Week 1 Lecture 2 Flashcards
attenuation
reduction of photons when x-rays travel through matter
What is radiation flux?
amount of photons that travels through a patient to hit the detector
What does radiation flux relate to?
patient dose and image quality
When viewing a CT image, the darker areas are representative of what?
increased radiation flux
high subject density = _____ radiographic density
high subject density = low radiographic density
what is the relationship between subject density and radiographic (image) density?
image density is inversely related to subject density
denser objects attenuates more photons = less image density = appear white
what does “polychromatic” xray beams mean?
x-ray beams made up of varying levels of energy
how do you determine the maximum energy any photon can have in the x-ray beam?
in high voltage generators, max energy is determined by kVp
detectors measure photon quantity and their energy. t/f
false.
detectors only measure the quantity NOT energy
Explain differential absorption.
it explains that low energy photons are more likely to be absorbed whereas high energy photons are likely to be transmitted
what is the relationship between Linear Attenuation Coefficient and radiation flux?
LAC gives predictive value to determine radiation flux at specific kVp selections
what are Linear Attenuation Coefficients (LAC)?
numerical values based on statistical analysis of measured photon attenuation through KNOWN substances at DIFFERENT kVp settings
What are Hounsfield Units (HU)?
numerical values based on measured radiation flux during a CT scan; measured through unknown objects
what is the relationship between LAC and HU?
HU obtained during a scan will be compared to LACs that way the appropriate shades will be displayed
what are the three different types of contrasts?
- subject contrast
- radiographic contrast
- contrast resolution
subject contrast
patient factors that affect attenuation
(ex. patient thickness/size, density, etc.)
radiographic contrast
relating to how many shades are seen on an CT image
what is long scale contrast? what type of image is it?
- long scale contrast describes shades that are similar in colour
- they are called LOW contrast image
what is short scale contrast? what type of image is it?
- short scale contrast describes shades that are easily distinguishable
- they are called HIGH contrast image
what is contrast resolution?
it relates to HOW many shades of a CT computer system can display different colours
low-contrast resolution scanners can make low-contrast images appear a high contrast images. t/f
true
what is volume averaging?
when the shade that is displayed on the pixel is the average of the radiation flux through both structures captured in that one pixel
what are artifacts?
when tissues appear darker than normal due to the “leftover” high energy photons’ ability to be transmitted through that tissue
what is beam hardening?
when lower energy photons are attenuated leaving the average energy of the remaining x-ray beam to travel through tissues
how do you minimize beam hardening?
by making the beam more homogenous
how do you make a beam more homogenous?
- higher kVps = less beam attenuation
- use of filters
- use of computer algorithms/correction software
- correct scan parameters
what are the pros of a filter?
- decreases the number of low energy photons
- increasing the average energy of the x-ray beam
- reduction of patient dose
*cupping artifacts
aka of artifacts in general
what are the main advantages of CT over conventional radiography?
- ability to get 3d view images of cross sections
- elimination of superimposed structures
- differentiation between structures with similar densities
- good image quality
what defines the Z axis?
the thickness of the cross section image slices
Define pixel.
2D square that consists of x and y elements
- “picture element”
Define voxel.
3D element that consists of X, Y, and Z axis
- “volume element”
Define matrix.
“grid formed from the rows and columns of pixels”
What affects the structure’s ability to attenuate x-ray beams?
- density, atomic number and thickness of the structure
What unit quantifies a structure’s ability to attenuate the x-ray beam?
cm^-1
Why does the slice thickness vary among examination protocols?
depending on the image you’re trying to get
ex. smaller structures will require smaller slices or else you might miss the structure completely
what is the anatomic position?
erect with arms to the side and with palms facing forward
how are x-ray photons produced?
when electrons strike the anode
what are the four things that affects beam attenuation??
- subject density
- atomic number
- tissue thickness
- energy of the photons
homogeneous beam aka?
monochromatic or monoenergetic
in a homogeneous beam, the photons reduce in number by the (different/same) percentage each time, but the energy (does/doesn’t) change each time.
homogeneous beam = same %, doesn’t change
in a heterogeneous beam, the photons reduce in number by the (different/same) percentage each time, but the energy (increases/decreases) each time.
different %, increases each time
petrous ridges
most common place to see beam hardening
what is the order of lowest to highest LAU for these tissues?
dense bone, grey matter, air, fat, white matter, blood, CSF, water, muscles
air, fat, water, csf, white matter, grey matter, blood, muscles, bone
↑ atomic number = what to subject density and radiographic density?
increase atomic number = increase subject density while decreasing radiographic density
what describes a ct image that includes a lot of soft tissue anatomy?
low contrast, due to differential absorption
how would ct images that contain tissue with similar (z) and uniform thickness appear?
long scale contrast
