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Flashcards in Week 6 Readings Deck (82)
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1

mA and scan time together are referred to as mAs and defines the _____ of the x-ray energy

Quantity

2

Total x-ray beam exposure in CT is dependent on a combination of ____, ______ and ____

mA setting, scan time, and kVp setting

3

Small filament size

-concentrates focal spot, reducing penumbra ( geometric unsharpness)
-which positively affects image quality
-do not tolerate heat well

4

In SDCT scan time is

The time the X-ray beam is on for the collection of data for each slice
-most often it is the time it takes the gantry to make a complete 360 degree rortation

5

In MDCT the scan time is

The time it takes the X-ray tube to make a 360 degree rotation, even though many slices may be produced

6

____mA allows for _____ scan times to be used. A _____ scan time is critical in avoiding image degradation as a result of patient motion

Higher, Shorter, short

7

The thicker and denser the part being examined, the more ____ that is required to produce an adequate image

mAs

8

True/false: it is more common practice to manipulate the mAs rather than the kVp

True!
-mA is more flexible (larger range 20-800 mA)
-the effect on image quality is more straightforward and predictable

9

Uncoupling effect

Using digital technology, the image quality is not directly linked to the dose, so even when an mA or kVp setting that is too high is used, a good image results
-this makes it difficult to identify when a dose that is higher than necessary is used
-does not play a role when the mA or kVp setting is too low because quantum noise will result and provide evidence of the inadequate exposure settings

10

Automatic tube current modulation

Software that automatically adjusts the tube current (mAs) to fit specific anatomic regions
-this software adjusts mAs during each gantry rotation to compensate for large variations in x-ray attenuation, such as when scans move from the shoulders to the rest of the thorax

11

SFOV and DFOV

Determines the area within the gantry, for which raw data are acquired
-scan data are always acquired around the gantrys isocenter
-the DFOV determines how much, and what section, of the collected raw data are used to create an image

12

Filter functions are only applied to _____ data

Raw

13

Reconstruction algorithms vs WW and WL

-changing the window setting merely changes the way the image is viewed
-changing the reconstruction algorithm will change the way the raw data are manipulated to reconstruct the image

14

Pitch

Relationship between slice thickness and table travel per rotation during a helical scan acquisition

15

Detail (high contrast resolution) aka spatial resolution

The level of detail that is visible on the image
-for ex if two thin wires lie close together in an object, will they be seen as two separate lines on the image?
-the ability to resolve (as separate objects) small, high-contrast objects
-the systems ability to resolve, as separate forms, small objects that are very close together

16

Contrast detectability (low contrast resolution) aka contrast resolution

The ability of the system to differentiate between objects with similar densities
-for ex consider an object that has nearly the same density as its background. Will this object be distinguishable on the CT image

17

How frequently an object will fit into a given space is its

Spatial frequency

18

A large object will have a _____ spatial frequency

Low

19

Small objects have a _____ spatial frequency

High

20

MTF

The ratio of the accuracy of the image compared with the actual object scanned
-MTF indicates image fidelity
-scale from 0 to 1
-if the image were blank and contained no information about the object, the MTF would be 0
-if the image reproduced the object exactly, the MTF of the system would have a value of 1

21

True/false: As objects become smaller (higher spatial frequency) they will not be as accurately depicted on the CT image

True!

22

As the size of the object increases, the MTF ____

Also increases

23

True/false: An object twice the size of another object may not necessarily possess twice the image fidelity

True!
-not a linear relationship

24

An MTF curve extending to the right indicates a system with _____ spatial resolution capabilities

Higher

25

what affects the quality of spatial resolution

-quality of the raw data
-reconstruction method

26

Matrix size and DFOV selection determine _____

Pixel size

27

The greater the total pixels present in the image, the ____ each individual pixel

Smaller

28

DFOV

-determines how much raw data will be used to reconstruct the image
-works like the zoom on a camera and can be used to show the entire area or to display a specific region of interest in greater detail
-increasing DFOV increases the size of each pixel in the image
-a larger pixel will include more patient data

29

_____ pixel size will increase spatial resolution

Smaller

30

Thinner slices produce _____ images

Sharper