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

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

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
Q

Flying focal spot technology influence on number of data samples

A

Essentially doubled d/t electronic switching b/w 2 focal spots

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26
Q

Impact of flying focal spot technology

A

This oversampling can be used to improve system’s temporal and spatial resolution

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27
Q

SSCT

A

Single Slice CT

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28
Q

MDCT

A

Multidetector CT

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29
Q

Efficient heat dissipation (for SSCT/MDCT) is required d/t

A

long acquisition times and relatively high exposure rates requiring x-ray tube with very high heat storage capacity

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30
Q

Methods for dissipating heat generated during CT

A

Oil cooling
Air cooling
Increased anode diameter
Conduction by tube rotation

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31
Q

MDCT demands high performance x-ray tube w characteristics including (3)

A

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
Q

Milliampere setting range

A

30 mA to upwards of 800 mA

33
Q

ATCM

A

Automatic Tube Current Modulation

34
Q

Effect of ATCM

A

automatic alteration of applied mA according to required noise index acceptable for appropriate image quality

35
Q

mA setting based on

A

Indication
Body Habitus
Required SNR

36
Q

SNR

A

Signal-to-Noise Ratio

37
Q

Total scan time equals

A

Sum of all acquisitions (exposure time) during a study

38
Q

Photon Fluence

A

quantity of x-radiation directed toward patient

39
Q

Photon Fluence is controlled by:

A

mA-seconds (mAs) setting in coordination with scan time gives constant milliampere

40
Q

Photon flux

A

Rate at which Photon Fluence passes through unit area over unit time

41
Q

mAs value for given acquisition is directly proportional to

A

patient radiation dose

42
Q

mAs value for given acquisition is inversely proportional to

A

Image Noise

43
Q

Effective mAs

A

calculated mAs value per qcquired slice

44
Q

Main controlling factor of “effective mAs”

A

Table speed

45
Q

Selected Pitch determines:

A

patient translation speed per tube revolution

46
Q

effective mAs =

A

mAs/pitch

47
Q

kVp

A

Peak Kilovoltage

48
Q

kVp controls

A

quality of x-ray beam/overal penetrating capabilities

49
Q

Higher kVp yeilds

A

x-ray beams with greater penetrating power

50
Q

Use of lower kVp

A

Decreases patient radiation dose
May improve contrast (esp during CTA)

51
Q

Streak artifact d/t very dense body parts (e.g. posterior fossa) mitigated by

A

higher kVp settiings

52
Q

The extremely efficient CT x-ray tube can generate beam using peak voltages in range of

A

70-150 kVp

53
Q

ATVS

A

Autimated Tube voltage Selection

54
Q

Effect of ATVS software

A

modulates tube potential based on changing patient attenuation along scan range

55
Q

Dual-energy CT

A

Apply multiple x-ray energies during a single acquisition

56
Q

Differentiation and characterization of tissue composition are made possible by

A

difference in attenuation b/w the high- vs low-kVp radiation of dual-energy CT

57
Q

Effect of complex voltage-switching systems employed by dual-energy CT

A

applied peak kVp is switched at extremely high rate for each succesive projection utilizing single x-ray tube

58
Q

Dual source CT systems

A

2 x-ray tubes at 90 deg from eo acquire simultaneously at different kVps

59
Q

Dual-energy CT expands clinical opportuniites to include

A

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
Q

HVL

A

Half-Value layer

61
Q

HVL definition

A

Thickness of material required to reduce x-ray beam intensity to 1/2

62
Q

HVL utility

A

Used a a measue of the overall quality of the beam
Helpful in determining amount of beam filtration necessary for given CT system

63
Q

Filtration removes

A

photons–from polyenergetic/heterogeneous x-ray beam–whose energy is insufficient to pass thru patient and still reach detector

64
Q

Beam Hardening

A

Increase of average intensity of beam as low-energy photons are absorbed along pathway resulting in artifactual image

65
Q

filtered beam that’s more homogeneous/monoenergetic is less susceptible to

A

artifacts from beam hardening

66
Q

Inherent Filtration (tube housing, cooling oil, etc) amounts to approx _____ aluminum-equivalent filtration

A

3 mm

67
Q

Energy quality of x-ray beam is improved by

A

“inherent” and “added” x-ray tube filtration

68
Q

Additional (minimal) filtration is added to inherent filtration in the form of

A

Thin (0.1-0.4 mm) copper sheets
bow-tie filters

69
Q

Thin copper sheets and bow-tie filters improve:

A

Beam utilization efficiency

70
Q

Bow-tie filter shape

A

thicker at ends than in middle

71
Q

Benefit of bow-tie filters

A

shape beam to reduce intensity toward outer margins thus decreasing patient exposure

72
Q

Why bow-tie filters are so effective on humans

A

Most body parts are circular or cylindrical, requiring less radiation on the perifery

73
Q

Tube warm-up calibration scans…

A

…include wide range of combos of kVp, mA, collimation settings

74
Q

CT generator includes a high-voltage transformer necessary to…

A

…convert low-frequency/low-voltage AC supply into high-frequency/high-voltage current required for efficient x-ray production

75
Q

Current CT scanners utilize high-frequency generators

A

which are modern, efficient, small enough to be housed within gantry

76
Q

Generator/Transformer power output is vendor specific with typical range of

A

60 to 100 kW

Mosby 5: Image Production (2 decks)

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