Contouring and Segmentation

PCARDP supports different segmentation models. Currently, the standard AHA 17-segment model is available, as well as the ASNC 20-segment model which is often used with SPECT data.

For using a particular segment model the corresponding tab should be selected. During the analysis one time-activity curve per segment is calculated by averaging the signals from all voxels belonging to the segment. It is then fit to a kinetic model, which results in one set of parameters. The result parameters from all segments are finally displayed as polar plots corresponding the segmentation scheme, and used for generating comprehensive reports.

CAUTION: A single segmentation must be employed in order to create a normal database, and when comparing patient data against such a database.

There are two options available for the myocardium detection procedure:

Myocardium centerline: the myocardium centerline detection is done by fitting an analytical LV model to the MYOCARDIUM images and then adjust it locally according to the tracer uptake pattern. The resulting outlines are shown as contour lines in the SA image. Note that this procedure should work reasonably with images showing most parts of the myocardium, but may result in a distorted shape in the case of severe defects. Generally, it is recommended to try improving the automatic result interactively, rather than outlining the center lines in a fully manual fashion.
EPI/ENDO outline: the myocardium center line is used to generate Epi/Endo contours. Initially, a normal vector is calculated for each vertex of the myocardium center line. Finally, the point belonging to the myocardium center line is translated along the normal vector by half of the heart wall thickness.

The advantage of the EPI/ENDO definition is that the sampling boundary is very clear and that both contours can individually be adjusted.

Polar Sampling

The Polar Sampling selection defines how the voxels used in the segment TAC calculation are located. PCARDP interpolates the axial range defined by the myocardium model into 22 slices. For each of these slices a radial sampling is performed every 10°. There are different ways how the myocardium samples are detected:

METHOD	MYOCARDIUM CENTERLINE	EPI/ENDO OUTLINE
Radial Maximum	Using the maximum value on the radial profile. The Wall thickness determines the range from the model within which the maximum is determined.	The maximum value is determined along the radial profile between the Epi and Endo profiles.
Model Crossing	Using the value at the intersection of the radial profile with the myocardial model.	Using the value in the middle between the Epi and Endo shapes.
Averaged on Model Crossing	Using a neighborhood of 4 voxels at the intersection of the radial profile with the myocardial model.	Using a neighborhood of 4 voxels in the middle between the Epi and Endo shapes.
Averaged on Radial Maximum	Using a neighborhood of 4 voxels around the determined profile maximum.	Using a neighborhood of 4 voxels around the determined profile maximum between the Endo and Epi shapes.

Note: the exact sampling points found by a prescribed sampling scheme can be visualized as spheres in a 3D rendering scene (see Examining the Results)

As a means to get a robust global result PCARDP allows averaging the TAC signals from all segments into a single TAC TOTAL MYOCARD. This TAC is only generated, if the box Calculate TAC of total myocardium is checked.

The Polar plot interpolation choice defines how the information calculated in the segments is presented in the polar plots. If it is set to NO, the segment structure is clearly visible. If set to 48x48, values at 48 radial distances and 48 angular increments are interpolated, and similarly with the or 60x60 setting. The effect is illustrated in the example below. While the raw polar plot represents the true numbers, the values are smeared by the interpolation filtering.

PCARD Polar Plot Interpolation