From Detecting Particles to Detecting Tumours

The CMS’s electromagnetic calorimeter uses scintillating crystal technology that is similar to the technology used in Positron Emission Tomography (PET) scans.

PET is a medical imaging device that uses antimatter to observe metabolic processes in the body, such as cancerous tumours. In this process, a radioactive “tracer” is injected into the body where it is absorbed by the targeted tissue. An isotope in the tracer will decay and produce antimatter particles called positrons, which will annihilate with an electron (its antimatter counterpart) in the body and release a pair of photons that fly in opposite directions. The ring-shaped PET detector surrounding the body detects the photons and uses their positions to reconstruct a 3D image of the target area. This image reveals information about the tissue by mapping the tracers concentrated there.

Credit: Imperial College London

The Crystal Clear Collaboration (CCC) was founded at CERN in 1990 to search for a suitable scintillator for use in the Large Hadron Collider’s electromagnetic calorimeter (ECAL). Among the various studied crystals, lead tungstate was found to be the most suitable and was then used by CMS and ALICE.

The CMS’s ECAL measures the energies of photons and electrons that are produced by high-energy collisions in the LHC. When electrons or high-energy photons pass through the ECAL, the scintillating crystals produce bursts of optical photons in proportion to the particles’ energy. These collisions in the LHC are like high-energy versions of the electron-positron collisions that occur in the body during a PET scan.

CMS ECAL crystals
Lead tungstate crystals used in the CMS ECAL. (Source: CMS/CERN)

Since 1996, the CCC has been working in the development of high-performance PET systems in addition to their generic research to better understand scintillator properties. One of these systems, ClearPET, is a small animal PET scanner that allows researchers to study the effects of radiopharmaceuticals on the brains of small animals. 

CMS ECAL crystal
CMS ECAL crystals next to matrix developed by CCC for a PET prototype.(Source: CMS/CERN)

In continuation of their PET research, the CCC launched the ClearPEM (Positron Emission Mammography) project in 2002 to develop a dedicated breast PET scanner. ClearPEM can detect cancer lesions as small as 1.3 millimetres, hopefully leading to earlier detection and subsequent lower breast cancer mortality. Its photodetector (the device that converts light from the scintillating crystals into electrical signals) is the same avalanche photodiode as the one developed for CMS’s ECAL! The ClearPEM system has gone through successful clinical trials in hospitals in Coimbra, Portugal, Marseille, France, and Monza, Italy.

More recently, some members of the CCC within an FP7 European project, EndoTOFPET-US (Endoscopic Time-Of-Flight PET & Ultrasound), developed a “time-of-flight” PET for the improved detection of pancreas and prostate cancer. The time-of-flight PET takes into account an additional measurement to more accurately triangulate the photons’ point of annihilation: the difference in photon arrival times to each side of the detector, or “time-of-flight”. This means it can achieve better-quality images in less time or reduce the dose of the radiopharmaceutical injected in the patient.

The development of faster detection being carried out by CCC would not only be valuable for medical imaging but also holds promise for future components of the CMS detector.


Find out more: