Week 2 Flashcards
CT image artifacts
patterns / structures that appear on the image but not found on the original object
CT image artifacts appear due to
1) patient based
2) physics principles based
3) hardware based
importance of identifying CT image artifacts
1) obscures important anatomical details
2) alters CT hounsfield units & tissue density
3) misdiagnosis
patient based artifacts
metallic implants, motion, MPR artifacts
hardware based artifacts
ring artifact, cone beam artifact
principles of physics based artifacts
beam hardening, partial volume, photon starvation
what is beam hardening effect
low energy photons are filtered out but high energy photons pass through, mean energy increases
2 types of artifacts from beam hardening effect
streak & cupping
how to fix cupping artifact
- higher kVp
- reconstruction algorithm
how to fix streaking artifact
- metal artifact algorithm
- higher kVp
- remove any artifacts
how do streaking artifacts appear
dark streaks between 1 or 2 dense object in an image (lots of lines converging)
how do cupping artifacts occur
x-ray hardens more than detector expects causing resultant attenuation profile to differ from ideal profile
how do metal streak artifacts occur
- low mAs (photon starvation)
- beam hardening
- scatter
- poisson noise
poisson noise AKA
white noise
Metal Streaking Artifacts: If scanned material includes features that are higher density than surrounding matrix, what occurs
‘starburst’ artifact formed in which bright streaks spread out from object for short distance into nearby material
why does Poisson Noise occur
statistical error of low photon count resulting in thin bright & dark streaks that occur along direction of greatest attenuation
why do contrast density artifacts occur
non-uniform distribution of contrast agents leading to streaks & variation in tissue enhancement
how to fix contrast density artifacts
optimize contrast injection protocols to ensure even distribution with saline flushes to clear contrast from vessels
what causes beam hardening artifact
hardened x-ray beam due to preferential absorption of low energy x-rays by dense tissues
how to fix beam hardening effect
- adjust exposure factors
- avoid high density objects in scan area
- use reconstruction algorithms
what causes metal implant artifact
metal implants absorb x-rays = causes streaks & shading
how to fix metal implant effect
- adjust scan parameters
- metal artifact reduction algorithms
- change imaging modality
what causes partial volume artifact
very dense structures partially included in the slice causing high contrast errors (off-center)
where do partial volume artifacts occur most frequently in
bony structures such as skull base & petrous bone region
how do noise / photon starvation artifacts appear as
streak artifacts in the direction of the widest part of object scanned due to less signal reaching detectors
what causes noise / photon starvation artifacts
- poor scan techniques
- reconstruction algorithm
how to correct noise / photon starvation artifacts
- iterative reconstruction
- patient positioning (hands up)
- increase dose
- use thicker slices
what causes patient motion artifacts
patient motion during scan disrupts spatial alignment of acquired data
how to fix patient motion artifacts
- ask patient to remain still
- sedation / gating techniques
- faster scan
how to fix ring artifacts
- recalibrate detector
- replace detector module
what causes ring artifacts
- common in rotate-rotate type
- mis-calibrated / defective DELs
how do ring artifacts appear as
rings centered on center of rotation
what causes cone beam artifacts
object seen by diff detectors at diff angles
what are cone beam artifacts similar to
partial effect artifacts for off-centered objects but more pronounced at image edges
what are cone beam artifacts prominent with
- large cone angle
- thinner slices
- 16+ slice scanner
how do cone beam artifacts appear as
streaks & altered density in peripheral part of image
how to fix cone beam artifacts
- manufacturer reconstruction algorithms (cone beam reconstruction algorithms)
- reposition ROI in isocenter
Low Contrast Detectability (LCD) performance of CT
imaging systems is influenced by CT system specification
- mAs
- kVp
- slice thickness
- pitch
- beam collimation
- processing & visualization
what is CT QC
series of distinct technical procedures that identifies defects or imperfections in a product that can be corrected to eliminate these defects
why is CT QC necessary
max image quality, low dose
what does CT QC include
monitoring, evaluation, maintenance
what is used to ensure accurate measurement of HU for diff tissues & materials
air calibration
what is HU value for air
-1000 HU
