PET is an abbreviation of Positron emission tomography and it provides a spatial image of where the cancer cells are located in the body. PET scans are harder to interpret if medical staff cannot situate the location of cancer cells in relation to the skeleton and soft tissue. This can be done by comparing PET images with an anatomical picture such as CT (computerised tomography) or MR (magnetic resonance) scans.
CT scans provide a three-dimensional x-ray image of the body. MR scans photograph the body using radio waves and a powerful magnetic field. MR provides far better images of soft tissue than CT does. The drawback of MR scans is that the examination is more expensive and takes much longer. The advantage is that MR does not emit ionising radiation.
Together with his three colleagues, Erlend Bolle has constructed a PET machine that is so small that it can be placed inside an MR machine. Both images can therefore be taken at the same time, and medical personnel do not have to correct the errors that occur when two images are combined after they have been taken.
In a standard PET examination, radioactive isotopes are attached to sugar molecules and injected into the body. The PET image is taken one hour later, when the sugar has been distributed to the entire body. Cancer cells burn sugar quicker than healthy cells. Radioactive gamma particles therefore accumulate in cancer cells. The gamma particles send out two sets of photons in opposite directions. This is called parallel photons.
In order to trace the radioactive source, the PET scanner must find which parallel photons are linked. This is one of the great challenges for current PET scanners. Only half of the photons deposit all their energy on first impact. On subsequent impacts, only some of the energy is deposited before the photons change direction and deposit the rest of the energy elsewhere. Current detectors have no depth information and therefore cannot reconstruct the positions of these photons.
The new detectors are made from entirely new crystals and light guides. In each of the five layers of the detectors, crystal pins are placed on top of a transverse layer of light guide fibres.
“Today, the scanners form a circle. This means that there is a gap between each detector block, and photons disappear through the gaps. Now, we have full coverage of crystals on all sides. We can capture several million particles a second. However, this does not happen at regular intervals. We measure each nanosecond. If we do not measure fast enough, we can get errors.”
COMPAMED.de; Source: University of Oslo