Cancer accounts for more than 20% of all deaths in Europe. PET, Positron Emission Tomography, scanners are commonly used to take 3D images of cancerous tumours. ‘When you have a main tumour somewhere in the body, what you are looking for typically is whether the tumour has spread to other sites in the body and those metatheses -spreading of the cancer from one organ to another- may be in the lymph nodes, in the lungs or in the brain,’ says Dennis Schaart, a researcher at Delft's University of Technology and participant in the SCISILIA project, backed by the pan-European platform public funding to applied research: EUREKA. The other project partners in this international research project were the University of Groningen, Amsterdam's free University’s Medical centre and Philips, including one of its German research branches which provided its expertise in simulations. SCISILIA made significant advances in PET technology improving the quality of the images to enable earlier diagnosis, improved quality of life for patients and a reduction in medical costs.
The new technology also makes it easy to monitor whether a patient is responding to a course of chemotherapy. Only a part of the patient population responds to most chemotherapies, but they still suffer the side effects. ‘When you rely only on anatomical information then you have to go through an entire course of chemotherapy and after 2-3 months nothing has worked,’ says Schaart. ‘With PET you can show in two, three weeks whether the patient is responding. Since oncology, the branch of medicine dealing with cancer, is essentially a fight against time this is very interesting for cancer patients.’
PET scanners are the result of research in a cutting-edge medical field called nuclear imaging. The purpose of a PET scan is generally to create a 3D image of one of our body functions, such as the spread of a cancer. A small amount of a radioactive tracer, targeted at a particular function, is injected into the body. ‘Tumorous cells need a lot of energy,’ explains Schaart. ‘You use a molecule which looks like glucose and the tumour cells think that it's glucose. Most of the cells of the body won't absorb much, so after a while all of these molecules have piled up inside the tumour cells from which the radiation is then emitted. If I can see where the radiation is coming from, I have an image of the tumour cells.’
When an isotope
emitting a very mild radiation coupled with the glucose-like molecule it becomes what is called a radiotracer. This element emits a positron: the electron’s anti-particle, when it decays. When a positron and one of the electrons present in our body meet they annihilate and are converted into energy in the form of two gamma photons emitted in opposite directions and detected on opposite sides of the patient. Draw a line between the points where the photons were detected and somewhere along that line there was an annihilation. Millions of these lines are used to create a 3D image of the positrons emitted inside the patient and therefore of the tumour from which they were emitted.
COMING TO YOUR CLINICS THIS YEAR
‘The essential part of any PET scanner is the detectors you use to detect the gamma photons,’ says Schaart. ‘Those are the eyes of the scanner.’ A scintillator, which is a form of crystal, absorbs the high-energy gamma photos and converts them to lower energy light photons. A light sensor then detects those tiny flashes of light in the crystal. ‘The EUREKA
project was targeted towards time of flight
: in the imaging process we measure when each of the gamma rays was emitted. That precision needs to be less than one nanosecond or 10-9 seconds.’ Accurate ‘time of flight’ translates directly into better images and earlier detection of metastasis.
The SCISILIA project used two techniques to improve the timing technique: a new scintillator material developed at the University of Delft
and a light sensing technology called SiPM, or silicon photomultiplier. ‘We had a somewhat spectacular result,’ says Schaart. ‘The systems that you buy today have a timing resolution in the order 500 picoseconds but using the combination of a new scintillator material we have created, the SiPM plus some clever signal processing, we were able to pass the 100 picosecond barrier. That was really a breakthrough.’
SCISILIA looked into the fundamentals of using SiPMs in PET, but a follow on project is now developing more practical solutions which can be scaled up to medical systems. Schaart expects to see SiPM-based PET scanners to be on the market very soon this year.