A new highly sensitive X-ray detector could allow medical imaging procedures to take place using far lower doses of ionising radiation than current systems, thereby reducing cancer risks for patients. Yet, there are many practical challenges to be faced before the new detector, which has been described by its creator as revolutionary, can be made available to patients.
The risks of ionising radiation are well known; while acute health effects such as skin burns or acute radiation syndrome can occur when doses of radiation exceed certain levels, even low doses of ionising radiation can increase the risk of longer-term effects such as cancer.
And while the data simply isn’t there to either confirm or refute the fact that the level of radiation patients receive from X-ray imaging falls into the latter category, the general consensus among the medical community is that there are some risks.
According to FDA, for example, the effective doses from diagnostic CTs, which use hundreds of X-rays to create incredibly detailed 3D images, are “not much less than the lowest doses of five to 20mSv received by some of the Japanese survivors of the atomic bombs” dropped over Hiroshima and Nagasaki in 1945. These survivors have “demonstrated a small but increased radiation-related excess relative risk for cancer mortality.”
Moreover, a study published in 2009 by the National Cancer Institute (NCI) predicted that up to 2% of future cancers – about 29,000 cases – might be caused by CT scans.
On the other hand, the overall risk from a single scan is small, with the NCI estimating that the additional risk of developing a fatal cancer from a scan is one in 2,000, while the lifetime risk of dying of cancer in the US is one in five. Some experts, like James Brink, radiologist-in-chief at Massachusetts General Hospital and chair of the board of the American College of Radiology, therefore believe the cancer risks associated with CT scans are “unproved” and “overemphasised”.
“In the absence of a proven radiation risk, our current efforts to protect patients could do more harm than good by discouraging clinically indicated diagnostic imaging or encouraging the substitution of suboptimal non-radiation-based imaging modalities,” he said in an article he co-authored in 2015.
That said, it is of course better to be safe than sorry; while we can’t categorically prove that radiation is a real problem, we do need to act as though it is. And it’s for this reason that reducing radiation dose by developing more sensitive X-ray detectors has become an important field of research for scientists.
Current limitations
As Professor Jinsong Huang, graduate chair of materials engineering at the Department of Mechanical and Materials Engineering at the University of Nebraska-Lincoln, who has been researching this area with colleagues around the world for several years, says: “We believe that the sensitivity of the current systems isn’t high enough yet.
“In order to get X-ray images in a regular medical examination, the X-ray machine needs to give a very large X-ray dose to obtain a regular image. You can compare X-ray imaging with a regular camera; if the camera is not sensitive enough, you need to use a stronger light to give enough signal.
“When it comes to X-rays, this unfortunately is not good for the human body, so we are trying to develop a new generation of materials for X-ray detection systems to overcome the limitations of the current systems in terms of sensitivity.”
In other words, although X-rays are extremely useful for medical diagnosis because they can pass through skin and soft tissue to reveal bone and deep tissue, they can also pass easily through the materials used in commercial X-ray detectors – usually amorphous selenium – meaning relatively high doses are required to acquire high-quality images.
For Huang, amorphous selenium simply isn’t the right material for the job. “It has really good advantages in terms of its stability, and the fact that it is easy to manufacture at a large scale and integrate into silicon circuits,” he notes. “Plus, as it’s been developed for many years, it’s a mature technology.
“However, it’s not a material that’s ideal for X-ray imaging because it doesn’t have the electronic properties required. It is a very poor semiconductor and it has low charge mobility, which means it is not a very good converter of X-rays.”
‘Revolutionary’ technology
It’s these limitations that Huang and his colleagues in the US, the Netherlands and China, have been working to overcome over recent years with their research on a very different substance, perovskite methylammonium lead tribromide, which they originally looked at in reference to solar cells but now believe could replace amorphous selenium as the go-to X-ray detector material in the years to come.
When the team had just started looking at solar cells in 2013, they found that the material contained many high-atomic-number items, such as lead and bromine, which was relevant because an atom’s X-ray absorbance is proportional to the fourth power of its atomic number. “This meant the material should be very effective in stopping X-rays,” Huang explains.
Secondly, it was discovered that its electronic properties were much more suitable for X-ray detection than amorphous selenium, and, thirdly, the material has a much longer charge-carrier lifetime. “Comparing charge mobility and lifetime, we found that this material would be excellent for X-ray detection,” Huang recalls.
The next step was to test it, but there was one more phase before fully investigating its potential as an X-ray detection material. “First, we began to look into this material for gamma-ray detection,” Huang explains. “We put thick crystals under gamma rays and found that a simple, single crystal could work like a solar cell, converting gamma-ray energy into electricity without applying any bias to the device.”
Following this initial finding, the team discovered a paper published in the journal Nature Photonics, where a polycrystalline version of the material they were investigating was being used for X-ray detection. In this case, the thickest films the researchers could produce were too thin to stop medical imaging X-rays but, undeterred, Huang and his colleagues decided to continue to develop their single crystal into a detector, too, and see what happened.
“We began to measure the sensitivity of this device under X-rays, and initial tests showed that the sensitivity was four times better than the best commercial amorphous selenium detectors,” Huang says, adding that it should therefore be able to detect weaker X-ray signals.
And since their work was published in Nature Photonics in March 2016, receiving acclaim from industry experts, Huang and his team have developed their technology further. “We’ve improved the detector and made it hundreds or thousands of times better than the commercial amorphous selenium detectors,” he says.
The implications of this for the medical imaging sector and patients are clear. “Our focus is on reducing the X-ray dose to the patient, so what we care about is sensitivity,” Huang explains. “If the sensitivity is high, we can use a lower-dose X-ray source, just like the regular camera I mentioned before; you don’t need to have such a strong light to get a high-quality image.
“A simple conversion of this is if you have a thousand-times-improved sensitivity, you can expect a thousand-times reduction of dose to the patient. However, this seems too ideal, so what we can realistically expect is to reduce the dose to the patient by ten or a hundred times. That would make a significant impact, and, to me, is a revolutionary result and would make X-rays much safer.”
Challenges ahead
Huang is realistic, however, about the challenges that remain before their single-crystal device could be used in commercial medical imaging machines. First, practical applications require an array of detectors rather than the single device that the researchers have so far experimented with. “I don’t know yet whether it will convert to a commercial product or not but we are ready to work with companies to make our detector into detector arrays so we can try to commercialise them,” he notes.
Partnerships will be absolutely key in ensuring the device can be developed into something suitable for medical applications. “The challenge is that, frankly, we don’t know what challenges we are going to face,” Huang admits.
“I work in devices and science, so definitely need to know more about the systems side and what is needed for X-ray detectors. And that’s exactly what we are doing with one company. We’ve started initial testing – for example, putting our materials on to the read-out circuits – to see if we can read out a good image.”
The next step, of course, would be scaling up, particularly for the device to be used in CT scanners. “For this, we’ll need to move our research from the lab to the industrial environment, so we will need much larger equipment than what we have currently to make the materials,” Huang explains.
In the meantime, initial feedback from medical imaging companies has been positive – “Just sitting in my office, I’ve been reached by at least three companies telling me they are interested in testing our detectors in their system,” Huang says – and the researchers continue to work on improving their technology to make it “even better than the state-of-the-art”.
