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Mosby 5: Image Production > Scan Modes/X-Ray Tube/Generators&Transformers > Flashcards

Scan Modes/X-Ray Tube/Generators&Transformers Flashcards

(76 cards)

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1
Q

CT fundamental principle

A

An image of an object may be recon’d on the basis of attenuation that occurs as x-ray is transmitted through it

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2
Q

CT summarized

A

x-ray beam rotated around Pt, exposing volume of tissue from all directions; detectors measure transmitted x-rays; image recon’d based on magnitude of x-ray atten that occurs at spatially dist’d points w/in Pt

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3
Q

CT General process

A

data acq–>data recon–>multidimensional image display–>image archival and communication

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4
Q

CT data acqusition

A

measurement of attenuation that occurs from x-ray tube along path through Pt to detector

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5
Q

CT data recon

A

computerized processing of transmission measurements into image(s)

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6
Q

Multidimensional image display

A

Disp of gray-scale image in 2D/3D format; representation of attenuation that occurred across scanned vol of tissue

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7
Q

CT image archival and communication

A

Display and storage of short/long-term (archival) of images on workstations

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8
Q

Scout/Scanogram/Localizer

A

Survey radiograph w x-ray in fixed position

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9
Q

Azimuth

A

Angle of tube/detectors in relationship to patient position during scout

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10
Q

Localizer

A

Scout used to prescribe CT acquisition

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11
Q

Translate/Index

A

movement of bed to next position

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12
Q

Conventional Axial Step-n-Shoot

A

complete revolution of x-ray around precise location w thickness determined by extent of collimation

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13
Q

Gantry

A

houses most mechanical parts: generator, x-ray tube, asst’d collimators, data acq syst (DAS), slip-rings, detectors

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14
Q

Slip-ring Technology

A

use of contact brushed (instead of fixed-length hard-wired cables) to supply electricity and enable transmission data to pass to comp syst: allows for helical/continuous acq

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15
Q

Helical CT

A

~continuous rot’n
~continuous bed mvn’t
~powerful ‘long-exposure’ x-ray
~specialized mathematical recon techniques

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16
Q

Allows for Volumetric Acquisistion

A

Helical Geometry

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17
Q

Universal section width of Helical Acquisition is controlled by

A

Collimation (slice thickness): chosen before acquisition

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18
Q

MDCT

A

Multidetector CT

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19
Q

Utility of MDCT for Helical Acquisition

A

Enables CT section of varying widths to be recon’d at any point along acquired volume

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20
Q

CT uses a metal-enclosed x-ray tube consisting of…

A

…cathode and rotating anode disc

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21
Q

Choice of foci is controlled by user via selection of (2+)

A

milliamperage (mA) setting, scan field of view (SFOV), etc

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22
Q

Dual filaments allow for…

A

…a choice of 2 focal spot sizes of 0.5-1.2mm diameter

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23
Q

Smaller focal spots improve:

A

Geometric efficiency of x-ray beam/greater spatial resolution

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24
Q

Flying focal spot technology involves…

A

…electromagnetic steering of electron beam emitted from cathode. Beam of e- is directed toward 2 separate locations on rotating anode, resulting in 2 sources of x-radiation

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25
Flying focal spot technology influence on number of data samples
Essentially doubled d/t electronic switching b/w 2 focal spots
26
Impact of flying focal spot technology
This oversampling can be used to improve system's temporal and spatial resolution
27
SSCT
Single Slice CT
28
MDCT
Multidetector CT
29
Efficient heat dissipation (for SSCT/MDCT) is required d/t
long acquisition times and relatively high exposure rates requiring x-ray tube with very high heat storage capacity
30
Methods for dissipating heat generated during CT
Oil cooling Air cooling Increased anode diameter Conduction by tube rotation
31
MDCT demands high performance x-ray tube w characteristics including (3)
High heat rating: absorb tremendous heat/dissipate quickly Small size/lightweight: must rotate within CT gantry at high speed Stable/long lasting: withstand huge centrifugal forces w/extended useful life
32
Milliampere setting range
30 mA to upwards of 800 mA
33
ATCM
Automatic Tube Current Modulation
34
Effect of ATCM
automatic alteration of applied mA according to required noise index acceptable for appropriate image quality
35
mA setting based on
Indication Body Habitus Required SNR
36
SNR
Signal-to-Noise Ratio
37
Total scan time equals
Sum of all acquisitions (exposure time) during a study
38
Photon Fluence
quantity of x-radiation directed toward patient
39
Photon Fluence is controlled by:
mA-seconds (mAs) setting in coordination with scan time gives constant milliampere
40
Photon flux
Rate at which Photon Fluence passes through unit area over unit time
41
mAs value for given acquisition is directly proportional to
patient radiation dose
42
mAs value for given acquisition is inversely proportional to
Image Noise
43
Effective mAs
calculated mAs value per qcquired slice
44
Main controlling factor of "effective mAs"
Table speed
45
Selected Pitch determines:
patient translation speed per tube revolution
46
effective mAs =
mAs/pitch
47
kVp
Peak Kilovoltage
48
kVp controls
quality of x-ray beam/overal penetrating capabilities
49
Higher kVp yeilds
x-ray beams with greater penetrating power
50
Use of lower kVp
Decreases patient radiation dose May improve contrast (esp during CTA)
51
Streak artifact d/t very dense body parts (e.g. posterior fossa) mitigated by
higher kVp settiings
52
The extremely efficient CT x-ray tube can generate beam using peak voltages in range of
70-150 kVp
53
ATVS
Autimated Tube voltage Selection
54
Effect of ATVS software
modulates tube potential based on changing patient attenuation along scan range
55
Dual-energy CT
Apply multiple x-ray energies during a single acquisition
56
Differentiation and characterization of tissue composition are made possible by
difference in attenuation b/w the high- vs low-kVp radiation of dual-energy CT
57
Effect of complex voltage-switching systems employed by dual-energy CT
applied peak kVp is switched at extremely high rate for each succesive projection utilizing single x-ray tube
58
Dual source CT systems
2 x-ray tubes at 90 deg from eo acquire simultaneously at different kVps
59
Dual-energy CT expands clinical opportuniites to include
Improved resolution of soft tissue (ligaments/tensons) during msk imaging Visualization of plaque within contrast-enhanced vessels (e.g., cardiac CT) Contrast medium subtraction techniques demonstrating "precontrast" images from single contrast enhanced scan Characterization of biochemical composition of UT calculi
60
HVL
Half-Value layer
61
HVL definition
Thickness of material required to reduce x-ray beam intensity to 1/2
62
HVL utility
Used a a measue of the overall quality of the beam Helpful in determining amount of beam filtration necessary for given CT system
63
Filtration removes
photons--from polyenergetic/heterogeneous x-ray beam--whose energy is insufficient to pass thru patient and still reach detector
64
Beam Hardening
Increase of average intensity of beam as low-energy photons are absorbed along pathway resulting in artifactual image
65
filtered beam that's more homogeneous/monoenergetic is less susceptible to
artifacts from beam hardening
66
Inherent Filtration (tube housing, cooling oil, etc) amounts to approx _____ aluminum-equivalent filtration
3 mm
67
Energy quality of x-ray beam is improved by
"inherent" and "added" x-ray tube filtration
68
Additional (minimal) filtration is added to inherent filtration in the form of
Thin (0.1-0.4 mm) copper sheets bow-tie filters
69
Thin copper sheets and bow-tie filters improve:
Beam utilization efficiency
70
Bow-tie filter shape
thicker at ends than in middle
71
Benefit of bow-tie filters
shape beam to reduce intensity toward outer margins thus decreasing patient exposure
72
Why bow-tie filters are so effective on humans
Most body parts are circular or cylindrical, requiring less radiation on the perifery
73
Tube warm-up calibration scans...
...include wide range of combos of kVp, mA, collimation settings
74
CT generator includes a high-voltage transformer necessary to...
...convert low-frequency/low-voltage AC supply into high-frequency/high-voltage current required for efficient x-ray production
75
Current CT scanners utilize high-frequency generators
which are modern, efficient, small enough to be housed within gantry
76
Generator/Transformer power output is vendor specific with typical range of
60 to 100 kW

Mosby 5: Image Production (2 decks)

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