The current version is a stacked system, consisting of the PET detector, crystals for gamma ray reading and integrated electronics, all caged inside an 8 cm long, 4 cm high box. The boxes can be scaled to increase power.

"A preclinical system uses 12 modules, and to go for a complete human system you need more for a ring," says Schaeffter.


Motion correction

One challenge of PET technology is photon attenuation, the intensity loss of gamma rays from the radioactive tracer (injected in the patient) while passing through tissue. The gamma rays get absorbed by different parts of the body at different rates, and this has to be corrected for when developing the image.

"You have a radioactive source inside the body, producing gamma rays with high energy," Schaeffter says. "Some of these gamma rays hitting on bones will be attenuated. So we have to put into the reconstruction process the different deterioration from the different organs. The lung has less deterioration than the bone, for instance," he adds.

Now, PET attenuation is often corrected for by using a CT scanner, which can compensate for the attenuation by helping to predict how much has occurred given the known attenuation rates of nearby tissue, and therefore the PET-CT modality has become a popular one. However, there is one drawback: PET is slow, taking anywhere from ten minutes to half an hour to acquire an image. While the patient lies under the scanner, the tumor can shift around from patient breathing, or from natural organ motion that happens in the body from the passage of gas or fluids. This movement can result in a blurred image. In addition, the tumor may move between areas with different PET attenuation that cannot readily be distinguished. And the CT scan, because it delivers an ionizing radiation dose, cannot be delivered continuously to provide completely accurate localization to correct for movement artifacts, according to Schaeffter.

But the team believes with MR, they can correct for both motion and attenuation at the same time: using time-of-flight PET combined with data reconstruction software running with the MR scan, they can, the idea goes, continuously monitor the patient during the time it takes for PET to work.

"We're able to derive motion-compensated attenuation corrections," says Schaeffter. "That means while we're doing a PET scan, we're also doing MR scanning, and then we can derive motion from MR scan, and we can use motion for two purposes: one to do proper attenuation correction, and two, to do motion-compensated PET reconstruction."

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