What is CZT, the amazing material that is generating a technological revolution (and why it is so difficult to obtain)

That’s what patients at London’s Royal Brompton Hospital had to do during certain lung scans, until the hospital installed a new device last year that reduced these exams to just 15 minutes.

This is partly due to the scanner’s image processing technology, but also due to a special material known as CZT (cadmium zinc telluride), which allows the machine to produce highly detailed three-dimensional images of patients’ lungs.

“You get beautiful images with this scanner,” says Dr. Kshama Wechalekar, head of nuclear medicine and PET (Positron Emission Tomography).

“It is a true feat of engineering and physics.”

The CZT on the machine, installed at the hospital in August, was manufactured by Kromek, a British company, and one of the few in the world that can make it.

You may have never heard of it, but – in Wechalekar’s words – it is causing a “revolution” in medical imaging.

The wonderful material also has many other uses, such as in X-ray telescopes, radiation detectors and airport security scanners.

And it is increasingly requested.

Research on patients’ lungs by Dr Wechalekar and her colleagues involves looking for the presence of many tiny blood clots in people with long Covid, or a larger clot known as a pulmonary embolism, for example.

The scanner, which costs £1 million (about $1.4 million), works by detecting gamma rays emitted by a radioactive substance injected into patients’ bodies.

But the sensitivity of the scanner means that less of this substance is needed than before.

“We can reduce the doses by approximately 30%,” says the doctor.

High demand, low supply

While CZT-based scanners are not new in general, large full-body scanners like this one are a relatively recent innovation.

CZT has been around for decades, but it is notoriously difficult to manufacture.

“It has taken a long time to develop it into an industrial-scale production process,” says Arnab Basu, founding CEO of Kromek.

At the company’s facility in Sedgefield, England, there are 170 small ovens in a room that Dr. Basu describes as “similar to a server farm.”

In these ovens, a special powder is heated, melted and then solidified, forming a monocrystalline structure.

The entire process takes weeks.

“Atom by atom, the crystals rearrange themselves […] until they are completely aligned,” explains Basu.

The newly formed CZT, a semiconductor, can detect tiny photon particles in X-rays and gamma rays with incredible precision, like 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 generate an image. Previous scanner technology used a two-step process, which was not as accurate.

“It’s digital,” says Basu.

“It’s a single conversion step. It retains all the important information, 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 for explosives detection at UK airports and for scanning checked luggage at some US airports.

“We expect CZT to enter the carry-on 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, USA, has previously used the material in space telescopes tethered to high-altitude balloons.

These detectors can capture X-rays emitted by both neutron stars and plasma around black holes.

Professor Krawczynski needs very thin pieces of CZT, 0.8mm, for his telescopes as this helps reduce the amount of background radiation they pick up, allowing for a clearer signal.

“We would like to buy 17 new detectors,” he says. “It’s really hard to get them thin.”

Kromek could not help him because, according to Basu, his company is currently in high demand.

“We support many research organizations,” he adds. “It’s very difficult for us to do a hundred different things. Each research project requires a very particular type of detector structure.”

For Krawczynski, it’s not a crisis: He says he could use CZT he has from previous research or cadmium telluride, an alternative, for his next mission.

However, there are more serious problems at the moment.

The next mission was due to leave Antarctica in December, but “all the dates are changing,” Krawczynski says, due to the US government shutdown in November.

Many other scientists use CZT.

In the UK, a major modernization 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 near the speed of light. The magnets cause these electrons, as they whiz by, to lose energy in the form of X-rays, and they are directed from the ring into lines of light to, for example, analyze materials.

Some recent experiments have involved the analysis of impurities in aluminum during its melting. Better understanding these impurities could help improve recycled forms of the metal.

With the Diamond Light Source upgrade, scheduled for completion in 2030, the X-rays produced will be significantly brighter, meaning existing sensors will not be able to detect them correctly.

“There’s no point in spending all this money upgrading these facilities if you can’t detect the light they produce,” says Matt Veale, leader of the detector development group at the Science and Technology Facilities Council, a stakeholder in Diamond Light Source.

Therefore, here too CZT is the material of choice.