IMAGE QUALITY

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					IMAGE QUALITY
SPATIAL RESOLUTION                     CONTRAST RESOLUTION




                                             RADIATION
  NOISE                                        DOSE




                     IMAGE ARTIFACTS
       QUALITY MEASUREMENT
             METHODS
•   PSF- POINT SPREAD FUNCTION
•   LSF- LINE SPREAD FUNCTION
•   CTF – CONTRAST SPREAD FUNCTION
•   MTF- MODULATION TRANSFER
    FUNCTION
        MTF
 MODULATION TRANSFER
      FUNCTION

MOST COMMONLY USED TO DESCRIBE
    SPATIAL RESOLUTION IN CT
              MTF
• MTF OF 1 – 100% TRANSFER OF
  OBJECT TO IMAGE

• MTF OF 0 – 0% TRANSFER OF
  OBJECT TO IMAGE
Spatial frequency



   Lp/cm
       Spatial Frequency
• LARGE OBJECTS – LOW S.F.

• SMALL OBJECTS – HIGH S.F.
    SPATIAL FREQUENCY




1              5
     ABSOLUTE SIZE OF AN
        OBJECT IN CT
• IT’S EQUAL TO THE RECIPROCAL OF
  THE SPATIAL FREQUENCY


     CT SCANNER SPATIAL FREQUENCY- 15 lp/cm.
  WHAT SIZE OF AN OBJECT CAN THE SCANNER RESOLVE

            1/15 = 0.06 cm/lp= 0.6 mm/lp

              0.6 /2 = 0.3 mm object
                Resolution in CT




Spatial Resolution          Contrast Resolution
    SPATIAL RESOLUTION
• DEGREE OF BLURRING IN THE IMAGE




MEASURE OF THE ABILITY TO DISCRIMINATE OBJECTS OF
     VARYING DENSITY A SMALL DISTANCE APART
         AGAINST A UNIFORM BACKGROUND
       GEOMETRIC FACTORS

• F. SPOT SIZE   BLURR
• APERTURE SIZE ( COLLIMATION) BLURR
• SLICE THICKNESS     BLURR
PARTIAL VOLUME EFFECT
• 10MM SLICE THICKNESS SPREADS
  2MM OBJECT OVER THE ENTIRE
  SLICE THICKNESS
# of projections   S. resolution
         ALGORITHM vs NOISE
DETAIL



                               EDGE-ENHANCEMENT




                            STANDARD




                SMOOTHING

                                       NOISE
  SPATIAL RESOLUTION AT
     HIGH CONTRAST
• HIGH CONTRAST RESOLUTION
       DISPLAY RESOLUTION
    • THE # OF PIXELS PER HORIZONTAL
      AND VERTICAL DIMENSIONS OF
      THE MATRIX ON THE MONITOR
      SCREEN OR FILM SHEET


MATRIX SIZE   DISPLAY RESOLUTION
DISPLAY RESOLUTION
HIGH RESOLUTION CT

               SLICE- 1 – 2 MM

           REDUCTION OF PARTIAL
             VOLUME ARTIFACT

          USE OF HIGH SPATIAL FREQ.
          ALGORITHM ( GOOD DETAIL)

              USE OF SMALL FOV

              ( SMALLER PIXEL)
    RECONSTRUCTION
     ( TARGETING) vs
 MAGNIFICATION IN SPATIAL
       RESOLUTION

• RECONSTRUCTION - VERY LITTLE
  DIFFERENCE

• MAGNIFICATION – SIGNIFICANT
  DIFFERENCE, SPATIAL RESOLUTION
CONTRAST RESOLUTION
• SENSITIVITY OF THE SYSTEM
• LOW CONTRAST RESOLUTION
• LOW CONTRAST DETECTIBILITY
       LOW CONTRAST
        RESOLUTION
   ( TISSUE RESOLUTION)
• THE ABILITY OF AN IMAGING
  SYSTEM TO DEMONSTRATE SMALL
  CHANGES IN TISSUE CONTRAST
    FACTORS AFFECTING LOW
     CONTRAST RESOLUTION
•   PHOTON FLUX
•   SLICE THICKNESS
•   PATIENT SIZE
•   DETECTOR SENSITIVITY
•   RECONSTRUCTION ALGORITHM
•   IMAGE DISPLAY
•   IMAGE RECORDING
•   QUANTUM NOISE
CONTRAST RESOLUTION
     PHANTOM
        PHOTON FLUX (P.F.)
• kVp       P.F.

• mAs       P.F.




 KVP, mAs          C. RESOLUTION
PATIENT SIZE       PHOTON FLUX




  PATIENT SIZE   C. RESOLUTION
SLICE THICKNESS   C. RESOLUTION
DETECTOR SENSITIVITY   C. RESOLUTION
      LOW CONTRAST
       RESOLUTION
• LOW SPATIAL FREQUENCY
  ALGORITHM
         ALGORITHM vs NOISE
DETAIL



                               EDGE-ENHANCEMENT




                            STANDARD




                SMOOTHING

                                       NOISE
QUANTUM NOISE   C. RESOLUTION
         NOISE IN CT
• FLUCTUATION OF
  CT# BETWEEN
  POINTS IN THE
  IMAGE FOR A
  SCAN OF
  UNIFORM
  MATERIAL SUCH
  WATER.

				
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