4.4. Image registration

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Functionality is available in the Pro edition
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This function cannot be used for establishing a diagnosis

This tools allows you to deform the images so that their coincidence is maximal. If the inconsistencies of the series are too large (for example, the patient turned), you must ﬁrst merge the layers manually or automatically (see Section 4.3) and then register the images.

If a study contains images with a varying density over time (for example, due to the introduction of contrast), then registration may not be performed correctly. To do this, exclude the tissue with varying density.

Set the maximum value of the density for which registration is performed. In this case, the tissues with a density higher than the speciﬁed value do not participate in the registration, and their deformation is performed according to another algorithm.
Check the Use image gradient in the Series registration settings dialog. This mode requires a large amount of video card memory. If the registration failed due to a lack of video card memory of the video card, turn oﬀ this mode or select a CPU(software) render device (see Section 15.6.3).

If the images contain air outside the patient’s body, then the density of such areas for CT examinations is usually less than or equal to -1024. It makes no sense to register these sections of images. Hence, for CT studies, the default minimum density value is -1023.

If the registration failed due to a lack of video card memory, select a CPU(Software) render device (see Section 15.6.3). In this case, the registration takes more time, but it is performed regardless of the amount of GPU memory.

4.4.1 Series registration

To register two series:

Create a fused series of at least two series.

Open the fused series in the Multiplanar Reconstruction window.

If the inconsistencies between the series are too large (for example, the patient turned over), you must ﬁrst merge the layers manually or automatically (see Section 4.3). More accurate merging of layers enables faster registration.
Open the segmentation panel by clicking the Segmented structure panel button.
Select a base layer on the layers panel (ﬁxed layer). Its images will not be changed.
Click the Volumes registration button. The dialog shown in Fig. 4.10 appears. If more than two series are fused, then the list from which you must select one series for registration (Choose moving seris layer).

Figure 4.10: Series registration settings dialog

The Output series description ﬁeld will be automatically ﬁlled in based on the name of the series being registered, and it will contain the "registered" mark. If necessary, change the description.

Select the desired registration options, checking the Aﬃne registration, Non rigid registration and Use image gradient boxes.

Set the minimum size of the parts that will aﬀect registration. The default size is 20 voxels. The smaller the value is, the more minor details are registered, but the risk of incorrect registration increases.

Set the percentage of voxels that are used during the registration. The larger the value is, the higher the accuracy is, but the lower the registration rate gets.

If it is necessary to perform registration for a speciﬁc range of densities, then set the minimum and/or maximum values. To do this, set the appropriate ﬂag and enter a numeric value. If the ﬂag is set, but the value is not set, then the OK button is unavailable. For CT series, the minimum default value is -1023.

Click OK to export the series or CANCEL to cancel. Registration may take a long time depending on the amount of RAM and the speed of the computer.

4.4.2 Phases registration

To register the phases of one series:

Open the multiphase series in the Multiplanar or Volume Reconstruction window.

Open the segmentation panel by clicking the Segmented structure panel button.

Click the Volumes registration button. The dialog shown in Fig. 4.11 appears.
The Output series description ﬁeld will be automatically ﬁlled in based on the name of the series being registered, and it will contain the "registered" mark. If necessary, change the description.
Select the base phase that should not be changed (ﬁxed phase).
Select the desired registration options, checking the Aﬃne registration, Non rigid registration and Use image gradient boxes.
Set the minimum size of the parts that will aﬀect registration. The default size is 20 voxels. The smaller the value the more minor details are registered, but the risk of incorrect registration increases.
Set the percentage of voxels that are used during the registration. The larger the value is, the higher the accuracy is, but the lower the registration rate gets.
If it is necessary to perform registration for a speciﬁc range of densities, then set the minimum and/or maximum values. To do this, set the appropriate ﬂag and enter a numeric value. If the ﬂag is set, but the value is not set, then the OK button is unavailable. For CT series, the minimum default value is -1023.
Click OK to export the series or CANCEL to cancel. Registration may take a long time depending on the amount of RAM and the speed of the computer.

Figure 4.11: Phases registration settings dialog