1. No category

Quality Management in CT
CT Prof. Stelmark
CT Prof. Stelmark
CT Prof. Stelmark
AVG. CT NUMBER
• WATER – CT# = 0 , +- 3 HU
• AIR – CT# = -1,000, +- 5 HU
CT Prof. Stelmark
AVERAGE CT NUMBER
CT Prof. Stelmark
Uniformity and noise
When imaged, a phantom of uniform material such as water should demonstrate
consistent CT values regardless of a pixel's position on the image matrix. Pixels at the
center of the image should measure as water density (∼ 0 HU), as should pixels at
various other points in the image.
Beam hardening occurs as the x-ray beam passes through the object (phantom, patient).
The beam exposing the center of the object has a higher average photon energy than the
beam exposing the periphery of the object. This beam-hardening effect may slightly alter
the pixel values at different points across an image of an object of uniform inherent
density.
The spatial uniformity of a CT system describes its ability to maintain relatively consistent
CT values across the entire image of an object of equal density.
CT Prof. Stelmark
4. Uniformity may be evaluated by positioning several ROI measurements at different
locations along the center and periphery of an image. The CT values should not differ by
more than 2 HU from one location to another
CT Prof. Stelmark
CT Prof. Stelmark
CAUSE OF FAILURE
• NOISE
CT Prof. Stelmark
FREQUENCY
• DAILY
CT Prof. Stelmark
Linearity
The relative accuracy between calculated CT numbers and their respective linear
attenuation coefficients is termed linearity.
The CT number for water is 0 HU. According to the formula for the calculation of
CT number relative to water discussed previously (see “Image Display”), the CT
number for air is −1000 HU, and the CT number for water is 0 HU.
The CT system is calibrated according to these two known values. Daily calibration
procedures should be performed using a water-filled phantom or a gantry cleared
of all objects and exposing only room air.
CT Prof. Stelmark
CT Prof. Stelmark
CT Prof. Stelmark
Linearity
Analysis of the values of the five pins should show a linear relationship between the
Hounsfield unit and electron density. The coefficient of correlation for this linear
relationship should be at least 0.96%, or 2 standard deviations.
CT Prof. Stelmark
LOW CONTRAST PHANTOM
CT Prof. Stelmark
CT Prof. Stelmark
EXPECTED RESULTS
• SMALLEST HOLES THAT COULD BE IMAGED
SHOULD BE 3 mm IN DIAMETER OR SMALLER
FOR 0.5% densityu difference.
CT Prof. Stelmark
CAUSE OF FAILURE
• NOISE
CT Prof. Stelmark
HIGH CONTRAST PHANTOM
CT Prof. Stelmark
CT Prof. Stelmark
CT Prof. Stelmark
SPATIAL RESOLUTION
• 0.45 –0.15 lp/mm (4.5 –15 lp/cm)
CT Prof. Stelmark
CAUSE OF FAILURE
• EXCESSIVE FSS
• MECHANICAL WEAR
• DETECTOR VIABRATIONS
CT Prof. Stelmark
FREQUENCY
• ONCE A MONTH
CT Prof. Stelmark
Slice Thickness
Slice thickness (sensitivity profile) is measured with the use of a specially designed test
object that incorporates a ramp, a spiral, or a step wedge. This assessment should be done
semiannually; the slice thickness should be within 1 mm of the intended slice thickness for
a thickness of 5 mm or greater. For an intended slice thickness of less than 5 mm, the
acceptable tolerance is 0.5 mm.
CT Prof. Stelmark
Accuracy of Distance-Measuring Device
EXPECTED RESULTS: The distance indicated by the CT scanner should agree with the true
distance as determined by counting the spaces between the two holes.
ACCEPTANCE LIMITS: Disagreement of 1 mm or less is good. Disagreement of greater
than 2 mm should be corrected.
POSSIBLE CAUSES OF FAILURE: Reconstruction algorithm may be improperly calibrated. If
the manufacturer has not provided the user with a means to recalibrate the algorithm, a
service person should be notified.
FREQUENCY: This should be performed at the time of installation and annually
thereafter.
CT Prof. Stelmark
Couch Incrementation
With automatic maneuvering of the patient through the CT gantry, the patient must be
precisely positioned. This evaluation should be done monthly. During a clinical examination
with a patient-loaded couch, note the position of the couch at the beginning and at the
end of the examination with the use of a tape measure and a straightedge on the couch
rails. Compare this with the intended couch movement. It should be within ±2 mm.
CT Prof. Stelmark