ct quality management. spatial resolution contrast resolution noise image artifacts radiation dose

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