Validation of the CT-MRI image registration with a dedicated phantom

The present study was aimed at verifying the automatic registration of the Focal (Elekta) platform with a dedicated phantom.

A phantom that simulates the pelvis region in a stylised way and finalised to the registration of computed tomography-magnetic resonance images was designed and realised. After acquiring the two sets of images, the registration was performed both in automatic and manual mode to verify whether they were comparable. To test the repeatability of the automatic registration, some known rigid transformations were imposed to the original images. If the registration method works correctly, parameters which bring the images into alignment must always be the same.

Automatic registration performed by the software did not prove satisfactory, whereas if a specific tool [volume of interest (VOI) tool] allowing the calculation to be limited to the landmark region was used, the registration parameters were comparable with those of the manual registration. Regarding the repeatability of the automatic registration, the software brought the images in the correct alignment performing translations and rotations along the longitudinal axis up to 40°, while it was not satisfactory for rotations along the transverse axes.

The experimental results showed that in clinical application automatic registration is reliable if the VOI tool that includes visible landmarks in both studies is used. However, because the algorithm did not prove sensitive to rotations along the transverse axes, the position of the patient during the examinations plays a crucial role.