Point Sampled Intercept

This method uses a randomly-placed array of uniformly spaced lines and points to measure the linear intercept across each object of interest. The points are represented by vertices with shorter lines spaced uniformly along the longer lines.

A particle is sampled if one of these vertices falls within its profile.

The linear intercept is the distance from one boundary to another along the randomly placed line that intersects the object at a specific point (vertex) on the line. The intercepts can be located anywhere in the profile of the object and don't need to be at the center or at any identifying point in the particle. This length is used to estimate a volume of the sampled objects.

The Point Sampled Intercept requires an isotropic intercept between the sampling line and the particles being sampled. That is, the direction of the intercept through the sampling point must be isotropic in 3D space with respect to the particles. There are three ways of fulfilling this condition:

1. The particles themselves are isotropic in the tissue (rare, and can be difficult to prove).
2. The sections are isotropic.
3. The sections are vertical—all parallel to a single, recognizable vertical axis. The sections must be isotropic around this axis.

Therefore, unless the isotropy of the particles can be demonstrated, the Point Sampled Intercept requires thin sections that are isotropic or vertical.

1. Click Probes>All probes>Volume>Point Sampled Intercept.
2. Choose a PSI type and specify the spacing between the lines. Experiment to determine an appropriate line spacing so that approximately 50-100 intercepts are sampled per specimen.
   • If you have vertical sections, right-click and select the direction of the vertical axis.
   • Change the size and shape of the tick marks (the tick marks are essentially markers) with File>Preferences>Stereology Preferences>Colors and Tick Marks.
3. Click a point on a vertex that falls within the profile of a particle to be sampled. Once the vertex is selected, all other lines and vertices are temporarily hidden; only the line and vertex over the particle are visible.
4. Click to mark the intersections of the line and the particle boundaries. If the line intersects the particle more than once, mark all intersections. Tick marks are displayed at the selected intersections.

   To delete a tick mark, right-click over the tick mark and select Delete?.

5. Once all intercepts for that particle have been marked, right-click and select Finish Current PSI. All lines and vertices are visible again.

   The vertex you just marked appears with a tick mark over it to prevent you from selecting it again.

6. Repeat steps 3–5 for each particle. If more than one vertex falls within a given particle, both points should be marked.
7. To end the probe run, right-click and uncheck Exit PSI.
8. To view the results, use Probes>Stereology results>Probe run list.

See Point sampled intercept formulas