The PFUS tool regards one image series as the Reference. All other series are adjusted in pixel size and slice thickness to the reference. This is done by interpolating their image information within oblique planes across the image volume, a process called reslicing. The images to be adjusted are called the Input studies in this text. As a result of reslicing, the Reference and the Inputs have identical resolution, and fusion images can be easily generated by combining the pixel values in the different series. For instance, the pixel colors (RGB values) can be added (blending mode), or only one of the image values can be shown depending on a threshold value (overlay mode). Furthermore, VOIs can directly be exchanged between all images, and arithmetical operations applied between images.
If the anatomic structures in the Reference and the Input series are already in spatial agreement, only a mere resolution adjustment of the Input series is required, for example to interpolate a 128x128 PET to the 512x512 matrix of the CT (Reference) in a hybrid PET/CT study. In other situations, however, an adjustment of the coordinate system is also required to bring the anatomy in both studies into agreement. For example, in most cases it is required to rotate and shift the images of a brain PET study to match them with the images of an independent MRI study, and interpolate them to the MRI matrix size.