatomic tissues that attenuate beam similarly

what causes high subject contrast

anatomic tissues that attenuate beam differently

what is image contrast

differences in brightness levels in an image that allows for types tissues to be distinguished

what is contrast resolution

ability of an image receptor to distingush between obaject with similar subject contrast

what is gray scale

number of different shades of gray that can be stored and displayed in a digital image

what does increasing kVp do

decrease attenuation, reduce absorption, increase x-ray transmission causing low brightness levels

what is displayed image contrast a product of

subject contrast and contrast resolution of digital receptor

what can be altered with digital imaging

grayscale

contrast

what is contrast resolution in digital imaging

ability of IR to distinguish between small objects that attenuate beam similarly

how is high contrast image caused

large differences in radiation absorption between tissues with varying composition

how is low contrast image caused

less differences in radiation absorption for tissue with similar composition

what does spatial resolution refer to

smallest object that can be detected in digital image receptor

what does increased unsharpness do

reduce overall visibility of structural lines

lower contrast → reduced visibility of structures

what is temporal resolution

inherent resolution on an image as a function of image acquisition time

what can decrease temporal resolution

increasing time of exposure during acquisition to reduce unsharpness

what kind of relationship do SID and OID have

geometric relationship

what does increased SID or decreased OID affect

reduces magnification

what factors affect distortion

CR alignment of x-ray tube

anatomic part

IR

what is scatter a result of

compton interactions

what does scatter do

degrade visibility of anatomic structures

reduce image contrast

what is an artifact

unwanted brightness level on an image

what are substances that affect visualizing anatomy

air (gas)

fat (adipose)

water

mineral

metal

what does visualizing anatomy depend on

five substances being in anatomical, physical contact with each other

how can dynamic imaging on internal anatomic structures be visualized

with a flat panel detector or image intensifier