image source, Kromek
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- Author, Chris Baraniuk
- Author title, BBC, Technology
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Reading time: 6 minutes
Lying as still as possible on your back in a large hospital scanner with your arms above your head for 45 minutes isn’t much fun..
That’s what patients at London’s Royal Brompton Hospital had to do for certain lung scans until the hospital installed a new machine last year that shortened these scans to just 15 minutes.
This is due in part to the scanner’s image processing technology, but also to a special material called CZT (cadmium zinc telluride), which allows the device to create very detailed three-dimensional images of patients’ lungs.
“With this scanner you get beautiful images,” says Dr. Kshama Wechalekar, head of nuclear medicine and PET (positron emission tomography).
“It is a real technical and physical masterpiece.”
The CZT of the machine installed at the hospital in August was made by Kromek, a British company and one of the few in the world that can produce it.
You may have never heard of it, but – in Wechalekar’s words – it is sparking a “revolution” in medical imaging.
The wonderful material also has many other uses, for example in X-ray telescopes, radiation detectors and security scanners at airports.
And it is increasingly in demand.
image source, Guy’s and St Thomas’ NHS Foundation Trust
During research on the lungs of patients by Dr. Wechalekar and her colleagues are looking for the presence of many tiny blood clots in people with long Covid, or for a larger blood clot known as a pulmonary embolism, for example.
The scanner, which costs a million pounds (about $1.4 million), detects gamma rays emitted from a radioactive substance injected into patients’ bodies.
However, due to the sensitivity of the scanner, less of this substance is needed than before.
“We can reduce the doses by about 30%,” says the doctor.
High demand, low supply
While CZT-based scanners in general are not new, large full-body scanners like this one are a relatively new innovation.
CZT has been around for decades, but is notoriously difficult to produce.
“It took a long time to develop it into an industrial-scale production process,” says Arnab Basu, founding CEO of Kromek.
At the company’s factory in Sedgefield, England, there are 170 small ovens in a room designed by Dr. Basu describes it as “similar to a server farm”.
In these ovens, a special powder is heated, melted and then solidified to form a monocrystalline structure.
The entire process takes weeks.
“The crystals are rearranged atom by atom (…) until they are completely aligned,” explains Basu.
The newly developed CZT, a semiconductor, can detect tiny photon particles in X-rays and gamma rays with incredible precision, similar to a highly specialized version of the light-sensitive silicon-based image sensor found in your smartphone camera.
Each time a high-energy photon hits the CZT, it mobilizes an electron and this electrical signal can be used to create an image. Previous scanner technologies used a two-step process that was not as accurate.
“It’s digital,” says Basu.
“It’s a single conversion step. All important information is retained, such as the timing and energy of the X-rays hitting the CZT detector; color or spectroscopic images can be created.”
He adds that CZT-based scanners are currently used to detect explosives at UK airports and to scan checked baggage at some US airports.
“We expect CZT to enter the carry-on luggage segment in the coming years.”
The chosen material
But it is not always easy to get CZT.
Henric Krawczynski of Washington University in St. Louis, US, has previously used the material in space telescopes attached to high-altitude balloons.
These detectors can detect X-rays emitted by both neutron stars and the plasma around black holes.
image source, Kromek
Professor Krawczynski requires very thin pieces of CZT (0.8mm) for his telescopes as this helps reduce the amount of background radiation they pick up and allows for a clearer signal.
“We want to buy 17 new detectors,” he says. “It’s really hard to make them thin.”
Kromek couldn’t help him because, according to Basu, his company is currently in high demand.
“We support a lot of research organizations,” he adds. “It’s very difficult for us to do a hundred different things. Every research project requires a very specific type of detector structure.”
For Krawczynski, this is not a crisis: He says he could use CZT from previous research or cadmium telluride as an alternative for his next mission.
However, there are currently more serious problems.
The next mission was scheduled to leave Antarctica in December, but “all the dates are changing,” Krawczynski says, due to the U.S. government shutdown in November.
Many other scientists use CZT.
In the UK, a major upgrade of the Diamond Light Source research center in Oxfordshire will improve its capabilities through the installation of CZT-based detectors.
Diamond Light Source is a synchrotron that shoots electrons around a giant ring at nearly the speed of light. Through the magnets, these flying electrons lose energy in the form of X-rays and are directed by the ring into lines of light, for example to analyze materials.
Some recent experiments have involved analyzing impurities in aluminum as it melts. A better understanding of these impurities could help improve recycled forms of the metal.
With the upgrade of the Diamond Light Source, scheduled for completion in 2030, the X-rays produced will be significantly brighter, meaning existing sensors will not be able to detect them properly.
“There’s no point in spending all this money upgrading these facilities if you can’t detect the light they produce,” says Matt Veale, head of the detector development group at the Science and Technology Facilities Council, a Diamond Light Source stakeholder.
Therefore, CZT is the material of choice here too.
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