L6.2. CT: Image Quality and Artefacts Flashcards
First generation CT
Detector: 1
Type of beam: pencil-like x-ray beam
Tube-detector movements: translate-rotate
Duration of scan (average): 25-30 mins
2nd Gen CT
Detector: multiple (up to 30)
Type of beam: fan-shaped x-ray beam
Tube-detector movements: translate-rotate
Duration of scan (average): less than 90s
3rd Gen CT
Detector: multiple, originally 288; newer use over 700 arranged in an arc
Type of beam: fan-shaped x-ray beam
Tube-detector movements: rotate-rotate
Duration of scan (average): approx 5 sec
4th Gen CT
Detector: >2000 arranged in an outer ring which is fixed
Type of beam: fan-shaped x-ray beam
Tube-detector movements: rotate-fixed
Duration of scan (average): few seconds
Originally, all CT scanners sequentially acquired axial slices according to what?
The stop-and-shoot principle
Spiral CT scanners allow for what?
Continuous tube-detector rotation and fast acquisition of data
What is the EBCT?
What was it developed for?
(Electron beam CT)
Developed to image the heart
Instead of physically rotating the tube-detector unit, it generates and directs electrons along a stationary tungsten ring
Increasing the number of detector rows means that?
Multiple channels of data can be acquired simultaneously
What are Dual-source CT scanners equipped with?
2 X-ray tubes rotating at a 90 degree angle to each other
What is image quality primarily determined by?
Detector size
Number of angular projections
Focal spot of the x-ray
What does higher slice thickness do?
Reduces radiation dose
Improves SNR at the expense of axial resolution
CT artifacts, what do they do?
Common, can occur for various reasons
Can mimic pathology (eg partial volume artefact) or degrade image quality to non-diagnostic levels
How can CT artifacts be classified?
According to the underlying cause of the underlying cause of the artifact:
-Patient-based artifacts
-Hardware-based artifacts
-Physics-based artifacts
-Helical and Multichannel artifacts
Patient-based artifacts:
-Motion artifact
-Transient interruption of contrast
Hardware-based artifacts:
-Ring artifacts
-Out of Field artifacts
-Tube arcing
Physics-based artifacts:
-Beam hardening
-Partial volume averaging
-Noise
-Photon starvation
-Aliasing in CT
Helical and multichannel artifacts:
-Windmill artifacts
-Cone beam effect
-Zebra artifact
-Stair step artifact
What happens in sequential scanning during the acquisition of a slice?
Table remains stationary
After completion of the acquisition: table moves to new position to perform the next scan
Disadvantage of sequential scanning
Relatively long scan time
How do spiral CT scanners allow for continuous tube-detector rotation and fast acquisition of data?
Slip-ring technology
What do sequential CT scanners rely on?
Physical connection in form of cables between the rotating elements (x-ray tube, detectors, stationary base)
Necessitates unwinding of wires after each acquisition
How do spiral CTs rotate continuously?
Transmit energy and data via electronically conductive brushes ans rotating rings
Table moves at a constant speed through the gantry
Path of acquisition relative to the subject resembles a spiral
Continuous acquisition of data allows for?
Coverage of larger sections in same time
What is required due to the energy generated by the X-ray tube for an extended period? (In spiral CT)
More heat storage capacity is required