VoxPlane

Measure point-source MTF

Select one point source, inspect sampling and read MTF50/10.

On this page

Before you start

Use an original 2D CT phantom slice with one bead or perpendicular wire cross-section, not MPR.

What you will achieve

Select one point source, inspect sampling and read MTF50/10.

1. Open the tool

Open the target slice and choose Analysis → MTF on the right. Expand More if the tool is there. Zoom in and identify one isolated bright target.

2. Include the target and background

Draw a rectangle containing the entire target and surrounding background. Both axes need at least 8 pixels; a square is not required. Exclude other bright targets and avoid clipping the source.

For numeric entry, open Set ROI geometry, enter center and dimensions, then Apply and calculate. Center X/Y use zero-based source pixel coordinates; width/height use mm.

3. Inspect actual sampling

Release the mouse and wait for calculation. Enable Show actual sampling support: the cyan solid boundary shows integration, pale dashed boundaries show the background band, and the cross marks the source.

The drawn rectangle locates the target; it is not the integration region. The default 110% refers to the adaptive baseline, not 110% of your rectangle. Its tapered edge can partially overlap the background band.

4. Read and check results

Read X/Y curves and MTF50/MTF10 for each direction; check lp/mm versus lp/cm. These frequencies correspond to normalized response falling to 50%/10%.

Open Calculation details to check the method, sampling regions, algorithm version and time. Interpret values with their measurement conditions. The software does not automatically judge compliance.

5. If no result appears

Read the error first. Check target completeness, other bright objects, background and image boundaries. Adjust and release to recalculate; do not treat an earlier result as the current measurement. Old records without sampling geometry need Recalculate before boundaries can be shown.

6. Save and understand the method

Save a workspace to retain ROI, curves, parameters and results; keep the source images. Reopening does not recalculate saved results.

The default uses 2D background correction and edge-weighted FFT. Gaussian-equivalent analysis has different assumptions. Finite-target correction and cross-slice averaging are not performed. See the reference chapter for details.