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mrad 251 - computed tomography > Week 1 Lecture 2 > Flashcards

Week 1 Lecture 2 Flashcards

1
Q

attenuation

A

reduction of photons when x-rays travel through matter

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2
Q

What is radiation flux?

A

amount of photons that travels through a patient to hit the detector

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3
Q

What does radiation flux relate to?

A

patient dose and image quality

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4
Q

When viewing a CT image, the darker areas are representative of what?

A

increased radiation flux

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5
Q

high subject density = _____ radiographic density

A

high subject density = low radiographic density

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6
Q

what is the relationship between subject density and radiographic (image) density?

A

image density is inversely related to subject density
denser objects attenuates more photons = less image density = appear white

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7
Q

what does “polychromatic” xray beams mean?

A

x-ray beams made up of varying levels of energy

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8
Q

how do you determine the maximum energy any photon can have in the x-ray beam?

A

in high voltage generators, max energy is determined by kVp

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9
Q

detectors measure photon quantity and their energy. t/f

A

false.
detectors only measure the quantity NOT energy

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10
Q

Explain differential absorption.

A

it explains that low energy photons are more likely to be absorbed whereas high energy photons are likely to be transmitted

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11
Q

what is the relationship between Linear Attenuation Coefficient and radiation flux?

A

LAC gives predictive value to determine radiation flux at specific kVp selections

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12
Q

what are Linear Attenuation Coefficients (LAC)?

A

numerical values based on statistical analysis of measured photon attenuation through KNOWN substances at DIFFERENT kVp settings

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13
Q

What are Hounsfield Units (HU)?

A

numerical values based on measured radiation flux during a CT scan; measured through unknown objects

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14
Q

what is the relationship between LAC and HU?

A

HU obtained during a scan will be compared to LACs that way the appropriate shades will be displayed

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15
Q

what are the three different types of contrasts?

A
  1. subject contrast
  2. radiographic contrast
  3. contrast resolution
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16
Q

subject contrast

A

patient factors that affect attenuation
(ex. patient thickness/size, density, etc.)

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17
Q

radiographic contrast

A

relating to how many shades are seen on an CT image

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18
Q

what is long scale contrast? what type of image is it?

A
  • long scale contrast describes shades that are similar in colour
  • they are called LOW contrast image
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19
Q

what is short scale contrast? what type of image is it?

A
  • short scale contrast describes shades that are easily distinguishable
  • they are called HIGH contrast image
20
Q

what is contrast resolution?

A

it relates to HOW many shades of a CT computer system can display different colours

21
Q

low-contrast resolution scanners can make low-contrast images appear a high contrast images. t/f

A

true

22
Q

what is volume averaging?

A

when the shade that is displayed on the pixel is the average of the radiation flux through both structures captured in that one pixel

23
Q

what are artifacts?

A

when tissues appear darker than normal due to the “leftover” high energy photons’ ability to be transmitted through that tissue

24
Q

what is beam hardening?

A

when lower energy photons are attenuated leaving the average energy of the remaining x-ray beam to travel through tissues

25
Q

how do you minimize beam hardening?

A

by making the beam more homogenous

26
Q

how do you make a beam more homogenous?

A
  • higher kVps = less beam attenuation
  • use of filters
  • use of computer algorithms/correction software
  • correct scan parameters
27
Q

what are the pros of a filter?

A
  • decreases the number of low energy photons
  • increasing the average energy of the x-ray beam
  • reduction of patient dose
28
Q

*cupping artifacts

A

aka of artifacts in general

29
Q

what are the main advantages of CT over conventional radiography?

A
  • ability to get 3d view images of cross sections
  • elimination of superimposed structures
  • differentiation between structures with similar densities
  • good image quality
30
Q

what defines the Z axis?

A

the thickness of the cross section image slices

31
Q

Define pixel.

A

2D square that consists of x and y elements
- “picture element”

32
Q

Define voxel.

A

3D element that consists of X, Y, and Z axis
- “volume element”

33
Q

Define matrix.

A

“grid formed from the rows and columns of pixels”

34
Q

What affects the structure’s ability to attenuate x-ray beams?

A
  • density, atomic number and thickness of the structure
35
Q

What unit quantifies a structure’s ability to attenuate the x-ray beam?

A

cm^-1

36
Q

Why does the slice thickness vary among examination protocols?

A

depending on the image you’re trying to get
ex. smaller structures will require smaller slices or else you might miss the structure completely

37
Q

what is the anatomic position?

A

erect with arms to the side and with palms facing forward

38
Q

how are x-ray photons produced?

A

when electrons strike the anode

39
Q

what are the four things that affects beam attenuation??

A
  1. subject density
  2. atomic number
  3. tissue thickness
  4. energy of the photons
40
Q

homogeneous beam aka?

A

monochromatic or monoenergetic

41
Q

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.

A

homogeneous beam = same %, doesn’t change

42
Q

in a heterogeneous beam, the photons reduce in number by the (different/same) percentage each time, but the energy (increases/decreases) each time.

A

different %, increases each time

43
Q

petrous ridges

A

most common place to see beam hardening

44
Q

what is the order of lowest to highest LAU for these tissues?

dense bone, grey matter, air, fat, white matter, blood, CSF, water, muscles

A

air, fat, water, csf, white matter, grey matter, blood, muscles, bone

45
Q

↑ atomic number = what to subject density and radiographic density?

A

increase atomic number = increase subject density while decreasing radiographic density

46
Q

what describes a ct image that includes a lot of soft tissue anatomy?

A

low contrast, due to differential absorption

47
Q

how would ct images that contain tissue with similar (z) and uniform thickness appear?

A

long scale contrast

mrad 251 - computed tomography (13 decks)

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