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Smart photochip for cheaper PET-scans

PET-scans can detect early tumours. A new detector module, co-developed by Dr Veerappan, makes PET-scanners cheaper, better and faster. Future patients will benefit, said Professor Edoardo Charbon.

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Patients with suspected tumours receive an injection of a substance that cancer cells imbibe. The simplest option is glucose for these ever-hungry cells. The injection carries an unstable isotope as a marker. It will emit a positron when it decays. Apositron doesn’t travel far in a wet environment like the human body. It will soon encounter an electron and disappear in a flash, creating two powerful gamma photonsin the process that fly away in opposite directions.

Positron cameras like PET (Positron Emission Tomography) distinguish annihilating positrons from random gamma flashes by ignoring all signals unless two opposite rays are detected simultaneously. Such flashes occur when a gamma photon hits a scintillation crystal in which it generates a shower of light photons. Until recently old fashioned electron tubes (photomultiplying tubes or PMTs) were the only option to detect these faint flashes of light.

“The first solid-state detectors compatible with PMTs, known as silicon photomultipliers or SiPMs, appeared around the year 2000”, recalled Professor Edoardo Charbon, chair of the Very Large Scale Integration group at the Faculty of Electrical Engineering, Mathematics and Informatics. Not only did solid-state detectors make PET-scanners lighter, smaller and cheaper, but for the first time, it became possible to combine PET imaging with an MRI-scanner, thus showing the tumour (with PET) plus the surrounding anatomy (with MRI).

The underlying technology to SiPMs is known as SPADs (Single-Photon Avalanche Diodes). These detectors can be cheaply made, said Charbon, because they are made with the same CMOS technology that makes smartphone cameras so cheap. Until now, however, SPADs also had some serious drawbacks. They were not very sensitive and generated much noise.

Dr Chockalingam Veerappan, PhD student with Professor Charbon, has “killed three birds with one stone.” Charbon counts on his fingers while summing up the achievements. Veerappan increased the SPADs sensitivity; he reduced the noise and designed and built a network into which modular detector units can be plugged. These modules contain an array of sensors on top of a processing unit and network card.

The network, or bus, is a very elegant way of detecting simultaneous gamma flashes. As soon as a sensor module ‘sees’ a gamma burst, it puts a ‘time stamp’ on it and places the data package onto the network. Other modules compare the time stamp with their observations. When two events coincide and the sensors are opposite each other, it must be a positron detection.

Thanks to the ultrafast electronics (a peak typically takes a few millionths of a millionth of a second) the place of origin can be located within a few millimetres.

Charbon expects the new SPAD technology to find useful applications in the Holland Particle Therapy Centre where complex cancers will be treated with proton beams. “Since it’s a novel therapy, researchers and oncologists will want to trace the effects of the treatment as accurately as possible. Our sensors can bring PET-scanning to the next level.”

As said, solid-state detectors in PET-scanners are not new. Nonetheless, Veerappan has significantly improved the technology regarding sensitivity, noise, modularity and localisation. Charbon trusts this technology will make PET-scanners cheaper and thus more generally accessible. Furthermore, the modularity of the system allows for simultaneous registration by multiple detector rings, or easy scaling up or down depending on if you want to scan a man or a mouse.

 Check the spadnet.eu website for more info

 Chockalingam Veerappan, Single-Photon Avalanche Diodes for Cancer Diagnosis, PhD supervisor Prof. Edoardo Charbon (EWI), March 24, 2016.

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