Computed Tomography Flashcards
In a CT image, dense structures appear ____, whereas less dense structures appear ____.
white (light gray)
dark
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
Why is there this residual superimposition in CT images?
CT iamges are taken in “slices”
CT image slices are anywhere from ___ to ___ mm thick
0.1 to 10 mm
In a CT image, the product of a pixel and slice thickness is referred to as what?
voxel
Can a voxel contain different tissues?
Yes
When a voxel contains different tissues what does its radiodensity represent?
The average value for radiodensities of all tissues contained in that voxel
The phenomenon of taking the average value for radiodensities of all tissues contained in a voxel is referred to as what?
volume averaging
Volume averaging can result in a ____ of contrast resolution
loss
How is volume averaging solved?
By taking thinner slices.
Thin slices have better ____, but result in loss of image ____
contrast
quality
How are axial images viewed?
looking upward at anatomic structures from below
How are sagittal images viewed?
from left toward right
Slices are arranged in reference to a ____ image.
scout
How many shades of gray can we distinguish?
32
What does windowing refer to?
The ability of the computer to select a range of radiodensities to display in an image
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
In order to distinguish between very similar radiodensities should there be a wide or narrow window setting?
narrow
What is the “level” of the window?
The central value between to similar radiodensities that represents the average attenuation
It is common practice to refer to window levels as either ____ windows or _____ windows, depending on which tissues are emphasized.
bone
soft tissue
Known factors that degrade image quality are referred to as what?
imaging artifacts
What are the 3 types of imaging artifacts?
- hardening
- streak artifacts
- motions artifacts
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
What does image hardening lead to?
dark bands in the image between areas of great radiodensity
What types of things lead to streak artifacts?
metals
What do streak artifacts look like on the image?
bright lines extending radially from the interfering metal
What causes motion artifacts?
patient movement
What do motion artifacts lead to?
shading or streaking in the image
What can reduce the risk of motion artifacts?
faster scanning times
Image volume is smaller with thinner slices, what does this lead to?
less radiodensity and increased “noise”
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
Slice thickness for small joints varies between __ to __ mm
0.5 to 2 mm
Slice thickness for the pelvis varies between __ to __ mm
2 to 3 mm
What does CT image best?
bone
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
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
What is another negative to CT?
There is relatively high radiation exposure associated with CT
Thinner slices and smaller pixels improve what?
spatial resolution
Thicker slices and larger pixels improve what?
contrast resolution
