CT QUALITY MANAGEMENT

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					CT QUALITY MANAGEMENT
SPATIAL RESOLUTION                     CONTRAST RESOLUTION




                                             RADIATION
  NOISE                                        DOSE




                     IMAGE ARTIFACTS
              TESTS
•   AVERAGE CT NUMBER AND NOISE
•   CROSS-FIELD UNIFORMITY
•   SPATIAL RESOLUTION
•   LOW CONTRAST RESOLUTION
•   SLICE THICKNESS
•   LASER LIGHT ACCURACY
         NOISE IN CT
• FLUCTUATION OF
  CT# BETWEEN
  POINTS IN THE
  IMAGE FOR A
  SCAN OF UNIFORM
  MATERIAL SUCH
  WATER.
NOISE CAN BE MEASURED BY
        SCANNING
    A WATER PHANTOM
AND COMPUTING THE MEAN
AND STANDARD DEVIATION
   FOR FOR A REGION OF
      INTEREST. (ROI)
AVERAGE CT NUMBER
AVG. CT #
     AVG. CT NUMBER
• WATER – CT# = 0 , +- 3 HU

• AIR – CT# = -1,000, +- 5 HU

• NOISE +- 10 HU
    CAUSE OF FAILURE
• MISCALIBRATION OF THE CT #
  GENERATING ALGORITHM
        FREQUENCY
• CT NUMBER OF WATER – DAILY
• CT NUMBER OF AIR – MONTHLY
    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
      UNIFORMITY
• NO MORE THAN 2 HU BETWEEN
  CENTER AND PERIPHERY
    CAUSE OF FAILURE
• NOISE
          FREQUENCY
• DAILY
      QUALITY MEASUREMENT
       METHODS OF SPATIAL
           RESOLUTION
•   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 RESOLUTION



      Lp/cm
   SPATIAL RESOLUTION
• LARGE OBJECTS – LOW S.F.

• SMALL OBJECTS – HIGH S.F.
SPATIAL RESOLUTION



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 mm/lp= 0.6 cm/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
HIGH CONTRAST PHANTOM
   SPATIAL RESOLUTION


• 0.45 –0.15 lp/mm (4.5 –15 lp/cm)
    CAUSE OF FAILURE
• EXCESSIVE FSS
• MECHANICAL WEAR
• DETECTOR VIABRATIONS
        FREQUENCY
• ONCE A MONTH
LOW CONTRAST RESOLUTION
  ( TISSUE RESOLUTION)

• THE ABILITY OF AN IMAGING
  SYSTEM TO DEMONSTRATE SMALL
  CHANGES IN TISSUE CONTRAST
LOW CONTRAST PHANTOM
    EXPECTED RESULTS
• SMALLEST HOLES THAT COULD BE
  IMAGED SHOULD BE 3 mm IN
  DIAMETER OR SMALLER FOR 0.5%
  DENSITYU DIFFERENCE.
    CAUSE OF FAILURE
• NOISE
      SLICE THICKNESS
• IF 5 MM THICKNESS OR MORE IS
  SELECTED IT SHOULD NOT VARY BY
  MORE THAN +- 1mm. IF IT IS 5 mm OR
  LESS IT SHOULD NOT VARY BY
  MORE THAN +- 0.5 mm.
    CAUSE OF FAILURE
• MISCALIBRATION OF THE
  COLLIMATION MECHANISM
       FREQUENCY
• SEMIANNUALLY
LASER LIGHT ACCURACY
• LIGHT AND RADIATION FIELD
  SHOULD COINCIDE TO 2mm
    CAUSE OF FAILURE
• TUBE MISALIGNMENT
• LASER MISALIGNMENT
       FREQUENCY
• ANNUALLY

				
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