Measure ramp FWHM and slice thickness
Select one ramp, check its direction/angle and distinguish FWHM from thickness.
On this page
Before you start
Use an original 2D CT phantom slice with one horizontal or vertical ramp and identify its physical angle.
What you will achieve
Select one ramp, check its direction/angle and distinguish FWHM from thickness.
1. Select FWHM
Open Analysis → FWHM. Its ROI, results and reset are independent of MTF.
2. Set direction and angle
Choose Horizontal X / Vertical Y to match the ramp. Check the angle in analysis tool settings: the default is 23°, with 45° available. Match the actual phantom rather than choosing an angle to improve the result.
3. Select one ramp
Draw a narrow rectangle along one ramp with background at both ends and at least 8 pixels along the profile. The transverse width determines averaging. Release to calculate or use Set ROI geometry. Rotated or multiple ramps are unsupported.
4. Read the profile and both results
Inspect the profile under Analysis & profile and choose Gaussian fitting or half-height crossings. FWHM is the profile width at half height; thickness is FWHM × tan θ, both in mm.
A single ramp does not correct phantom tilt. Measured thickness is not slice spacing.
5. Troubleshoot and save
If calculation fails, check direction, background, pixel count and whether one ramp is selected. Adjust and retry. Save the workspace and source images; use Recalculate only when you want saved values updated with current settings.