Textbook - Chapter 1 Flashcards
spatial resolution
ability of a system define small objects distinctly
low-contrast solution
ability of a system to differentiate between objects with similar densities
temporal resolution
speed that the data can be acquired
z-axis
thickness of a cross-sectional slice
pixel
2D picture element consisting of x and y
x-axis
width
y-axis
height
voxel
3D volume element consisting of x, y, & z
what’s the most common matrix size for ct?
512x512
what is attenuation?
degree which an x-ray beam is reduced
when photons pass through something unimpeded, what colour will it show up on the ct image?
black
colour of low attenuation objects
black
colour of high attenuation objects
white
colour of intermediately attenuating objects
differing shades of grey
number of photons that interact with an object is dependent on?
- thickness
- density
- atomic number
*of the object
what is linear attenuation coefficient (µ)?
amount of the x-ray beam that is scattered or absorbed per unit thickness of the absorber
____ µ = _____ photon energy
↓µ = ↑photon energy
____ µ = _____ atomic number
↑ µ = ↑ atomic number
____ µ = _____ density
↑ µ = ↑ density
image contrast is determined by?
difference in linear attenuation coefficients among tissues
soft tissues’ linear attenuation coefficient is ____ (dis/proportional) to physical density
proportional
what are positive agents?
contrast agents that are higher in density than structures they’re introduced to
what examples of positive agents?
barium sulfate and iodine
what is an example of a negative agent?
water (low-density)
what are Hounsfield units also known as?
CT numbers or density values
what are HUs?
- indicates the degree that a structure attenuates an x-ray beam in comparison to water
- average of all the attenuation measurements for the pixel
HU of dense bone
+1000
HU of air
-1000
HU of distilled water
0
what are the main advantages of CT over conventional radiography?
- can get the 3d view –> get images of cross sections
- elimination of superimposed structures
- differentiation between structures with similar densities
- good image quality
matrix
grid that is formed from the rows and columns of pixels
what are image artifacts?
things that are on the image that isn’t actually there
what causes artifacts?
polychromatic X-rays and beam hardening
what causes beam hardening?
when low energy photons are absorbed leaving high energy ones to strike the detector
what is the anatomic position?
erect with arms to the side and palms facing forward
how are x-ray photons produced?
when electrons strike the anode
1 HU = ___% difference between the linear attenuation coefficient of water
0.1%
what are cupping artifacts?
vague areas of decreased density
Raw data
Data acquired by the system
Scan data
Aka raw data
Raw data have not been assigned HUs. Why?
Haven’t been sectioned to creat pixels
Image reconstruction
Using raw data to create an image
Done by processing the raw data, assigning HUs to the pixels
Image data
Data included in the image through image reconstruction
Prospective reconstruction
Reconstruction automatically produced during scanning
Retrospective reconstruction
Using the raw data later to generate new images (even after they have undergone perspective reconstruction)
Stop and shoot, aka?
Axial scanning or conventional scanning or serial scanning
2 reasons for changing image plane from axial to coronal?
- Anatomy of interest lies vertically instead of horizontally
- Reduction of artifacts caused by surrounding structures
mA
quantity of electrons
kVp
voltage = intensity of x-ray beam
