Quantum PET Scanners

Positron Emission Tomography (PET) scanners use radioactive chemicals (isotopes) to help us see how the body functions, for example monitoring brain activity or finding cancerous tumours.

PET isotopes are attached to biologically active molecules (e.g. sugars) that naturally travel to specific parts of the body, such as the brain, heart, or cancer cells. 

The radioactive chemical decays and gives out a positron (a positively charged particle), which annihilates with an electron in the body. This gives out a pair of gamma-ray photons, which travel in opposite directions. These are picked up by a ring of detectors around the patient. To create a PET image, straight lines are drawn between the detectors that receive the gamma rays.

The video below, produced by Australia's Nuclear Science and Technology Organisation (ANSTO), explains how a PET scan works. 

In our lab, we use two detectors like the ones found in hospital PET scanners. This means that our research is directly translatable into hospital settings, without the need for new technology.  

The small green stand in the middle holds a radioactive material called Sodium-22. Although different to the radioactive isotopes used to create PET scans, it also gives off positrons, which annihilate with electrons to create the gamma rays we are studying.

Each detector in our setup has a fan on the back to keep the electronics and detectors at a stable temperature and prevent overheating, which helps them work more accurately and reliably. The detectors are also carefully mounted so we can measure their positions with high precision.

Inside the detectors:

When the entangled gamma-ray photons enter the detector crystal, they can undergo a process called Compton scattering. This means that the photon hits an electron, gives it some of its energy, and changes direction. The more energy the photon transfers, the bigger the angle at which it scatters.

The electron that absorbs the energy leaves a trail or signal inside the crystal. This signal is picked up by very sensitive sensors called SiPMs (silicon photomultipliers), which convert it into an electrical signal we can measure.

When a photon hits one of the detector crystals, it Compton scatters and we record its location. The photon travels through the detector until its remaining energy is deposited in a second 'hit'.  Altogether, we get four hits across the two detectors - two from each back-to-back photon. We can represent the hits as bright spots on these screen displays (above).

By measuring the difference in scattering angles of the two back-to-back photons, we can witness their entanglement. Entanglement means any effect on one of the photons is immediately felt on the the other, so when one photos scatters, this influences the angle the other scatters in.

If we plot all of the hits we receive from the back-to-back photons, our detectors show images like these:

You can see that there are many hits scattered across the screens. You'll notice that the detector screens are mirror images of each other.  This happens because the photons are emitted back-to-back.

The distribution of hits from the scattered photons is show below. After the first interaction, the photons can scatter in any direction. This makes it difficult to spot any clear patterns by eye.

In order to see the pattern that is characteristic of the quantum entangled gamma-ray photons, we can look at particular hits.  If we only look at hits in the centre of detector 1 that scatter in a particular direction, we can look the the pattern formed on detector 2.  

For quantum entangled gamma-ray photons, the bright spots, where the photons are most likely to scatter, will always have 90° between detector 1 and detector 2. In contrast, if the gamma-ray photons were not quantum entangled, we would see an even colour distribution around the ring.  

Random Coincidence

Sometimes two photons that originated from two different annihilation events are detected at the same time, falsely appearing as if they came from the same event. This creates an incorrect line of response between the detectors, which creates noise in the image.  

By using only the photons most likely to be entangled, we can create clearer images. 

This images below shows a circular 'phantom' - a test object used in PET imaging to evaluate how accurately the scanner produces images. This phantom contains five rods, each filled with a radioactive source. Both images were taken using the same scanner and the same phantom. However, the difference lies in the data used. On the left, we used data that is least likely to be entangled (with an angle difference (Δɸ) around 0°). On the right, we used data most likely to be entangled (with Δɸ around 90°). The right-hand image is noticeably clearer and shows less noise. 

Photons scattering before the detector

Sometimes the photons scatter before they reach the detector, for example off the skull of the patient. This also leads to incorrect lines of response and blurry images. It was previously assumed that any scattering, either in the patient, the table, or in the scanner itself, would lead to the quantum entanglement being broken. However, recent research at the University of York has shown that the quantum entanglement is preserved for photons scattered in the forward direction at angles of 0° - 60°.

An experiment was carried out in which one small extra crystal was placed between the source and one of the detectors. This enabled the scattering location to be controlled, prior to reaching the detector. The energy deposited in the scatterer crystal, as well as within the detector, could also be measured.

When the distribution of hits was measured, it showed the characteristic pattern that indicated that the entanglement of the gamma-ray photons still existed, and is therefore relatively robust. 

In current PET imaging, CT scans are used to identify probable scattering locations, such as bone, and this is used to filter the PET data and remove scattered events. We want to use the information from the quantum entanglement (which we now know is preserved) of the scattered photons to recycle this data. By training AI with the quantum information, we hope it will be able to predict the scatter locations without the need for CT scans, and build anatomical maps of the patient from PET data alone. This would reduce the time, cost and radiation dose to the patient.