Working flat out

The X-ray tube has been the workhorse of medical imaging systems for nearly 100 years. In such a tube, a high-voltage potential is applied across a cathode and an anode, accelerating electrons towards the anode or metal target.

When the electrons collide with the target, they lose their energy and X-rays are emitted. Although effective, X-ray tubes have some limitations.

They are expensive to manufacture and replace, they are fragile and they burn out frequently. In addition, vacuum tubes require high-voltage electronic support systems to create and accelerate the electron beam, as well as associated shielding, which increases their capital cost and weight.

Vacuum-tube-based solutions are also heavy; even existing ’portable’ solutions can weigh 100kg or more. As a result, taking radiological equipment based on them to a patient can be impractical. As even portable systems emit a conical beam of X-rays, it is vital to keep a specificdistance from a patient to obtain an image and to avoid radiation over-exposure to the skin.

For these reasons, many organisations are aiming to create flat, solid-state X-ray generation systems that address the limitations of the unwieldy X-ray tube-based devices, while enabling them to efficiently marry up to the latest generation of predominantly flat X-ray detection systems. One company, Radius Heath, plans to develop a lightweight, flat-panel X-ray source based on the pioneering work undertaken by Gil Travish, a research scientist at the University of California Los Angeles (UCLA).

The company’s Microemitter Array X-ray (MAX) combines the advantages of field-enhanced emitters with pyroelectric crystal field generators to create a flat-panel device that can generate X-rays from very low-power sources. Popularly used in flat-panel plasma screen television sets, field-emitter arrays comprise many individual, identical, small, field-enhanced electron emitters in a 2D pattern.

These coneshaped emitters have high field-enhancement properties; by applying just a few volts to a given electrode, the electric field that is produced at their tips can be millions of times higher than on a normal X-ray cathode. However, field-emitter arrays also have limitations.

Like existing X-ray vacuum tubes, it is still obligatory to build a secondary high-voltage structure around them to accelerate the electrons from the electron sources at a high enough velocity to cause Xrays to be emitted from a target. To resolve this, Travish’s solution was to create an array of microfabricated emitters from pyroelectric crystals.

While these can also generate large electric field potentials when repeatedly heated and cooled in a vacuum, producing beams of electrons that accelerate towards an array of metal targets to generate parallel X-rays uniformly across a flat panel, they can do so without the need for high voltages to be applied to initially generate the electrons or to accelerate them.

When a pyroelectric field emitter made of such a crystal is heated in a vacuum, a large electric field develops across it. The top surface of the crystal becomes positively charged and attracts electrons. When the crystal is then cooled, the electrons from the top surface of the crystal accelerate towards a metal target, which is at ground potential, producing X-rays.

By thermally cycling the pyroelectric emitter, a constant beam of X-rays can be produced. Travish said that although many pyroelectric materials exist that could be used to fabricate the emitters, Radius Heath has favoured just two, whose properties have been proven in the telecommunications sector.

The first is lithium niobate, single crystals of which have been exploited to create optical waveguides, and the second is lithium tantalate, which has been used to build pyroelectric detectors. ’The use of pyroelectric emission from crystals as the basis for our display is critical,’ he said.

’As the voltage required to generate energetic electrons is produced within the crystals, this eliminates the need for high-voltage power supplies, transformers and high-voltage electronics.’ While previous efforts have harnessed the characteristics of such pyroelectric-based sources to create X-rays, the non-uniform output from such devices over a period of time has been a limiting factor.

However, Travish claims that with the MAX technology he is able to control the emission of electrons from such sources well enough to maintain a stable source of X-rays that could be applied to clinical applications. In the system Radius Health proposes, pyroelectric emitters would be fabricated on a single tile around 1cm2 in size using conventional photolithographic semiconductor wafer fabrication techniques.

As the voltage required to generate energetic electrons is produced within the crystals, this eliminates the need for high-voltage power supplies, transformers and high-voltage electronics

Gil Travish, UCLA

The work to scale up the tile to such a size is an extension of the work Travish has already undertaken proving that the concept works by building a 1mm2 tile with 64 electron emitters in an 8x8 array. On the bottom of each tile, a resistive heater takes all the emitters on the tile in and out of thermal equilibrium.

Each of the larger 1cm2 tiles, whose emitter arrays would be addressable in groups, would then be butted together to form a larger structure upon which individual targets, filters and collimators would be built. The company aims to create a commercial X-ray source of at least 10 x 20cm in size, with the possibility of developing a system up to 30 x 40cm in size.

Travish said that, in contrast with a conventional X-ray tube that has reflective metal targets, the targets of the new system would be transmissive. So, unlike tubes, where electrons from a target are emitted at 90° to the direction of the electrons that hit it, in the MAX system the X-rays would be emitted in parallel to them.

With suitable collimation, this would enable such an array of tiles to produce a large area of parallel X-rays at a size that matches that of a flat-panel detector, enabling a compact, portable system to be created. Another advantage of the new design is that, since it employs many emitters and targets spread over a large surface area, the energy density is also low.

This, said Travish, minimises the load induced by changes in temperature of the materials. Furthermore, since the new system produces a large parallel number of X-rays, rather than a cone-shaped beam, it eliminates the need for stand-off distances.

Not only could a MAX plane parallel-emission flat panel be placed much nearer a patient, it could potentially touch a patient. While acknowledging that medical devices traditionally have a long development period, Travish said that Radius Health plans to demonstrate the effectiveness of its MAX technology this year and will next year produce a prototype pre-clinical device upon which it will perform studies to prove the commercial viability of the system.

’Since it requires fewer parts than traditional X-ray systems, our MAX technology will be rugged, lightweight and small, able to be transported easily and set up quickly, heralding a new era of X-ray systems,’ he said.