Phantom Laboratory Catphan 503 - Scan Incrementation Accuracy Test

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Now that you have determined the half maximum CT number, you can measure the
full width at half maximum of the ramp. Set the CT scanner level at the half maximum
CT value and set your window width at 1. Measure the length of the wire image to
determine the FWHM. Multiply the FWHM by 0.42 to determine the slice width.
Schematic illustration of two sequential 5mm scans superimposed. L1 is
the center point on the 23° ramp in the rst scan image and L2 is the
center point on the 23° ramp on the second image.
Scan incrementation
Use the wire ramps to test for proper scanner incrementation between slices, and for
table movement.
Scan section 1 using a given slice width, (e.g. 5mm). Increment the table one slice width
(e.g. 5mm) and make a second scan. Establish the x and y coordinates for the center of
each ramp image. Calculate the distance between these points and multiply by the 23°
ramp angle correction factor of 0.42.
0.42(L1 - L2) = scan incrementation
This test can also be used to test table increment accuracy. Scan the section and
increment the table 30mm in and out of the gantry and scan again. The ramp centers
should be the same on both images.
0.42(L1 - L2) = 0