ct quality management. spatial resolution contrast resolution noise image artifacts radiation dose
TRANSCRIPT
CT QUALITY MANAGEMENT
SPATIAL RESOLUTION CONTRAST RESOLUTION
NOISE
IMAGE ARTIFACTS
RADIATIONDOSE
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
MTFMODULATION 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