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
