‘It’s amazing’ – the wonder material very few can make

Lying on your back in a big hospital scanner, as still as you can, with your arms above your head – for 45 minutes. It doesn’t sound much fun.

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

It is partly thanks to image processing technology in the scanner but also a special material called cadmium zinc telluride (CZT), which allows the machine to produce highly detailed, 3D images of patients’ lungs.

“You get beautiful pictures from this scanner,” says Dr Kshama Wechalekar, head of nuclear medicine and PET. “It’s an amazing feat of engineering and physics.”

The CZT in the machine, which was installed at the hospital last August, was made by Kromek – a British company. Kromek is one of just a few firms in the world that can make CZT. You may never have heard of the stuff but, in Dr Wechalekar’s words, it is enabling a “revolution” in medical imaging.

This wonder material has many other uses, such as in X-ray telescopes, radiation detectors and airport security scanners. And it is increasingly sought-after.

Investigations of patients’ lungs performed by Dr Wechalekar and her colleagues involve 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 £1m scanner works by detecting gamma rays emitted by a radioactive substance that is injected into patients’ bodies.

But the scanner’s sensitivity means less of this substance is needed than before: “We can reduce doses about 30%,” says Dr Wechalekar. While CZT-based scanners are not new in general, large, whole-body scanners such as this one are a relatively recent innovation.

CZT itself has been around for decades but it is notoriously difficult to manufacture. “It has taken a long time for it to develop into an industrial-scale production process,” says Arnab Basu, founding chief executive of Kromek.

In the company’s facility at Sedgefield, there are 170 small furnaces in a room that Dr Basu describes as looking “like a server farm”.

A special powder is heated up in these furnaces, turned molten, and then solidified into a single-crystal structure. The whole process takes weeks. “Atom by atom, the crystals are rearranged […] so they become all aligned,” says Dr Basu.

The newly formed CZT, a semiconductor, can detect tiny photon particles in X-rays and gamma rays with incredible precision – like a highly specialised version of the light-sensing, silicon-based image sensor in your smartphone camera.

Whenever a high energy photon strikes the CZT, it mobilises an electron and this electrical signal can be used to make an image. Earlier scanner technology used a two-step process, which was not as precise.

“It’s digital,” says Dr Basu. “It’s a single conversion step. It retains all the important information such as timing, the energy of the X-ray that is hitting the CZT detector – you can create colour, or spectroscopic images.”

He adds that CZT-based scanners are currently in use for explosives detection at UK airports, and for scanning checked baggage in some US airports. “We expect CZT to come into the hand luggage segment over the next [few] years.”

But it’s not always easy to get your hands on CZT.

Henric Krawczynski at Washington University in St Louis in the US has used the material before on space telescopes attached to high altitude balloons. These detectors can pick up X-rays emitted by both neutron stars and plasma around black holes.

Prof Krawczynski wants very thin, 0.8mm pieces of CZT for his telescopes because this helps to reduce the amount of background radiation they pick up, allowing for a clearer signal. “We’d like to buy 17 new detectors,” he says. “It’s really difficult to get these thin ones.”

He was unable to source the CZT from Kromek. Dr Basu says his firm has high demand at the moment. “We support many, many research organisations,” he adds, “It’s very difficult for us to do a hundred different things. Each research [project] needs a very particular type of detector structure.”

For Prof Krawczynski, it’s not a crisis – he says he might use either CZT that he has from previous research, or cadmium telluride, an alternative, for his next mission.

However, there are bigger headaches at the moment. That upcoming mission was due to fly from Antarctica in December but “all the dates are in flux”, says Prof Krawczynski, because of the US government shutdown.

Many other scientists use CZT. In the UK, a major upgrade of the Diamond Light Source research facility in Oxfordshire – costing half a billion pounds – will improve its capabilities thanks to the installation of CZT-based detectors.

Diamond Light Source is a synchrotron, which fires electrons around a giant ring at nearly the speed of light. Magnets cause these whizzing electrons to lose some energy in the form of X-rays, and these are directed off from the ring in beamlines so that they may be used to analyse materials, for example.

Some recent experiments have involved probing impurities in aluminium while it melts. Understanding those impurities better could help improve recycled forms of the metal.

With Diamond Light Source’s upgrade, due to complete in 2030, the X-rays produced will be significantly brighter, meaning that existing sensors would not be able to detect them properly.

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

That’s why, here too, CZT is the material of choice.