CT QUALITY MANAGEMENT
SPATIAL RESOLUTION
CONTRAST RESOLUTION
RADIATION
DOSE
NOISE
IMAGE ARTIFACTS
TESTS
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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
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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