CT Basics And Model Flashcards
What are the 3 segments of CT imaging?
- Data acquisition
- Image recons
- Image display
What is data acquisition?
X-rays passing through the patient and hitting the detector
What is the gantry?
Ring shaped frame of the CT scanner.
It may be tilted forward or backward depending on patient or exam type.
What are slip rings?
Help the gantry frame continuously rotate for helical scans via electronically powered communication
What is the generator?
Power the scanner that control the exposure factors and are:
- High frequency
- High voltage
- High beam energy
- High penetrating ability
- High patient dose
What exposure technique helps to extend generator life?
High kV and low mAs
What are cooling systems?
Cool the scanner due to high heat from producing x-rays and include…
- blowers
- filters
- oil to air heat exchange
What material is the x-ray source usually made from?
Tungsten because it has a high atomic number and high melting point
What size focal spot is better for image quality and why?
A SMALLER focal spot produces sharper images because it reduces penumbra
(Heat concentrates on smaller portion of anode —> cannot tolerate as much heat)
What is a flying focal spot?
Multiple focal spots that create 2 beams and overlap directed at the same single detector array
What role does filtration have and what are the benefits?
- Filters out low energy x-rays
- Shapes the beam
Reduced patient dose and less image artefact
What filter is commonly used for body imaging?
Bow tie filter (thicker to lessen the beam for the body periphery)
What benefits does collimation have?
- Reduce scatter radiation
- Lower patient dose
- Increases image quality
What does collimation do?
Determine slice thickness and dose profile
What are detectors?
Array/collection of detectors that capture data from x-rays after passing through the patient
What are optimal characteristics of detectors?
- High detector efficiency
- Low/no afterglow
- High scatter suppression
- High stability
What type of detector was and is used?
Past: xenon gas
Now: solid-state crystal
What are different types of detector arrays?
- Matrix array - evenly sized elements
- Hybrid array - most elements the same size with smaller rows in the middle
- Adaptive array - smaller elements in the middle and progressively getting larger the farther it goes out
What determines the scanner generation?
Tube and detector configuration
What is the 1st generation scanner generation?
Thin linear beam over the patient with a single detector opposite to the tube
What beam style is used for a 1st gen scanner generation?
Pencil beam
What is the 2nd generation of the scanner?
Fan shaped beam that is linear over the patient before rotating with single detector array opposite tube
What is the 3rd generation of scanner?
Fan shaped beam travels linearly over patient with detector array opposite to the tube.
Both tube and detectors rotate in the gantry.
What is another name for the 3rd gen scanner?
Rotate rotate scanner
What are pros of 3rd gen scanners?
- Shorter scan time
- Less artefacts
- More collimation ability
- Reduced scatter
- Better image quality
What are cons of 3rd gen scanners?
Increased ring artefact because same detector is overused and misalignment occurs
What is the 4th generation scanner?
Tube is fixed and detector array moves 360 degrees in the gantry with a wide fan shaped beam
What are pros of 4th gen scanners?
Really quick scan times
What are cons of 4th gen scanners?
- More motion artefact
- Increased patient dose because of overscanning and tube closer to the patient
What is incrementation?
Table movement during scanning
What is the full length the table can move?
Scannable range
What units are used for helical scan table movements?
mm/sec
What is the pre patient collimation and what does it determine?
Between the x-ray source and patient
Limits the beam that hits and exposes the patient, determines slice thickness and patient dose
What is the post patient collimation and what does it determine?
Between the patient and detector array
Restricts the area that the beam can hit the detector array, determines slice thickness and reduces scatter
What is automatic tube modulation?
Adjusts the tubes current to maintain image quality while reducing patient dose
How are images produced?
Raw data (no pixels or HU assigned) —> recon (pro or retrospective) —> image data
How are images produced from the source?
- Tube produces x-ray photons
- Filters will filter out low energy photons
- Beam passes through the collimator to shape it
- Beam passes though the patient
- Beam hits and absorbed by the detector array
- Photons are converted to electronic signal
- Photons are absorbed and converted to digital signals by an analog to digital converter as raw data
- Digital data is assigned HU values
- The computer applies filters to remove artefacts and better visualise the subject
What is window width?
The number of HU of pixels in an image
Quantity
What is window length?
Centre of HU in an image
Range
What is the SFOV?
Area within the gantry from which raw data are collected
What is DFOV?
Amount of raw data that creates an image
_FOV cannot be larger than _FOV…
DFOV; SFOV
