Measure water noise
Measure water noise with its independent central ROI and save a report.
On this page
Before you start
Open a 2D CT cross-section with a successfully detected complete water phantom.
What you will achieve
Measure water noise with its independent central ROI and save a report.
1. Switch to water noise
Open Analysis → QA → Water noise. The canvas shows this group’s large ROI; other groups remain saved.
2. Check the large central ROI
The initial diameter is 40% of the phantom diameter. Check its intended position inside the phantom. Move or resize, release and wait for automatic calculation. Press Esc during dragging to cancel.
3. Read noise results
Noise SD is the population standard deviation of valid pixels inside the large ROI, in HU. Noise % = SD / 1000 × 100%. It describes this slice and sampling configuration, not external-air SD or an automatic compliance decision.
Troubleshooting
Wait while values show “—” during recalculation. On failure, read the reason, adjust the region and click Calculate. Each ROI requires at least 16 valid pixels. Regenerate current group ROIs replaces the group layout; use Calculate if you only need new values.
Save or export
Click Export report in the footer, preview, then save. Check the image, ROI placement and values in the PDF. Water reports contain CT value/uniformity and water-noise groups; air reports are separate. Save a workspace and retain the source images to continue editing. Reports do not automatically determine compliance.