Adelaide’s Micro-X has re-engineered the X-ray tube from the electron source up, radically reshaping imaging systems.
For more than a century, X-ray tubes have relied on the same core architecture: a thermionic filament heated until it “boils off” electrons, which are then accelerated at high voltage into a tungsten anode to generate X-rays.
Since 2011, high-tech company Micro-X has been trying to solve the two problems with this approach: it is inherently energy-intensive and thermally constrained.
Micro-X has designed and manufactured innovative, ultra-lightweight and mobile X-ray imaging systems for medical and security applications using proprietary carbon nanotube (CNT) technology.
Their products include portable hospital imaging units known as Rovers, brain CT scanners for stroke diagnosis and security scanners.
COO Anthony Skeats explains that the engineers have “replaced that hot filament with what’s called a cold cathode field emitter, which operates more like an LED”.
The technology sees high-voltage electric fields concentrate at the tips of conductive carbon nanotubes and this draws electrons directly from their structure – reshaping the size, weight and control of X-ray systems.
Overcoming barriers
Field emission from carbon nanotubes has been understood in principle for decades, but the research was stuck due to problems with current density and stability.
“The best way to describe our CNT is that each one’s a mini lightning rod, but at a nanoscopic scale,” Skeats said. “They’re very strong, like a spider web, with a very high strength-to-size ratio.”
However, under strong electric fields the structures can ablate. Micro-X’s invention lies in figuring out the mechanism behind that, and solving it with materials engineering.
“We found a solution to essentially share the current balance between each nanotube, which acts like a series of infinite resistors.”
Emitters that once delivered only a few milliamps can now operate in the hundreds of milliamps range under test conditions, with sustained stability.
The manufacturing challenge extends well beyond making nanotube emitters. The engineers have also had to leverage expertise in vacuum physics, high-voltage design and how electrons migrate across ceramics.
To this end, Skeats said that they still have blindspots. “We honestly don’t always know what’s going on inside these tubes,” he said. “And that is quite fun.”
Micro-X manufactures the core emitter, builds the X-ray tube, designs the high-voltage power electronics and integrates the system into finished products. “For an engineer, that end-to-end capability is almost unheard of.”
System architecture
Removing the hot filament has a cascading effect. A cold cathode generates negligible heat at the emission source, eliminating warm-up time and ensuring no fluctuations in current and voltage.
“We’re still using CT and imaging people the same way, but the way we do it has scaled down significantly in both weight, size, cost and complexity,” Skeats said.
A conventional mobile X-ray tube weighs around 25 kg, while Micro-X’s equivalent is approximately 2.5 kg. In CT, the contrast is even starker: traditional tube assemblies range from 50-60 kg, while the CNT technology tube weighs around 200 g.
Tubes are as small as a can of coke or a golf ball. Rather than spinning a heavy gantry, the emitters can be arranged sequentially around a patient, resulting in a CT unit that weighs only 70 kg – less than one tenth of the traditional machine weight.
Real-world application
Lower weight and smaller footprints support mobile and point-of-care imaging – a potential boon for emergency departments, disaster recovery and health care in regional and remote areas.
Digital control allows rapid cycling and lower energy consumption, while simplified mechanics reduce maintenance costs.
However, building a global customer base has taken Micro-X around a decade. “When you’re manufacturing in Australia, it’s hard to demonstrate to overseas customers that you have local commitment.”
The company has persisted because of its belief in its revolutionary technology – with applications well beyond health care.
Cold cathode emitters have relevance in fields as diverse as manufacturing, security screening, food inspection and aerospace.
“Specific industries use X-ray to just help them speed up detecting defects. What is common among them is that they have inspection systems relying on what we call non-destructive testing.”
In semiconductor manufacturing, X-ray systems are used for advanced packaging inspection, solder joint analysis and failure diagnostics.
“We’re also exploring communications, as spacecraft when coming back into orbit lose contact for a period of time because radio frequency can’t penetrate the atmosphere – something X-rays may be able to solve,” Skeats said.
One thing’s for certain: Micro-X’s engineers are just getting started. “Our team is motivated by the fact things go wrong on a daily basis. We have to fix these problems, and we love to fix these problems.”
The 12th Australasian Congress on Applied Mechanics is on next month – there’s still time to register.
Great to see the innovation, which like other medical conceptions, i.e. PAUT, DRT etc., have cascaded into the industrial sector.