Unit 5: Image Quality in CT (Part 1)

Question 1

T/F

The aspects that control exposure in X-ray is the same in CT

Answer 1

True

Question 2

T/F

Increasing mA decreases the number of electrons that will produce x rays

Answer 2

False; Increasing mA increases the number of electrons that will produce x rays

Question 3

Why is having a smaller filament better for increasing spatial resolution?

Answer 3

Using a smaller filament concentrates the focal spot, which in turn results in increased spatial resolution

Question 4

What are the disadvantages of a smaller filament?

Answer 4

-Does not tolerate high mA values a well as large filaments so time must be increased to maintain mAs

Question 5

In CT, what is scan time?

Answer 5

The time it takes for the x ray tube to make one 360 degree rotation

Question 6

What is the typical scan time in CT?

Answer 6

2s-0.5s

Question 7

T/F

Shorter scan times completely eliminate artifacts caused by motion entirely.

Answer 7

False; Shorter scan times reduce, but do not eliminate artifacts caused by motion entirely.

(peristalsis, cardiac motion)

Question 8

T/F

As a rule, if mAs can be maintained, it is preferable to use the shortest scan times available.

Answer 8

True

Question 9

What happens to heat produced as mAs increases?

Answer 9

As mAs increases, so does the amount of heat being generated within the x-ray tube.

Question 10

What is the main limiting factor for exposure in all scanners?

Answer 10

Heat

Question 11

T/F

There is no correlation between mAs levels and interscan delay

Answer 11

False; There is a direct correlation between mAs levels and the interscan delay.

Question 12

What is Interscan Delay?

Answer 12

Interscan delay is the time between scans (when the tube is not energized).

Question 13

T/F

The lower the mAs setting, the longer the interscan delay needed to allow for longer tube cool-off time between scanning sequences.

Answer 13

FALSE; The higher the mAs setting, the longer the interscan delay needed to allow for longer tube cool-off time between scanning sequences.

Question 14

What 2 main factors determine the mAs levels selected?

Answer 14

Patient thickness and density.

Question 15

T/F

All CT scanners today have AEC systems

Answer 15

True

Question 16

What is this describing?

The automatic control of the mA intwo directions of the patiention during data acquisition that take into consideration the patient size and the attenuation differences of the various tissues.

Answer 16

Automatic tube current modulation (ACTM)

Question 17

What is the main goal of ATCM?

Answer 17

To provide consistent image quality despite the size of the patient and the tissue attenuation differences

Question 18

T/F

ATCM often optimizes patient dose.

Answer 18

True

Question 19

What is angular TC modulation?

Answer 19

The automatic control of tube current in the x-y axis (in plane)

Question 20

mA is modulated as a function of what in x-y axis TCM?

Answer 20

mA is modulated as a function of projection angle

Question 21

What is the purpose of the x-y axis TCM algorithm?

Answer 21

Ensures that a constant (uniform) noise level is maintained during the scanning process

Question 22

What is longitidinal (z axis) TCm based on?

Answer 22

Longitidinal (z axis) TCm based on differences in attenuation along body parts

Question 23

What algorithim is responsible for this?

Designed to change the mA automatically as the patient is scanned from head to toe while maintainting constant noise level for different thickness of body parts examined.

Answer 23

Z-axis TCM

Question 24

mA is modulated as a function of what along the patient in Z axis TCM?

Answer 24

mA is modulated as a function of longitudinal location along the patient

mA changes correspondingly as the patient is moving along the table

Question 25

What are the 3 ways in which software automatically adjusts mA to fit specific anatomic regions?

Answer 25

1. Obtaining attenuation characteristics by analyzing AP and lateral scouts
2. obtaining attenuation charecteristics by analyzing (in real time) signal from the detector rows in the first half (180 degrees) of the rotaion
3. 1 and 2

Question 26

What are attenuation values compared to for obtaining references?

Answer 26

1. The target mAs level
2. The amount of noise in the image

(based on the maufacturer)

Question 27

What is the oval ratio?

Answer 27

A ratio of the long axis to the short axis of an ellipse/circle
a/b

Question 28

T/F

From any given z location the system estimates the attenuation level and the oval ratio from the scan.

Answer 28

False; From any given z location the system estimates the attenuation level and the oval ratio from the scout

Question 29

What does the oval ratio reflect?

Answer 29

How circular or elliptical the patient is at a given location and is based on width information in the scout

Question 30

T/F

The oval ratio changes based off of changing the isocenter.

Answer 30

True

Question 31

What is the reference noise measurements in the system determined from?

Answer 31

A set of phantoms representing a wide range of patient sizes and shapes

Question 32

What is the primary aim of tube current (mA) modulation?

Answer 32

To adjust mA according to the patient’s attenuation and ultimately to reduce the radiation dose to the patient while sustaining diagnostic image quality

Question 33

What are the choices of kVp in CT system scanners?

Answer 33

80, 100, 120 and 140

Fewer than x ray

Question 34

T/F

Excessively reducing kVp would result in increased radiation dose to the patient due to a dramatic increase in attenuation of the x ray beams by the patient

Answer 34

True

(especially for larger patients)