Week 10 - Quality Resolution And Artifacts

Question 1

What is image quality?

Answer 1

-comparison of actual object to image
-subjective
-in CT: the quality that is directly related to diagnosis accuracy

Question 2

T/F
Quality of image should serve the purpose of why it was acquired

Answer 2

True

Question 3

What are the two main features of image quality?

Answer 3

-detail (high contrast resolution)
-contrast detectability (low contrast resolution)

Question 4

Seeing two separate small structures

Answer 4

Detail (high contrast resolution)

Question 5

Differentiating between structures with similar densities

Answer 5

Contrast detectability (low contrast resolution)

Question 6

Identifying two very small structures close together

Answer 6

Spatial resolution

Question 7

What are the 2 methods of spatial resolution measurement?

Answer 7

-direct measurement
-modulation transfer function
*useful in evaluating CT performance on a different day or to compare 2 systems

Question 8

What is used for the direct measurement test of spatial resolution?

Answer 8

-phantom used with line pairs
-# of line pairs are counted

Ex. If 20 lines seen in 1cm = 20lp/cm

Question 9

-how frequently an object fits into a given space

Answer 9

Direct measurement -spatial frequency

Question 10

What level of frequency do large objects have?

Answer 10

Low frequency

Question 11

What level of frequency do small objects have?

Answer 11

High spatial frequency
*thinner slices

Question 12

Does CT or radiography have better spatial resolution?

Answer 12

Radiography

Question 13

-better spatial resolution
-5-7 lp/mm

Answer 13

Radiography

Question 14

-less spatial resolution
-1 lp/mm

Answer 14

CT

Question 15

T/F
Spatial resolution can be viewed from two dimensions

Answer 15

True

Question 16

What are the 2 dimensions spatial resolution can be viewed in?

Answer 16

-in plane resolution: resolution in xy direction

-longitudinal: resolution in x direction

Question 17

What factors affect spatial resolution?

Answer 17

-matrix size and DFOV
-slice thickness

Question 18

What is the equation for pixel size?

Answer 18

Pixel size = FOV/matrix size

Question 19

T/F
Smaller pixel size is better to see two distinct shapes

Answer 19

True

Question 20

T/F
Objects smaller than the pixel will have density averaged with other objects in the pixel

Answer 20

True
*less accurate image

Question 21

Why are smaller pixels better to see two distinct shapes?

Answer 21

With larger pixels, small objects in the same pixel are averaged

With smaller pixels, small objects are seen distinctly

Question 22

T/F
-thinner slices produce sharper images
-narrow slice thickness improve resolution

Answer 22

True

Question 23

If a matrix is made of x and y, what axis is added to create a voxel?

Answer 23

z axis

Question 24

-determines which part of data to be enhanced or optimized
-smoothing: reduce difference between pixels, ex. Dental fillings
-bone or detail filters: used with great density differences between inherent densities, more noise produced

Answer 24

Reconstruction algorithms

Question 25

T/F A larger focal spot reduces spatial resolution

Answer 25

True

Question 26

T/F Increasing pitch reduces spatial resolution (more pronounced in SDCT)

Answer 26

True

Question 27

T/F Patient motion reduces spatial resolution

Answer 27

True

Question 28

-distinguishing between similar densities

Answer 28

Contrast resolution

Question 29

Ability to distinguish object that has nearly similar density as its background

Answer 29

Low contrast detectability

Question 30

T/F CT can differentiate 0.5% contrast difference (5% in general rad)

Answer 30

True

Question 31

How is contrast resolution tested?

Answer 31

Through a (phantom) cylinder with small differences -dependent on operator characteristic *subjective

Question 32

-undesirable fluctuations of pixel values in a homogeneous material -caused mainly by quantum mottle

Answer 32

Noise Smaller SD = less noise = better contrast

Question 33

T/F Doubling mAs increases SNR by 40%

Answer 33

True

Question 34

What algorithm is contrast resolution better with?

Answer 34

Soft tissue algorithm

Question 35

T/F When imaging a larger patient, contrast resolution is decreased (SNR reduced)

Answer 35

True *less photons reaching IR

Question 36

What are some factors affecting contrast resolution?

Answer 36

-contrast detail response: relationship between object size and visibility *small object will cause contrast to decrease -WW/WL must be considered

Question 37

Why is contrast media not always required for lung scanning?

Answer 37

Differences in densities

Question 38

T/F A window width is narrowed to view pathologies

Answer 38

True

Question 39

-how rapidly data is acquired -gantry speed -# of detector channels -speed of state detecting change in signal

Answer 39

Temporal resolution

Question 40

What is increased temporal resolution important for imaging?

Answer 40

-involuntary moving structures -dynamic flow of contrast (ex. CTA)

Question 41

-the ability of the system to accurately portray an object varies according to the size of the object -as objects become smaller, they will not be as accurately depicted on the CT image -the ratio of accuracy of image compared to actual object scanned

Answer 41

Modulation transfer function

Question 42

MTF - 0

Answer 42

No information about the object ( not accurate)

Question 43

MTF - 1

Answer 43

Accurate representation (perfect)