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IMAGE QUALITY . NOISE LINEARITY CROSS-FIELD UNIFORMITY IMAGE ARTIFACTS. NOISE IN CT. FLUCTUATION OF CT# BETWEEN POINTS IN THE IMAGE FOR A SCAN OF UNIFORM MATERIAL SUCH WATER. NOISE IN CT. NOISE LEVEL. PERCENTAGE OF CONTRAST OR IN CT NUMBERS. NOISE LEVEL %= STANDARD DEVIATION /CT # RANGE.

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image quality
IMAGE QUALITY
  • NOISE
  • LINEARITY
  • CROSS-FIELD UNIFORMITY
  • IMAGE ARTIFACTS
noise in ct
NOISE IN CT
  • FLUCTUATION OF CT# BETWEEN POINTS IN THE IMAGE FOR A SCAN OF UNIFORM MATERIAL SUCH WATER.
noise level
NOISE LEVEL
  • PERCENTAGE OF CONTRAST OR IN CT NUMBERS

NOISE LEVEL %= STANDARD DEVIATION /CT # RANGE

slide5
NOISE CAN BE MEASURED BY SCANNING A WATER PHANTOMAND COMPUTING THE MEAN AND STANDARD DEVIATION FOR FOR A REGION OF INTEREST. (ROI)
the noise levels for ct scanners vary and depend on
THE NOISE LEVELS FOR CT SCANNERS VARY AND DEPEND ON:
  • MAS
  • SCAN TIME
  • KVP
  • SLICE THICKNESS
  • OBJECT SIZE
  • ALGORITHM
algorithm vs noise
ALGORITHM vs NOISE

DETAIL

EDGE-ENHANCEMENT

STANDARD

SMOOTHING

NOISE

noise in ct is mainly related to
NOISE IN CT IS MAINLY RELATED TO
  • PHOTON FLUX ( # OF DETECTED PHOTONS)
  • MATRIX SIZE (PIXEL SIZE)
  • SLICE THICKNESS
  • ALGORITHM
  • ELECTRONIC NOISE ( DETECTOR ELECTRONICS)
  • SCATTER RADIATION
noise
WIDTH OF PIXEL

SLICE THICKNESS

DOSE

NOISE SPATIAL R.

NOISE SPATIAL R.

NOISE

NOISE
linearity
LINEARITY
  • RELATIONSHIP OF CT# TO THE LINEAR ATTENUATION COEFFICIENT
cross field uniformity
CROSS-FIELD UNIFORMITY
  • THE UNIFORMITY OF CT # THROUGHOUT THE ENTIRE SCAN FIELD OF VIEW IS ONE INDICATION THAT CT SCANNER IS PERFORMANCE IS ACCEPTABLE.
  • IT REFERS TO THE VALUES OF THE PIXELS IN THE RECONSTRUCTED IMAGE

THEY SHOULD BE CONSTANT AT ANY POINT IN THE IMAGE OF PHANTOM

cross field uniformity18
CROSS-FIELD UNIFORMITY

CROSS-FIELD UNIFORMITY

image artifacts
IMAGE ARTIFACTS
  • A DISTORTION OR ERROR IN AN IMAGE THAT IS UNRELATED TO THE SUBJECT BEING STUDIED
image artifacts20
IMAGE ARTIFACTS

STREAKS

RING

CT# DISTORTION

sources of artifacts
SOURCES OF ARTIFACTS
  • PATIENT
  • IMAGING PROCESS
  • EQUIPMENT
types and causes of artifacts
STREAKS

IMPROPER SAMPLING OF DATA

PARTIAL VOLUME AVERAGING

PATIENT MOTION

METAL & BEAM HARDENING

NOISE

SPIRAL SCANNING

MECHANICAL FAILURE

TYPES AND CAUSES OF ARTIFACTS
types and causes of artifacts23
SHADING

PART. VOLUME AVG.

BEAM HARDENING

SPIRAL SCANNING

SCATTER

OFF-FOCUS RADIATION

INCOMPLETE PROJECTION

TYPES AND CAUSES OF ARTIFACTS
types and causes of artifacts24
RINGS AND BANDS

BAD DETECTOR IN III

GENERATION CT

TYPES AND CAUSES OF ARTIFACTS
motion artifact voluntary involuntary
MOTION ARTIFACT- VOLUNTARY & INVOLUNTARY

CORRECTION:

  • IMMOBILIZATION
  • PROPER EXPLANATION OF PROCEDURE
  • SHORT SCAN TIME
  • MOTION REDUCTION SOFTWARE (SHIMADZU –MAC)

I

metal artifacts
METAL ARTIFACTS

INCOMPLETE PROJECTION PROFILES

STAR SHAPED ARTIFACTS

beam hardening artifact cupping artifact
BEAM HARDENING ARTIFACT-CUPPING ARTIFACT
  • INCREASE IN THE MEAN ENERGY OF THE X-RAY BEAM AS IT PASSES THROUGH THE PATIENT
  • IT CAN OCCUR WHEN RADIATION BEAMS HAVE DIFFERENT PATH LENGTHS
reduction of beam hardening
REDUCTION OF BEAM HARDENING
  • BOW TIE FILTER UTILIZATION
  • SPECIAL CORRECTION SOFTWARE
partial volume artifact

PARTIAL VOLUME ARTIFACT

AVERAGE CT NUMBERS IN SLICE

equipment induced artifacts streak artifacts
EQUIPMENT INDUCED ARTIFACTSSTREAK ARTIFACTS
  • MECHANICAL FAILURE
  • POOR GANTRY RIGIDITY
  • MECHANICAL ASSIGNMENT
  • TUBE ROTOR WOBBLE
  • POOR SAMPLING OF THE DETECTOR
  • BAD DETECTOR-RING ARTIFACTS ( III GEN.)
  • TUBE ARCING
correction of ring artifact
CORRECTION OF RING ARTIFACT
  • DETECTOR RECALIBRATION
  • BALANCING ALGORITHM UTILIZATION
aliasing artifact
ALIASING ARTIFACT
  • NOT ENOUGH SAMPLES
correction of aliasing artifact
CORRECTION OF ALIASING ARTIFACT
  • INCREASE NUMBER OF VIEWS OR RAY SAMPLES PER VIEW.
  • CONVOLUTION FILTER (SOFTWARE METHOD)
spiral artifacts
SPIRAL ARTIFACTS
  • WINDMILL
  • MOTION
  • STAIRCASE
  • SHADING