Week 2 - Attenuation

Question 1

T/F
Relative transmission or attenuation measurements are sent to the computer as raw data

Answer 1

True

Question 2

T/F
Several hundred views are needed to reconstruct the CT image

Answer 2

True

Shades of grey create the matrix and then create images

Question 3

Reduction in the intensity of the beam as it passes through matter

Answer 3

Attenuation

Question 4

The degree to which a beam is reduced

Answer 4

Attenuation

Question 5

The mass of a substance per unit volume

Answer 5

Density

Question 6

T/F
The number of photons that interact depends on thickness, density, atomic number of the object

Answer 6

True

Question 7

Why does the number of photons that interact increase with density?

Answer 7

The more electron, neutrons, protons in each atom, the higher the likelihood of interaction

Question 8

The amount of xray beam that is scattered or absorbed per unit thickness of the absorber is expressed by what?

Answer 8

Linear attenuation coefficient, represented by Greek letter ‘u’

Question 9

When interacting with matter, photons are either:

Answer 9

Absorbed
Transmitted
Scattered

Question 10

What does an area of low attenuation look like on an image?

Answer 10

Darker
(More photons to the detector)

Question 11

Various shades of grey

Answer 11

Intermediate attenuation

Question 12

What will an area of high attenuation look like on an image?

Answer 12

White

Question 13

What are factors that affect attenuation?

Answer 13

-effective atomic density (atoms/vol)
-atomic number (Z)
-density of matter
-thickness
-photon energy (125kvp, 140kvp)

Question 14

All xray photons have the same energy

Answer 14

Homogenous (monochromatic, monoenergetic)

*gamma

Question 15

What type of beam did hounsfield use in his study?

Answer 15

Monochromatic beam
(Gamma source)
Satisfied Lambert-Beer law

Question 16

What is Euler’s constant?

Answer 16

2.718

Question 17

Absorber attenuates equal amounts
20% of remaining photons removed per 1cm

Answer 17

Homogenous beam
*beam energy does not change

Question 18

T/F
‘U’ is inversely proportional to photon energy

Answer 18

True
As photon energy increases, attenuation decreases

Question 19

T/F
‘U’ is directly proportional to atomic number and density

Answer 19

True
As density/atomic number increases, attenuation increases

Question 20

The photons have different energies

Answer 20

Heterogeneous beam
(Polychromatic)

Question 21

T/F
In a heterogenous beam, photons decrease but quality increases - low energy photons are removed

Answer 21

True
The beam gets “harder”

Question 22

What changed during heterogeneous attenuation?

Answer 22

Both quality and quantity

Question 23

Where does photoelectric interactions occur?

Answer 23

Absorption , inner shells

Question 24

Where does Compton interaction occur?

Answer 24

Scatter, outer shells

Question 25

T/F PE occurs mainly in tissues with high Z (bone, positive contrast media…) Minimal in soft tissue

Answer 25

True

Question 26

T/F Compton effect occurs in soft tissue

Answer 26

True Difference in tissue density produces differences in Compton interaction

Question 27

Why does a heterogenous beam not have a straightforward relationship between intensity and attenuation?

Answer 27

Because beam intensity differs

Question 28

How is the linear attenuation coefficient determined in CT?

Answer 28

Using the number of photons instead of the intensity

Question 29

Quality does not change

Answer 29

Monochromatic

Question 30

Quality and quantity change Mean energy increases

Answer 30

Polychromatic

Question 31

Given a constant kvp, place in order of increasing ‘U’ for the following Water, bone, air, fat, blood

Answer 31

Air Fat Water Blood Bone

Question 32

T/F Differences in ‘U’ cause image contrast

Answer 32

True Image is a direct reflection of ‘U’

Question 33

T/F Density differences are needed to observe differences between structures

Answer 33

True

Question 34

What is the atomic number (Z) of barium

Answer 34

56

Question 35

What is the atomic number of iodine

Answer 35

Z=53

Question 36

Raw data (data received from detectors) undergo various pre and post processing (Necessary for image reconstruction)

Answer 36

Data processing

Question 37

The scan data, which represent attenuation data, are converted into digital image characterized by CT numbers

Answer 37

Image reconstruction

Question 38

Conversion of the attenuation readings into CT image is accomplished by what mathematical procedure?

Answer 38

Reconstruction algorithms

Question 39

T/F Each pixel in the reconstructed image is assigned a CT number

Answer 39

True

Question 40

What does a lower CT number represent?

Answer 40

Darker (less attenuation)

Question 41

What does a higher CT number represent?

Answer 41

White (higher attenuation)

Question 42

How are CT image attenuations quantified on any area?

Answer 42

Expressed in Hounsfield Units (H.U.)

Question 43

T/F Hounsfield units quantify the degree that a structure attenuates an xray beam

Answer 43

True

Question 44

What HU numbers do human anatomy fall between?

Answer 44

Air (-1000) and bone (1000)

Question 45

T/F 1HU = 0.1% difference between ‘U’ of the tissue of interest compared ‘U’ of H2O

Answer 45

True

Question 46

HU inaccuracies can be due to:

Answer 46

-poor equipment calibration -image artifacts -volume averaging (voxel containing different material will average them) Voxel = 3D

Question 47

Range of CT numbers

Answer 47

Window width

Question 48

Center of range of CT numbers

Answer 48

Window level

Question 49

Window width

Answer 49

Controls image contrast

Question 50

Window level

Answer 50

Controls image brightness

Question 51

What are the 3 types of windows?

Answer 51

Bone Lung Mediastinal