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
What does the large amount of data being produced in a very short period lead to in spiral CT scanning?
Expanded storage
Expanded processing capacity
What happens in mechanical CT
X-ray tube and detector array physically rotate around table
Forces created during rotation restrict rotation speed and temporal resolution of mechanical
In the absence of rotating parts, what is the temporal resolution of EBCT?
100 ms
What happens to emitted x-rays from the target ring in EBCT?
They are collimated
Then after passing through the patients are collected by the stationary detectors on the opposite side
When was there an enormous improvement of the resolution of mechanical CT
1980’s
When is EBCT used?
Can be used for non-invasive coronary angiography
Most often used for quantification of coronary calcium
Advantage of multi-detector CT acquisition?
Increased longitudinal coverage that can be achieved per rotation
Particularly useful for cardiac acquisitions (have longer scans than non-moving organs)
Current CT tech simultaneously acquires how many slices?
320
-> $2.5 mill
320-slice scanners are physically equipped with 320 detector rows
What other detector systems are offered?
16, 32, 64, 128, 256
Some systems allow for twice their slice acquisition by double sampling in the longitudinal direction
-> rapid, longitudinal alternation of focal spot during acquisition, partially overlapping sets of projections from slightly differing positions are acquired
Single z-Sampling vs Double z-Sampling
Double:
-Improved longitudinal resolution
-Longer total scan time
