Week 1 Lecture 2

Question 1

attenuation

Answer 1

reduction of photons when x-rays travel through matter

Question 2

What is radiation flux?

Answer 2

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

Question 3

What does radiation flux relate to?

Answer 3

patient dose and image quality

Question 4

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

Answer 4

increased radiation flux

Question 5

high subject density = _____ radiographic density

Answer 5

high subject density = low radiographic density

Question 6

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

Answer 6

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

Question 7

what does “polychromatic” xray beams mean?

Answer 7

x-ray beams made up of varying levels of energy

Question 8

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

Answer 8

in high voltage generators, max energy is determined by kVp

Question 9

detectors measure photon quantity and their energy. t/f

Answer 9

false.
detectors only measure the quantity NOT energy

Question 10

Explain differential absorption.

Answer 10

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

Question 11

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

Answer 11

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

Question 12

what are Linear Attenuation Coefficients (LAC)?

Answer 12

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

Question 13

What are Hounsfield Units (HU)?

Answer 13

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

Question 14

what is the relationship between LAC and HU?

Answer 14

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

Question 15

what are the three different types of contrasts?

Answer 15

1. subject contrast
2. radiographic contrast
3. contrast resolution

Question 16

subject contrast

Answer 16

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

Question 17

radiographic contrast

Answer 17

relating to how many shades are seen on an CT image

Question 18

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

Answer 18

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

Question 19

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

Answer 19

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

Question 20

what is contrast resolution?

Answer 20

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

Question 21

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

Answer 21

true

Question 22

what is volume averaging?

Answer 22

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

Question 23

what are artifacts?

Answer 23

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

Question 24

what is beam hardening?

Answer 24

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

Question 25

how do you minimize beam hardening?

Answer 25

by making the beam more homogenous

Question 26

how do you make a beam more homogenous?

Answer 26

- higher kVps = less beam attenuation - use of filters - use of computer algorithms/correction software - correct scan parameters

Question 27

what are the pros of a filter?

Answer 27

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

Question 28

*cupping artifacts

Answer 28

aka of artifacts in general

Question 29

what are the main advantages of CT over conventional radiography?

Answer 29

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

Question 30

what defines the Z axis?

Answer 30

the thickness of the cross section image slices

Question 31

Define pixel.

Answer 31

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

Question 32

Define voxel.

Answer 32

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

Question 33

Define matrix.

Answer 33

"grid formed from the rows and columns of pixels"

Question 34

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

Answer 34

- density, atomic number and thickness of the structure

Question 35

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

Answer 35

cm^-1

Question 36

Why does the slice thickness vary among examination protocols?

Answer 36

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

Question 37

what is the anatomic position?

Answer 37

erect with arms to the side and with palms facing forward

Question 38

how are x-ray photons produced?

Answer 38

when electrons strike the anode

Question 39

what are the four things that affects beam attenuation??

Answer 39

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

Question 40

homogeneous beam aka?

Answer 40

monochromatic or monoenergetic

Question 41

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.

Answer 41

homogeneous beam = same %, doesn't change

Question 42

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

Answer 42

different %, increases each time

Question 43

petrous ridges

Answer 43

most common place to see beam hardening

Question 44

what is the order of lowest to highest LAU for these tissues? dense bone, grey matter, air, fat, white matter, blood, CSF, water, muscles

Answer 44

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

Question 45

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

Answer 45

increase atomic number = increase subject density while decreasing radiographic density

Question 46

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

Answer 46

low contrast, due to differential absorption

Question 47

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

Answer 47

long scale contrast