Image Viewer > Advanced Viewing

Image Fusion is the combination of multiple images to be displayed as a single image. Currently, PET-CT fusion is the only supported mode of operation (with the PET images being recoloured and overlaid on top of the CT images).

Fusion occurs automatically upon opening a study with valid combinations of overlaying and underlying image series. For example, upon opening a single-patient study with a single CT series and two PET series, there will be two additional Fusion display sets generated and available for selection on the context menu.

There are some restrictions on the kinds of source images upon which Image Fusion will happen:

•	There must be at least five overlaying images and five underlying images.

•	All the overlaying and underlying images must come from the same study.

•	All the overlaying images must be in the same frame of reference.

•	All the underlying images must be in the same frame of reference. They do not have to be in the same frame of reference as the overlaying images.

•	All the overlaying images must have the same image orientation (relative to patient coordinates).

•	All the underlying images must have the same image orientation (relative to patient coordinates). They do not have to be in the same orientation as the overlaying images.

•	The spacing between consecutive overlaying images must be consistent.

•	The overlaying images may not have a horizontal gantry tilt 1.

If a particular combination of image series do not meet the above requirements, then the corresponding Fusion display set will not appear on the list of display sets.

Adjusting the Window

You can manually adjust the window/level parameters of the fused images independently by adjusting the window/level on the corresponding source, unfused display set. The Fusion display set is synchronized to always use the same window/level settings as the source images. If the source images are not currently visible, add an image box or two by changing the layout and selecting the source display set.

Adjusting the Opacity

You can adjust the opacity of the overlaying image by selecting the Adjust Opacity Tool. This tool is similar in operation to the Window/Level Tool, except that it adjusts the transparency of the overlaying image, allowing you to change the degree to which the images are fused, or even to switch completely to one image or the other.

PET and CT Image Co-registration

Image co-registration (i.e. the alignment of one image relative to the other) is performed automatically using the patient coordinate space specified in the image headers. No manual registration is necessary.

Although PET-CT setups are typically calibrated to produce images using the same patient coordinate space, there are some configurations that do not write the image headers to indicate this fact (specifically, the Frame of Reference UID (0020,0052) attributes are not given identical/correlated values). In this case, if all other requirements are satisfied, image fusion is still performed but a yellow warning label will appear on the text overlay to indicate that the images' frames of reference do not strictly match.

Limitations

There are a few limitations to the current implementation of Image Fusion, and we hope to eliminate these in a future version.

•	Currently does not support horizontally gantry tilted overlaying images.

1 As the overlaying images are reformatted using MPR in order to achieve best co-registration on the underlying images, the same horizontal gantry tilt restriction for MPR also applies for the overlaying images.