Computed Tomography Flashcards Preview

633: Human Imaging II > Computed Tomography > Flashcards

Flashcards in Computed Tomography Deck (39):
1

In a CT image, dense structures appear ____, whereas less dense structures appear ____.

white (light gray)

dark

2

What is the major difference between radiodensity presented on a CT vs. an x-ray?

In a CT image the shades of gray accurately reflect the radiodensity of tissue relatively free from superimposed radiodensities of other tissues

3

Why is there this residual superimposition in CT images?

CT iamges are taken in "slices"

4

CT image slices are anywhere from ___ to ___ mm thick

0.1 to 10 mm

5

In a CT image, the product of a pixel and slice thickness is referred to as what?

voxel

6

Can a voxel contain different tissues?

Yes

7

When a voxel contains different tissues what does its radiodensity represent?

The average value for radiodensities of all tissues contained in that voxel

8

The phenomenon of taking the average value for radiodensities of all tissues contained in a voxel is referred to as what?

volume averaging

9

Volume averaging can result in a ____ of contrast resolution

loss

10

How is volume averaging solved?

By taking thinner slices.

11

Thin slices have better ____, but result in loss of image ____

contrast

quality

12

How are axial images viewed?

looking upward at anatomic structures from below

13

How are sagittal images viewed?

from left toward right

14

Slices are arranged in reference to a ____ image.

scout

15

How many shades of gray can we distinguish?

32

16

What does windowing refer to?

The ability of the computer to select a range of radiodensities to display in an image

17

What is the significance of windowing?

It allows us to distinguish between tissues of similar densities such as gray matter and white matter in brain

18

In order to distinguish between very similar radiodensities should there be a wide or narrow window setting?

narrow

19

What is the "level" of the window?

The central value between to similar radiodensities that represents the average attenuation

20

It is common practice to refer to window levels as either ____ windows or _____ windows, depending on which tissues are emphasized.

bone

soft tissue

21

Known factors that degrade image quality are referred to as what?

imaging artifacts

22

What are the 3 types of imaging artifacts?

- hardening
- streak artifacts
- motions artifacts

23

What causes hardening of an image?

As photons in an x-ray beam pass through structures such as the skull, the beam becomes “harder” because lower-energy photons are absorbed more readily

24

What does image hardening lead to?

dark bands in the image between areas of great radiodensity

25

What types of things lead to streak artifacts?

metals

26

What do streak artifacts look like on the image?

bright lines extending radially from the interfering metal

27

What causes motion artifacts?

patient movement

28

What do motion artifacts lead to?

shading or streaking in the image

29

What can reduce the risk of motion artifacts?

faster scanning times

30

Image volume is smaller with thinner slices, what does this lead to?

less radiodensity and increased “noise"

31

Do thinner slices require more or less radiation? Explain...

The duration of the exam must be increased because there is an increased number of slices per area which results in an increase in the total radiation dose

32

Slice thickness for small joints varies between __ to __ mm

0.5 to 2 mm

33

Slice thickness for the pelvis varies between __ to __ mm

2 to 3 mm

34

What does CT image best?

bone

35

What are the 10 circumstances in which CT is chosen first?

1) for identifying subtle fractures and/or complex fractures
2) for evaluating degenerative changes, such as spinal arthritic changes
3) in serious trauma, since multiple injuries to both osseous and soft tissue structures can be determined from one imaging series
4) to evaluate spinal stenosis
5) to exam the condition of IVD when combined with diskogram
6) for evaluation of loose bodies in a joint
7) less time-consuming than MRI or ultrasound
8) to accurately measure osseous alignment in any plane
9) less expensive than MRI
10) less problematic for patients with claustrophobia

36

What is the major limitation of a CT?

It has limited capabilities for determining histological makeup of imaged tissues because it identifies tissues primarily on basis of radiodensity.
- for example, a tumor that has the same radiodensity as the muscle surrounding it may be missed because the computer assigns the same shade of gray to the tumor as it does to the muscle

37

What is another negative to CT?

There is relatively high radiation exposure associated with CT

38

Thinner slices and smaller pixels improve what?

spatial resolution

39

Thicker slices and larger pixels improve what?

contrast resolution