Week 2 - Attenuation Flashcards
T/F
Relative transmission or attenuation measurements are sent to the computer as raw data
True
T/F
Several hundred views are needed to reconstruct the CT image
True
Shades of grey create the matrix and then create images
Reduction in the intensity of the beam as it passes through matter
Attenuation
The degree to which a beam is reduced
Attenuation
The mass of a substance per unit volume
Density
T/F
The number of photons that interact depends on thickness, density, atomic number of the object
True
Why does the number of photons that interact increase with density?
The more electron, neutrons, protons in each atom, the higher the likelihood of interaction
The amount of xray beam that is scattered or absorbed per unit thickness of the absorber is expressed by what?
Linear attenuation coefficient, represented by Greek letter ‘u’
When interacting with matter, photons are either:
Absorbed
Transmitted
Scattered
What does an area of low attenuation look like on an image?
Darker
(More photons to the detector)
Various shades of grey
Intermediate attenuation
What will an area of high attenuation look like on an image?
White
What are factors that affect attenuation?
-effective atomic density (atoms/vol)
-atomic number (Z)
-density of matter
-thickness
-photon energy (125kvp, 140kvp)
All xray photons have the same energy
Homogenous (monochromatic, monoenergetic)
*gamma
What type of beam did hounsfield use in his study?
Monochromatic beam
(Gamma source)
Satisfied Lambert-Beer law
What is Euler’s constant?
2.718
Absorber attenuates equal amounts
20% of remaining photons removed per 1cm
Homogenous beam
*beam energy does not change
T/F
‘U’ is inversely proportional to photon energy
True
As photon energy increases, attenuation decreases
T/F
‘U’ is directly proportional to atomic number and density
True
As density/atomic number increases, attenuation increases
The photons have different energies
Heterogeneous beam
(Polychromatic)
T/F
In a heterogenous beam, photons decrease but quality increases - low energy photons are removed
True
The beam gets “harder”
What changed during heterogeneous attenuation?
Both quality and quantity
Where does photoelectric interactions occur?
Absorption , inner shells
Where does Compton interaction occur?
Scatter, outer shells
T/F
PE occurs mainly in tissues with high Z (bone, positive contrast media…)
Minimal in soft tissue
True
T/F
Compton effect occurs in soft tissue
True
Difference in tissue density produces differences in Compton interaction
Why does a heterogenous beam not have a straightforward relationship between intensity and attenuation?
Because beam intensity differs
How is the linear attenuation coefficient determined in CT?
Using the number of photons instead of the intensity
Quality does not change
Monochromatic
Quality and quantity change
Mean energy increases
Polychromatic
Given a constant kvp, place in order of increasing ‘U’ for the following
Water, bone, air, fat, blood
Air
Fat
Water
Blood
Bone
T/F
Differences in ‘U’ cause image contrast
True
Image is a direct reflection of ‘U’
T/F
Density differences are needed to observe differences between structures
True
What is the atomic number (Z) of barium
56
What is the atomic number of iodine
Z=53
Raw data (data received from detectors) undergo various pre and post processing
(Necessary for image reconstruction)
Data processing
The scan data, which represent attenuation data, are converted into digital image characterized by CT numbers
Image reconstruction
Conversion of the attenuation readings into CT image is accomplished by what mathematical procedure?
Reconstruction algorithms
T/F
Each pixel in the reconstructed image is assigned a CT number
True
What does a lower CT number represent?
Darker (less attenuation)
What does a higher CT number represent?
White (higher attenuation)
How are CT image attenuations quantified on any area?
Expressed in Hounsfield Units (H.U.)
T/F
Hounsfield units quantify the degree that a structure attenuates an xray beam
True
What HU numbers do human anatomy fall between?
Air (-1000) and bone (1000)
T/F
1HU = 0.1% difference between ‘U’ of the tissue of interest compared ‘U’ of H2O
True
HU inaccuracies can be due to:
-poor equipment calibration
-image artifacts
-volume averaging (voxel containing different material will average them)
Voxel = 3D
Range of CT numbers
Window width
Center of range of CT numbers
Window level
Window width
Controls image contrast
Window level
Controls image brightness
What are the 3 types of windows?
Bone
Lung
Mediastinal
