Computed Tomography

Question 1

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

Answer 1

white (light gray)

dark

Question 2

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

Answer 2

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

Question 3

Why is there this residual superimposition in CT images?

Answer 3

CT iamges are taken in “slices”

Question 4

CT image slices are anywhere from ___ to ___ mm thick

Answer 4

0.1 to 10 mm

Question 5

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

Answer 5

voxel

Question 6

Can a voxel contain different tissues?

Answer 6

Yes

Question 7

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

Answer 7

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

Question 8

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

Answer 8

volume averaging

Question 9

Volume averaging can result in a ____ of contrast resolution

Answer 9

loss

Question 10

How is volume averaging solved?

Answer 10

By taking thinner slices.

Question 11

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

Answer 11

contrast

quality

Question 12

How are axial images viewed?

Answer 12

looking upward at anatomic structures from below

Question 13

How are sagittal images viewed?

Answer 13

from left toward right

Question 14

Slices are arranged in reference to a ____ image.

Answer 14

scout

Question 15

How many shades of gray can we distinguish?

Answer 15

32

Question 16

What does windowing refer to?

Answer 16

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

Question 17

What is the significance of windowing?

Answer 17

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

Question 18

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

Answer 18

narrow

Question 19

What is the “level” of the window?

Answer 19

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

Question 20

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

Answer 20

bone

soft tissue

Question 21

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

Answer 21

imaging artifacts

Question 22

What are the 3 types of imaging artifacts?

Answer 22

• hardening
• streak artifacts
• motions artifacts

Question 23

What causes hardening of an image?

Answer 23

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

Question 24

What does image hardening lead to?

Answer 24

dark bands in the image between areas of great radiodensity

Question 25

What types of things lead to streak artifacts?

Answer 25

metals

Question 26

What do streak artifacts look like on the image?

Answer 26

bright lines extending radially from the interfering metal

Question 27

What causes motion artifacts?

Answer 27

patient movement

Question 28

What do motion artifacts lead to?

Answer 28

shading or streaking in the image

Question 29

What can reduce the risk of motion artifacts?

Answer 29

faster scanning times

Question 30

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

Answer 30

less radiodensity and increased “noise”

Question 31

Do thinner slices require more or less radiation? Explain…

Answer 31

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

Question 32

Slice thickness for small joints varies between __ to __ mm

Answer 32

0.5 to 2 mm

Question 33

Slice thickness for the pelvis varies between __ to __ mm

Answer 33

2 to 3 mm

Question 34

What does CT image best?

Answer 34

bone

Question 35

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

Answer 35

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

Question 36

What is the major limitation of a CT?

Answer 36

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

Question 37

What is another negative to CT?

Answer 37

There is relatively high radiation exposure associated with CT

Question 38

Thinner slices and smaller pixels improve what?

Answer 38

spatial resolution

Question 39

Thicker slices and larger pixels improve what?

Answer 39

contrast resolution