week 2, lec 2: systems and components Flashcards
what is the physical process of CT
Gantry (x-ray tube to filter to collimator to detector array) to data acquisition to the operating console for post processing
how are CT images formed?
- High voltage generator
- CT gantry (patient and table), attenuation measurements, digital data)
- Analogue to digital conversion (ADC)
- Computer processing of this data
- numerical CT image (DAC)
- CT console: image display
- PACS: electronic communication
High voltage generator:
what do slip rings do
-Transmit data; electrical connections made by stationary brushes pressing against rotating circular conductor
high voltage generator:
what are some consideration for slip ring scanners and what do we need to do to help
- Continuous rotation and scanning; increased heat being added to the tube and no cooling time between rotation
- What we need to do: greater heat capacity, faster cooling method (internal cooling fans and cold room)
high voltage generator:
what is the process of the x-ray generator
- main power
- full wave rectifiers (ADC)
- capacitors
- invert circuit
- high voltage transformer
- full wave rectifiers (DAC)
- high voltage capacitors
- high frequency high voltage current
- x-ray tube
x-ray tube(s):
what are the demands of the x-ray tube
High power levels leads to problems with: heat generation, heat storage and heat dissipation
x-ray tube(s):
what was the problem with 1st/ 2nd gen x-ray tubes
1st and 2nd generation scanners had stationary anodes, oil tubes
- Problem: Poor heat dissipation
x-ray tube(s):
what are the benefits of the modern (3rd gen) x-ray tubes
use rotating anodes; creates an increased heat loading on the tube
- Uses a much larger anode compared to earlier models (increase spatial resolution)
- Benefit: produces a heterogeneous x-ray beam
x-ray tube(s):
3 characteristics of the straton x-ray tube
- Siemens innovation; revolutionary design
- Entire tube body rotates; allows all the bearings located outside the evacuated tube, and enables the anode to be cooled efficiently
- Oil cooled; anode directly oil cooled
- Enabled high speed volume scanning
- Utilises electron beam to strike anode at 2 focal spots
what is a flying focal spot and what advantages does it have
Electronically change from 1 focal spot to another
-With the change of the FS, there is a resultant off-set of the x-ray beam
- Double the no. of slices / detectors
- Increases the spatial resolution
- Significantly suppresses artefacts
- Improves z axis resolution
collimation and filtration:
what is filtration and why do we use it
Filtration removes greater % of low energy photon to create a monochromatic beam
collimation and filtration:
problems with filtration
beam hardening; can lead to artefact
collimation and filtration:
what is a bowtie filter used for and what are its benefits?
- head scans Bowtie filter: -removes low energy photons - creates greater attenuation at the edges to compensate the body shape - Hopefully a more uniform exit beam
collimation and filtration:
what are pre and post patient collimation used for (3 for pre, 2 for post)
Pre patient:
- defines slice thickness
- affects patient dose and amount of scatter present (prevents patient over-irradiation)
Post patient:
- determines the beam width after passing through patient
- ensures proper beam width at the detector end
- reduces the number of scattered photons from entering detectors
detectors:
how do CT detectors work
- Collects photon intensity and converts it to electrical signal for reconstruction
- Capture radiation that’s passed through the patient and converts it to electrical signal for reconstruction
detectors:
what is needed for CT detectors to work
- Efficiency: ability to capture, absorb and convert x-ray photons to electrical signals
- Must have a high capture / absorption and conversion efficiency rate
- Dynamic range: ratio of the largest to smallest discernible
- High reproducibility and accuracy, stability and minimal afterglow
detectors:
what are gas ionisation detectors?
- Individual gas chambers separated by plates: tungsten
- When attenuated, photons interact with the charged plates and the Xenon gas; ionisation occurs
- The ionisation of ions produces an electrical current
- The signal produced varies directly with the number of photons absorbed
detectors:
how do solid state detectors work and why are they better than gas ionisation detectors
-All new MSCT use Solid State Detectors (99-100% efficient)
X-ray to crystals to light photodiode to electrical signal to data recon
detectors:
how does scintillation work
-A scintillator emits light when it is struck by x-ray photons
detectors:
how is the slice thickness determined in multiple detector array scanners
-Minimum slice thickness determined by the width of the detectors in the slice thickness dimension
detectors:
give an example of how slice thickness is determined in multiple detector array scanners
-Minimum slice thickness e.g. 0.5mm can bind to create larger slice/ thickness, e.g. 1mm thick slice
detectors:
what is detector configuration
- Rows vs slices vs coverage
- 64 ‘Slice’ machines can have different coverage
- Coverage = Slice thickness x no. of detector rows
- No. of ‘slices’ can be twice the number of detector rows due to sampling
- More slices does not always mean more detail
detectors:
what are 3 advantages of multi-slice detectors
- Increase speed and volume coverage
- Increase pitch and tube rotation times: shorter scan times
- Improved spatial resolution
- Thinner slices have better isotropic resolution: flawless MPR and 3D images
- Efficient x-ray beam: larger anodes have better heat dissipation increases tube life
- Reduction in radiation exposure using dose reduction software
data acquisition system:
what is it?
measures transmitted radiation, converts analogue measurements to digital (ADC) and transmits data to the computer
data acquisition system:
how does it work and where is it located?
- Located between detector and computer
- Samples each detector and transmits the signal to the computer
- Detector
- Current to voltage converter
- Pre amplifier
- Analogue integrator 5. ADC
- Computer
-It amplifies the electrical signal due to it being very weak
data acquisition system:
what does the computer do
- Data processing
- Image reconstruction
- Image display
- Image transmission, storage etc.
- Work station: setting protocols, deciding scanned body part/ how much is getting scanned, exposure factors etc.
Couch:
what are its characteristics
- Must be radiolucent; carbon fibre
- Strong
- RT: Flat couch required (reproducible position)
- Various lengths available, depends on scans being performed (longer for ED scans if possible)
