20. Coregister with DICOM Tags

20.1. About Coregistration

FireVoxel offers powerful tools for 3D and 4D image coregistration. These tools enable superimposing images of different dimensions, orientations, and even different imaging modalities (e.g., CT and MRI).

In the description below, the stationary image (that remains unchanged during coregistration) will be called the target image.

The source image is the image that is transformed (translated, rotated, scaled, etc.) to match the target image.

20.2. [Coregister] Using Orientation\Position tags

This coregistration command uses DICOM orientation and position attributes and matrix algebra to transform the source image and align it with the target. This command is the fastest and simplest method that requires no adjustable parameters and is suitable for coregistering 3D images acquired during the same exam. The source and target images may have different matrix, field of view, and orientation. The patient is assumed to have maintained the same position between the two acquisitions.

Requires two 3D images in DICOM format. The source document may also contain segmentation masks in ROI layers, which will be coregistered together with the images. This method works primarily for MRI and CT images in DICOM format and also for some NIfTI images that unambiguously record the image orientation in the image header. This method does not work for simpler image formats, such as ANALYZE.

To use, first, make sure that the layer with the source image is visible and active, especially if the source document window contains multiple layers, such as ROI layers. Next, activate the target window. Then select Register, Coregister using Orientation\Position tags.

A dialog window (Registered Volume) will appear prompting the user to select an option for adding the coregistered volume:

  • Add as a new layer – Coregistered image will be added to the target document window and placed on top of all existing layers.

  • Create new Document – Coregistered image will be placed in a new document window, or

  • Cancel the operation.

A new layer containing a coregistered image will be created (depending on the user’s choice, either in the target document window or in a new document window). Use Layer Control to view the layers. The coregistered image will have the same matrix dimensions and resolution as the target image. The coregistered layer will be named after the source image with a suffix added (_reg). Any ROI layers present in the source window (and containing the same DICOM tags as the source image) will also be coregistered together with the source image.

The user may check the accuracy of the coregistration by reducing the opacity of the new layer (using the Alpha slider on the Layer Control panel). If the coregistration is not inadequate, the user may need to use more advanced coregistration commands, such as coregistration with AutoFocus described below.