Topic 13: CT image quality + artefacts Flashcards
(29 cards)
Describe 1st gen CT scanners
- Detector = 1
- Beam type = pencil-like X-ray
- Tube detector movements = translate-rotate
- Duration = 25-30 mins
Describe 2nd gen CT scanners
- Detector = multiple
- Beam type = fan-shaped X-ray
- Tube detector movements = translate-rotate
- Duration = less than 90 secs
Describe 3rd gen CT scanners
- Detectors = multiple = 288 > 700
- Beam type = fan-shaped X-ray
- Tube detector movement = rotate-rotate
- Duration = 5 secs
Describe 4th gen CT scanners
- Detectors = multiple in fixed outer ring = 2000+
- Beam type = fan-shaped X-ray
- Tube detector movements = rotate-fixed
- Duration = few secs
Describe sequential scanning
- During acquisition of slice = table stationary
- Post acquisition = table moves to new position = next scan
What is the disadvantage of sequential scanning?
- Long scan time
Describe spiral scanning
- Continuous tube detector rotation = fast acquisition
- Using slip-ring technology
- Continuous acquisition = coverage of larger sections in same amount of time
- More heat storage capacity required = generates energy for extended period
- More storage + processing capacity required = large amount of data produced in short time
Compare the mechanisms of sequential + piral CT scanners
SEQUENTIAL:
- Rely on = physical connection = cables between rotating elements
- Have to unwind post each acquisition
SPIRAL CT:
- Transmit energy + data via electric conductive brushes + rotating rings
- Scanners can rotate continuously
Describe electron beam CT scanning
- Developed to image heart
- Instead of physical rotation tube-detector unit = generates = directs e- along stationary tungsten ring
- X-rays from ring = collimated = pass through patient
- Use = non-invasive coronary angiography + quantification of coronary calcium
What is the temporal resolution of EBCT?
100 ms
Describe multi-slice CT scanning
- Increased number of detector rows = multiple channels of data aquired simultaneously
- Use = cardiac acquisitions = need longer scan times compared to non-moving organs
- Acquires up to 320 slices simultaneously BUT option = 16/32/64/128/256
What is the advantage of multi-slice CT scanning?
- Increased longitudinal coverage achieved per rotation
What is the benefit of focal point alteration in multi-slice CT?
- Rapid + longitudinal change of X-ray source/focal point during acquisition = 2 projections = different angles = slightly overlap = 3D image
- Single z-sampling = 1 set images = less detail
- However double z-sampling = 2 sets = more detail = better logitudinal resolution BUT longer duration
Describe the multi-slice CT detectors
- Increased number = individual detector width decreased
- 4-64 slice CT = width decrease 1>0.4 + 1.25>0.5
- Combined with overlapping slices = improves spatial resolution + longitudinal direction
Describe dual-source CT scanning
- 2 X-ray tubes = rotating at 90° to each other
- Both tube detector system = at different voltages = improves tissue differentiation
- Application = dual-energy CT
What is the advantage of dual-source CT scanning?
- Improvement of temporal resolution
Factors affecting image quality
- Detector size
- Number of angular projections
- Focal spot of X-ray tube
What is the resolution + slice thickness of clinical CT scanners?
- Resolution = 0.2-1 mm
- Slice thickness = 0.4-5 mm
- Higher slice thickness = reduce radiation dose + improve SNR = BUT loss of axial resolution
What is the focal point size of clinical CT scanners?
- Larger spot = blurs image
- High resolution CT tubes = 3–5 µm
Describe the function of collimators
- X-ray + tissue = random scattering = image noise
- Located = front of detector = anti-scatter grid
- Eliminate deviating photons from source-detector path
- Reduce beam size + thickness = small focal spot
Define artefacts
- Discrepancy between CT numbers in image + expected number = HU
Give common artefacts
- Beam hardening
- Partial volume effect
- Bad detector
- Metal
- Patient motion
Describe beam hardening effect
- When X-rays pass through strongly absorbing materials
- Detector + signal amplifiers = noise source
- Electronic noise-suppression filters + software noise suppression = reduce image noise
- Operator = reduces noise BUT loss of detail
- X-ray = not monochromatic beam = beam hardening common
- Lower energy X-ray absorbed = energy peak to higher energies = absorption values underestimated = artefact
How can beam hardening be reduced?
- By pre-hardening beam
- Place thin metal plate in front of tube = molybdenum/tungsten
