Scan Modes/X-Ray Tube/Generators&Transformers Flashcards
CT fundamental principle
An image of an object may be recon’d on the basis of attenuation that occurs as x-ray is transmitted through it
CT summarized
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
CT General process
data acq–>data recon–>multidimensional image display–>image archival and communication
CT data acqusition
measurement of attenuation that occurs from x-ray tube along path through Pt to detector
CT data recon
computerized processing of transmission measurements into image(s)
Multidimensional image display
Disp of gray-scale image in 2D/3D format; representation of attenuation that occurred across scanned vol of tissue
CT image archival and communication
Display and storage of short/long-term (archival) of images on workstations
Scout/Scanogram/Localizer
Survey radiograph w x-ray in fixed position
Azimuth
Angle of tube/detectors in relationship to patient position during scout
Localizer
Scout used to prescribe CT acquisition
Translate/Index
movement of bed to next position
Conventional Axial Step-n-Shoot
complete revolution of x-ray around precise location w thickness determined by extent of collimation
Gantry
houses most mechanical parts: generator, x-ray tube, asst’d collimators, data acq syst (DAS), slip-rings, detectors
Slip-ring Technology
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
Helical CT
~continuous rot’n
~continuous bed mvn’t
~powerful ‘long-exposure’ x-ray
~specialized mathematical recon techniques
Allows for Volumetric Acquisistion
Helical Geometry
Universal section width of Helical Acquisition is controlled by
Collimation (slice thickness): chosen before acquisition
MDCT
Multidetector CT
Utility of MDCT for Helical Acquisition
Enables CT section of varying widths to be recon’d at any point along acquired volume
CT uses a metal-enclosed x-ray tube consisting of…
…cathode and rotating anode disc
Choice of foci is controlled by user via selection of (2+)
milliamperage (mA) setting, scan field of view (SFOV), etc
Dual filaments allow for…
…a choice of 2 focal spot sizes of 0.5-1.2mm diameter
Smaller focal spots improve:
Geometric efficiency of x-ray beam/greater spatial resolution
Flying focal spot technology involves…
…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
Flying focal spot technology influence on number of data samples
Essentially doubled d/t electronic switching b/w 2 focal spots
Impact of flying focal spot technology
This oversampling can be used to improve system’s temporal and spatial resolution
SSCT
Single Slice CT
MDCT
Multidetector CT
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
Methods for dissipating heat generated during CT
Oil cooling
Air cooling
Increased anode diameter
Conduction by tube rotation
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
Milliampere setting range
30 mA to upwards of 800 mA
ATCM
Automatic Tube Current Modulation
Effect of ATCM
automatic alteration of applied mA according to required noise index acceptable for appropriate image quality
mA setting based on
Indication
Body Habitus
Required SNR
SNR
Signal-to-Noise Ratio
Total scan time equals
Sum of all acquisitions (exposure time) during a study
Photon Fluence
quantity of x-radiation directed toward patient
Photon Fluence is controlled by:
mA-seconds (mAs) setting in coordination with scan time gives constant milliampere
Photon flux
Rate at which Photon Fluence passes through unit area over unit time
mAs value for given acquisition is directly proportional to
patient radiation dose
mAs value for given acquisition is inversely proportional to
Image Noise
Effective mAs
calculated mAs value per qcquired slice
Main controlling factor of “effective mAs”
Table speed
Selected Pitch determines:
patient translation speed per tube revolution
effective mAs =
mAs/pitch
kVp
Peak Kilovoltage
kVp controls
quality of x-ray beam/overal penetrating capabilities
Higher kVp yeilds
x-ray beams with greater penetrating power
Use of lower kVp
Decreases patient radiation dose
May improve contrast (esp during CTA)
Streak artifact d/t very dense body parts (e.g. posterior fossa) mitigated by
higher kVp settiings
The extremely efficient CT x-ray tube can generate beam using peak voltages in range of
70-150 kVp
ATVS
Autimated Tube voltage Selection
Effect of ATVS software
modulates tube potential based on changing patient attenuation along scan range
Dual-energy CT
Apply multiple x-ray energies during a single acquisition
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
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
Dual source CT systems
2 x-ray tubes at 90 deg from eo acquire simultaneously at different kVps
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
HVL
Half-Value layer
HVL definition
Thickness of material required to reduce x-ray beam intensity to 1/2
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
Filtration removes
photons–from polyenergetic/heterogeneous x-ray beam–whose energy is insufficient to pass thru patient and still reach detector
Beam Hardening
Increase of average intensity of beam as low-energy photons are absorbed along pathway resulting in artifactual image
filtered beam that’s more homogeneous/monoenergetic is less susceptible to
artifacts from beam hardening
Inherent Filtration (tube housing, cooling oil, etc) amounts to approx _____ aluminum-equivalent filtration
3 mm
Energy quality of x-ray beam is improved by
“inherent” and “added” x-ray tube filtration
Additional (minimal) filtration is added to inherent filtration in the form of
Thin (0.1-0.4 mm) copper sheets
bow-tie filters
Thin copper sheets and bow-tie filters improve:
Beam utilization efficiency
Bow-tie filter shape
thicker at ends than in middle
Benefit of bow-tie filters
shape beam to reduce intensity toward outer margins thus decreasing patient exposure
Why bow-tie filters are so effective on humans
Most body parts are circular or cylindrical, requiring less radiation on the perifery
Tube warm-up calibration scans…
…include wide range of combos of kVp, mA, collimation settings
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
Current CT scanners utilize high-frequency generators
which are modern, efficient, small enough to be housed within gantry
Generator/Transformer power output is vendor specific with typical range of
60 to 100 kW
