Display of positron emission tomography with Cadplan

Recent clinical experience at Peter MacCallum Cancer Institute (PMCI) with the use of unregistered Positron Emission Tomography (PET) images for radiotherapy target marking in the lung suggests that co-registered PET images would be invaluable. PMCI has three radiotherapy treatment planning systems but none of them currently is able to display or co-register PET images with Computed Tomography (CT) images. This paper details the approach taken to display co-registered PET images with the CADPLAN treatment planning system. CT Image files are normally transferred to Cadplan by DICOM transfer, but the Cadplan DICOM server will not receive (has no presentation context for) PET images. The fundamental design of the CADPLAN system envisages display of only a single image dataset, which must be a CT scan for planning reasons. The problem of data transfer is crudely solved by File Transfer Protocol (FTP) over the network. Fortunately the multislice format of the PET image files makes individual transfer manageable. A menu based C program running at the same time as Cadplan is invoked to sample the DICOM PET Image and create multiple Cadplan CART image format files that are co-registered with each existing transverse CT slice. With the Cadplan in contour mode, the program allows the co-registered PET images to be swapped in and out of the image section of the CART files promptly, while keeping the contour information. This allows radiotherapy target volumes to be marked using transverse PET emission images, and effectively circumvents the design constraints prohibiting the display of more than one image set. Contours can be over-laid for review on reconstructed sagittal or coronal views of CT or PET images constructed using the standard Cadplan tools. Co-registration is facilitated by identical positioning with the aid of lasers and FDG loaded fiducial markers on the PET scanner and CT couch. A polyurethane cast fixed with EFFILOCK is used to ensure identical patient orientation on the CT and PET couches. Since both imaging modalities are without significant geometric distortion the co-registration is then simply a translation. PET transmission images can be used for co-registration verification. The practical implementation of display of PET images with CADPLAN has enabled us to begin a trial of 10 patients, the results of which will be reported separately.