Misregistration

Because the resulting CT images and PET images will be fused, it is imperative that there be as little patient movement between the time of the CT scan and the PET scan, minimizing malalignment (“misregistration”) of the resulting images.  This is especially difficult because the PET portion of the exam often requires the patient to remain still for up to 30 minutes.

While the hybrid nature of the machine and its shared patient bed certainly help to limit this malalignment of images, subtle unintended patient repositioning during the exam is a frequent occurrence.

Additionally, even when the patient remains perfectly still, bowel peristalsis over the 30-minute exam and respiratory motion regularly cause misregistration of the CT and PET images.

Respiratory motion is a particular imaging problem because the patient must breathe during the long acquisition time required for the PET imaging of the chest. If a normal full inspiratory breath-hold technique is utilized for the CT portion of the exam, marked misregistration will occur.

Breathing Techniques: To minimize lung misregistration, either shallow tidal breathing or a shallow end-expiratory breath hold during the CT exam is recommended.