Introduction to PET-CT multi-modality imaging

From the physicians’ point of view it is highly beneficial that these two imaging techniques appear in a single device. The patients’ body parts cannot move between the taking of two images because they do not change their position between the two examinations, since they lie in the same bed.

Figure 1. PET/CT scanner (from Siemens home page)

 
As far as the detectability of the contrast agents is concerned, the sensitivity of the PET is the highest of all known medical imaging devices. The table below contains the sensitivity of the different imaging techniques depending on the concentration of the contrast agents.

So it can be seen that PET is not particularly good as far as spatial resolution is concerned, but its sensitivity is exceptional.

As mentioned previously, combining the images is made very easy by the presence of two modalities in the device, but there are further advantages to that, which need to be pointed out. With the aid of the CT image the reconstruction of the PET image can be performed much more precisely, since the CT image is practically a photon attenuation map. This can be integrated in the reconstruction process of the PET image, this way the reconstruction will be more accurate. However, it is important to mention that the CT uses X-ray photons generated by electrons accelerated through 140 kV, while in PET 511-keV photons are detected. That is why the coded attenuation coefficient map in the CT image needs to be scaled so that it will be appropriate for the 511-keV photons.

Figure 2. Function of CT attenuation

 
In the figure below a PET image obtained after corrections is depicted on the left, the middle part of the figure shows a PET image without corrections, while the CT image, based on which the corrections were performed, is shown on the right. It is obvious that there are several structures that are not visible in the uncorrected image, which appear in the corrected one.

Figure 3. CT-AC Corrected-non Corrected PET acquisiton, as well as the fundament CT image of the correction (image is coming from Scanomed Ltd.)