How to

Measure length and angle on a CBCT slice

After this page you will be able to read a distance between two points in millimetres and an angle between two lines in degrees in any MPR pane in JawAtlas, and you will know why the slice you choose matters more than the tool.

Where the tools live

Open a case and choose MPR from the segmented control at the top. The toolbar at the bottom of the pane has four chips: "Move", "Adjust", "Measure" and "Angle". Move drags the white crosshair; Adjust changes window and level. Measure and Angle are the two measurement modes, and they work in the axial, coronal and sagittal panes alike. The underlying geometry is described on Measurements.

A 41.3 mm length placed across the arch on the axial pane with two taps.

Steps

Navigate to the slice first. In Move mode, drag the crosshair to the structure and use the vertical slider on the right to scroll the stack until the structure is sharply in the plane. The readout at the top left, for example "Axial +10.2 mm", tells you where you are.
Tap "Measure". A blue prompt appears above the chips: "Tap where the measurement should start".
Tap the first point, then the second. After the first tap the prompt changes to "Tap where it should end". After the second, a green line is drawn with its length in millimetres, formatted like "41.3 mm". The points are stored in volume coordinates, so the measurement stays attached to the anatomy if you move the crosshair.
For an angle, tap "Angle". The prompts walk you through three taps: "Tap the first point of the angle", then "Tap the corner, where the two lines meet", then "Tap the last point of the angle". The result is shown in degrees, for example "92.5°", at the corner.
Clear with the trash button. As soon as a measurement or a half-finished one exists, a round trash button appears next to the chips. Tapping it clears every measurement in the pane. Switching tools also discards an unfinished measurement.

Check your scale

A scale ruler sits at the bottom left of each MPR pane and of the Pano cross-section. It is drawn from the DICOM spacing and picks a round length that fits the current zoom, 20 mm on a typical view. Use it as a sanity check: a lower first molar crown is about 10 to 11 mm mesiodistally, and the demo scan's voxels are 0.266 mm in plane. If a measurement disagrees with the ruler by a factor of two, you tapped the wrong points.

Why the slice matters more than the tool

The arithmetic is exact: the app converts your taps to millimetres with the voxel spacing from the header, and that spacing is known to three decimals. The error comes from geometry. A 2D distance is the projection of the true 3D distance onto the slice, so a structure that runs obliquely through the plane reads shorter than it is, and a line drawn across a canal that is partly out of plane reads longer than the canal's true width. So before you measure, scroll the pane until the whole structure lies in the slice, and pick the plane that contains it: a canal running buccolingually belongs in a coronal pane, a root length in a sagittal or coronal pane, an arch width in an axial pane. The oblique Slice stage and the perpendicular cross-section of the Pano stage are the places to work out which plane that is; they show you the geometry so that you tap the right pane in MPR.

Voxel size sets the floor. With 0.266 mm voxels you cannot meaningfully report tenths of a millimetre, even though the readout shows one decimal. See field of view and voxel size, and remember that a series imported with gaps is flagged for this reason: interpolated slices stretch or shrink distances along the stack.

If it does not work

The second tap moved the crosshair instead. You are still in Move mode. Check that the "Measure" chip is highlighted white before tapping.

The line disappears when I scroll. It did not disappear; it is drawn only on slices close to the plane it was placed in. Scroll back and it returns.

The readout looks far too big or small. Compare with the scale ruler. If both disagree with anatomy, the DICOM spacing in the export was wrong; re-export the series from the scanner software.

Next, learn to orient yourself before measuring on radiological orientation conventions.

Questions students ask

How accurate are measurements in JawAtlas?

They are calibrated from the voxel spacing in the DICOM header, so the arithmetic is exact to the voxel. The real uncertainty comes from where you tap and which slice you tap on: a canal measured on an oblique slice reads longer than it is. Treat the number as a study value, not a clinical one.

Can I measure on the panoramic view or in 3D?

The Measure and Angle chips live in the MPR stage, in any of the three planes. The panoramic cross-section and the Slice stage show a scale ruler or a depth bar for orientation, and the 3D volume rendering has no measurement tools because a projected image has no single depth to measure in.

Can I delete just one measurement?

No. The trash button that appears next to the tool chips clears all measurements in the current pane, including a half-finished one. Measurements are meant as quick checks while you study; save a moment if you want to keep the view.

JawAtlas is an education and training aid. It is not a medical device and must not be used for diagnosis, treatment planning or any clinical decision.

Last reviewed 2026-10-13