Photoacoustic imaging : The sound of light

Jun 4th 2009 The Economist (print edition)

Biomedical technology: A novel scanning technique that combines optics with ultrasound could provide detailed images at greater depths

IF LIGHT passed through objects, rather than bouncing off them, people might now talk to each other on “photophones”. Alexander Graham Bell demonstrated such a device in 1880, transmitting a conversation on a beam of light. Bell’s invention stemmed from his discovery that exposing certain materials to focused, flickering beams of light caused them to emit sound—a phenomenon now known as the photoacoustic effect.

It was the world’s first wireless audio transmission, and Bell regarded the photophone as his most important invention. Sadly its use was impractical before the development of optical fibres, so Bell concentrated instead on his more successful idea, the telephone. But more than a century later the photoacoustic effect is making a comeback, this time transforming the field of biomedical imaging.

A new technique called photoacoustic (or optoacoustic) tomography, which marries optics with ultrasonic imaging, should in theory be able to provide detailed scans comparable to those produced by magnetic-resonance imaging (MRI) or X-ray computerised tomography (CT), but with the cost and convenience of a hand-held scanner. Since the technology can operate at depths of several centimetres, its champions hope that within a few years it will be able to help guide biopsy needles deep within tissue, assist with gastrointestinal endoscopies and measure oxygen levels in vascular and lymph nodes, thereby helping to determine whether tumours are malignant or not. There is even scope to use photoacoustic imaging to monitor brain activity and gene expression within cells.

To create a photoacoustic image, pulses of laser light are shone onto the tissue being scanned. This heats the tissue by a tiny amount—just a few thousandths of a degree—that is perfectly safe, but is enough to cause the cells to expand and contract in response. As they do so, they emit sound waves in the ultrasonic range. An array of sensors placed on the skin picks up these waves, and a computer then uses a process of triangulation to turn the ultrasonic signals into a two- or three-dimensional image of what lies beneath.

The technique works at far greater depths (up to seven centimetres) than other optical-imaging techniques such as confocal microscopy or optical-coherence tomography, which penetrate to depths of only about a millimetre. And because the degree to which a particular wavelength of light is absorbed depends on the type of tissue and, in the case of blood, on whether it is oxygenated or deoxygenated, there is, in effect, a natural contrast agent. This makes the technique superior to ultrasound alone when it comes to picking out detailed features such as veins.

MRI and CT scans are also capable of delivering this kind of detail. But they usually require contrast dyes to be injected into the bloodstream, says Lihong Wang, a photoacoustic researcher at Washington University in St Louis, Missouri. CT scans also involve potentially harmful ionising radiation. And MRI and CT scans are very expensive, using machines that cost millions of dollars and require dedicated staff to operate them. Photoacoustic tomography, by contrast, could eventually be performed using portable hand-held devices, similar to those used for ultrasound scanning. This would allow doctors to diagnose and monitor patients in clinics, and reduce the need to refer them to consultants. “Photoacoustics provides greater access at a much lower cost than these other technologies,” claims Michael Thornton of Endra, a medical-imaging company based in Ann Arbor, Michigan.

Shining a light

A pioneer of the technique in the late 1980s was Alexander Oraevsky, who was based at the Soviet Academy of Sciences in Moscow at the time. He had been evaluating lasers as a means of removing tissue, but in the course of his experiments he realised that his samples were producing ultrasound, and began exploring the potential of this effect for imaging. Since then the technology has come a long way, not least because of the development of nanosecond pulsing lasers. Being able to deliver such brief pulses of energy to the sample being imaged—a nanosecond is a thousand-millionth of a second—has helped improve the resolution of the resulting images. Dr Oraevsky and other researchers have shown that it is possible to image the entire blood-supply system of a mouse, for example, down to a resolution of about half a millimetre.

One of the most promising applications for photoacoustics is in the treatment of cancer. Since blood cells are natural absorbers of light, photoacoustics is particularly good at providing high-contrast images of the formation of blood vessels (angiogenesis) and detecting increased metabolic activity (hypermetabolism), both of which are hallmarks of cancer, notes Dr Wang. Preliminary clinical research is now under way to look at how the technology can be used to monitor the development of breast cancer and identify how far it has progressed.

Even with mammography and ultrasound, the current gold standards for breast-cancer screening, doctors cannot tell if a tumour is malignant or benign without performing an invasive and expensive biopsy. “About eight out of ten patients who undergo a biopsy come back negative,” says Dr Oraevsky, who now works for Fairway Medical Technologies, a company based in Houston, Texas. Photoacoustic tomography could potentially be used to diagnose women in the doctor’s surgery.

One approach being explored by Michael Pashley, head of ultrasound imaging and therapy at Philips Research in Briarcliff Manor, New York, is to develop a hybrid ultrasound scanner that can produce ordinary ultrasound scans as well as photoacoustic images. In theory the two images could even be superimposed, he says. At the moment the work, which is being carried out in collaboration with Dr Wang, is geared towards monitoring the development of breast cancers that have already been diagnosed, says Dr Pashley. But if the technology proves successful, he hopes to move on to using it for the initial diagnosis.

Lihong V. Wang

Getting the picture

Although the different absorption characteristics of oxygenated and deoxygenated blood provide an extremely good natural contrast agent, this approach has its limits. So some companies are exploring the use of photoacoustics in conjunction with artificial contrast-agents introduced to the bloodstream. VisualSonics, an ultrasound-imaging company based in Toronto, has been evaluating contrast agents made up of gold nanorods attached to antibodies that bind to specific targets found in cancer cells. Ultrasound is already used to detect such agents but its resolution is sufficient to show only the structure of blood vessels. Dr Wang reckons that if contrast agents that are too small to be picked up by ordinary ultrasound were introduced into a patient’s bloodstream, they could be detected using photoacoustic imaging. Furthermore, it would be possible to see where the contrast agents built up, and hence determine the extent of a tumour. And by creating contrast agents that bind to specific genetic targets, the same technique could be used to monitor gene expression, he suggests.

Room for improvement

Despite its potential and its many advantages over other methods, there are some difficulties with photoacoustic imaging that have not yet been resolved. As light penetrates deeper into tissue, the resulting ultrasonic signal diminishes. This is partly because some of the light has been absorbed by the preceding tissue, but it is also because the laser light is dispersed, diffused and back-scattered. This places limits on just how deeply photoacoustic imaging can delve. In the future it might be possible to go a little deeper, says Dr Wang, but probably not by much. “If light is delivered from both sides of the tissue, ten-centimetre-thick tissue can potentially be imaged,” he says.

Bone tissue represents another obstacle to the technology, but not for the reason you might think. Laser light usually passes easily through bone, but sound does not. The speed at which sound travels through bone is different from the speed at which it travels through soft tissue, and as the ultrasound passes from one medium to the next it is distorted. Air cavities, many of which are found inside the human body, pose a similar problem, says Dr Wang.

Even so, VisualSonics and other companies are keen to explore the use of photoacoustics for neuroimaging. It is not an insurmountable problem, says Dr Wang, who is working on a technique to model the skull so that its effects on the ultrasonic waves can be predicted and eliminated in software, restoring clarity to the signals. If he can get this approach to work, it would further extend the revolutionary potential of photoacoustic imaging in the coming years. Doctors would not merely be able to diagnose cancer in the comfort of their own surgeries—they would be able to perform brain scans, too. A technology that traces its roots to a stillborn 19th-century communications device would have taken another step towards the futuristic dream of the all-purpose hand-held medical tricorder seen in “Star Trek”.

Solar-thermal technology

The other kind of solar power
Jun 4th 2009 The Economist (print edition)

Energy: Think of solar power, and you probably think of photovoltaic panels. But there is another way to make electricity from sunlight, which arguably has even brighter prospects

IN THE past few months BrightSource Energy, based in California, has signed the world’s two largest deals to build new solar-power capacity. The company will soon begin constructing the first in a series of 14 solar-power plants that will collectively supply more than 2.6 gigawatts (GW) of electricity—enough to serve about 1.8m homes. But to accomplish this feat BrightSource will not use photovoltaic cells, which generate electricity directly from sunlight and currently constitute the most common form of solar power. Instead, the company specialises in “concentrating solar-thermal technology” in which mirrors concentrate sunlight to produce heat. That heat is then used to create steam, which in turn drives a turbine to generate electricity.

Solar-thermal power stations have several advantages over solar-photovoltaic projects. They are typically built on a much larger scale, and historically their costs have been much lower. Compared with other renewable sources of energy, they are probably best able to match a utility’s electrical load, says Nathaniel Bullard of New Energy Finance, a research firm. They work best when it is hottest and demand is greatest. And the heat they generate can be stored, so the output of a solar-thermal plant does not fluctuate as wildly as that of a photovoltaic system. Moreover, since they use a turbine to generate electricity from heat, most solar-thermal plants can be easily and inexpensively supplemented with natural-gas boilers, enabling them to perform as reliably as a fossil-fuel power plant.

Getty Images

A power of tower near Seville

Besides these benefits, the main drivers for the growth of the solar-thermal industry are moves to limit carbon-dioxide emissions and requirements to increase the proportion of electricity produced from renewable sources. According to New Energy Finance, about 12GW of concentrating solar-thermal power capacity is being planned worldwide—a vast amount, given that only about 500 megawatts (MW) of such capacity has been built to date. To maximise the energy that can be collected from the sun, solar-power facilities are being constructed in regions that enjoy daily uninterrupted sunshine for much of the year. According to Mark Mehos of America’s National Renewable Energy Laboratory, solar-thermal power could in theory generate 11,000GW in America’s south-west. That is about ten times America’s entire existing power-generation capacity.

Simple techniques for concentrating sunlight to generate heat date back thousands of years. In China and ancient Greece, people focused the sun’s rays with mirrors or glass to light fires. In times of war, the same approach is said to have been used to set enemy ships ablaze. By the early 20th century several scientists had built simple machines that could run on concentrated heat from the sun.

A significant milestone was reached in 1913 when Frank Shuman, an American inventor, created the first large solar-thermal pumping station in Meadi, Egypt. He designed a system based on five large reflectors, each 62 metres long and made of glass mirrors arranged to form a trough in the shape of a parabola. Each parabolic trough focused sunlight onto a tube running along its length, heating the water inside it. The resulting steam powered an engine connected to a pump capable of delivering 6,000 gallons of water a minute from the Nile to nearby fields.

Do try to concentrate

The modern history of solar-thermal power began after the oil crises of the 1970s, which prompted many nations to start to investigate clean and renewable energy sources as alternatives to fossil fuels. Over the following decades America, Spain and a handful of other countries built solar-thermal pilot plants for research purposes. The first company to implement the technology on a commercial scale was Luz International, an Israeli company founded in 1980.

Drawing on prior research, Luz began building a series of solar-thermal power stations in California’s Mojave desert in the mid-1980s. Like Mr Shuman before, the company used parabolic troughs to focus sunlight on to liquid-filled tubes, but instead of water they used oil as the heat-transfer fluid. Once it reached a temperature of about 390°C, the hot oil was pumped to a so-called “power block” where it went through a series of heat exchangers, turning water into steam and powering a conventional steam-turbine. The turbine then turned a generator to produce electricity.

By 1990 Luz had constructed nine plants with a total capacity of 354MW. At the time, solar-thermal power was producing about 90% of all solar electricity in the world, says Arnold Goldman, the former chief executive of Luz, who is now chairman of BrightSource. But when the price of natural gas fell and America’s tax incentives for solar power were not renewed, the industry came to a grinding halt. For nearly two decades no new commercial solar-thermal plants began operating. In the meantime, solar-photovoltaic technology slowly took over the market, and by 2007 worldwide installed capacity reached 9.2GW. Although it is more expensive per kilowatt-hour, solar panels can be deployed in small, modular systems, and thus require much less capital investment. Moreover, they can generate power off the grid, which turned out to be an important market for solar power in its early days.

Now, as the solar-thermal industry is experiencing a revival, parabolic-trough projects are garnering much of today’s investment money because of their proven track record. To improve the economics still further, SkyFuel, a firm based in New Mexico, is replacing curved glass mirrors, which are expensive to make, with a thin, reflective low-cost film. And other competing solar-thermal technologies that were developed in parallel with trough-based systems, but never commercialised, are also ready to be deployed.

Among them is an approach that BrightSource uses, in which a field of small, flat mirrors called “heliostats” redirect and concentrate sunlight onto a central receiver at the top of a tower. The tower contains a fluid, typically water, which boils and the resulting steam is then transferred to a nearby “power block”, where it spins a conventional turbine. The advantage of this “power tower” approach is that it can produce steam at a temperature of 550°C and can thus achieve a higher thermal-to-electric efficiency than trough-based systems, says John Woolard, the chief executive of BrightSource. In addition, he says, power-tower systems suffer from fewer pumping losses than trough-based designs. The first commercial power-tower began operating in Spain in 2007.

Another advance that makes solar-thermal power more economically and technologically viable than in the past is the ability to use a large number of smaller and less expensive mirrors, steered by computer systems, to ensure more accurate and automatic tracking and redirection of sunlight than was ever possible before. Bill Gross, the chief executive of eSolar, a developer of “power tower” technology based in Pasadena, California, says his firm is using software to turn thousands of flat mirrors and shape them into a continuously evolving parabola around the tower.

Storage and hybrids

Both power-tower and parabolic-trough systems can store thermal energy in the form of hot, molten salt. It is then possible to generate steam, and thus electricity, even when the sun is not shining. Solar-thermal plants without storage can operate about 30% of the year; but with storage that number could climb to 70% or higher. Unfortunately storage is expensive, and is only economical when regulators provide incentives. In Spain, for example, producers of solar-thermal power receive a guaranteed feed-in tariff. That makes it particularly appealing for Spanish plants to have storage capabilities, to maximise their ability to sell electricity to utilities. In America the main incentives for solar-thermal projects are a 30% investment-tax credit or an equivalent cash grant. As a result, American plants have to be built more cheaply in order to make a profit, and thus typically do not include storage.

Eyevine

Power from a parabola

A cheaper alternative to storage is hybridisation. All the original Luz plants also have natural-gas boilers that can generate steam when the sun is not shining. Because solar-thermal plants have a power block and turbine already in place, the extra cost is marginal. Hybridisation could also be done the other way around, by using steam generated from solar-thermal collectors to help drive the turbines at existing coal or gas plants. The Electric Power Research Institute, based in Palo Alto, is studying the feasibility of this approach as a means of reducing fuel costs and emissions at existing power stations.

In addition to parabolic troughs and power-towers there is also a third solar-thermal technology, which combines curved, dish-shaped mirrors with heat engines. In a dish-engine design, the mirrors concentrate sunlight to generate heat, which then typically powers a Stirling engine—a machine that converts heat into mechanical energy by compressing and expanding a fixed quantity of gas. The change in pressure drives the engine’s pistons, which drive a shaft that turns a generator to produce electricity.

Although they are highly efficient, Stirling engines have seen little practical use since their invention nearly two centuries ago, and so far there are no commercial solar-thermal systems that use this approach. Critics of the technology say it involves too many moving parts, making it more complex and expensive to operate and maintain than competing technologies. Stirling Energy Systems, based in Phoenix, Arizona, hopes to prove the doubters wrong. It has signed two large power-purchase agreements, for up to 1,750MW, and plans to fulfil them using dish-engine systems built in conjunction with its sister company, Tessera Solar. Both projects are due to start construction as early as 2010.

One obstacle hampering the growth of the entire field is the difficulty of obtaining financing for solar-thermal projects in the current economic climate, says Thomas Mancini, programme manager for concentrating solar-power at Sandia National Laboratories. As a result, some announced projects may be delayed or perhaps never be built. The situation has prompted some companies to change their business models: Ausra, a solar-thermal company based in Mountain View, California, has switched from being an independent power-producer to being mostly an equipment supplier, for example.

Although solar-thermal power produces no carbon-dioxide emissions, it can have some negative environmental impacts. Both power-tower and trough-based systems are typically water-cooled, and require millions of gallons of water annually. That can cause big problems, especially in desert environments. The California Energy Commission recently urged NextEra Energy Resources, a renewable-energy company, to consider dry cooling instead of using water for its proposed solar-thermal power project in Kern County, California. (Stirling-engine designs do not require water for cooling.) Another potential problem when building power plants in remote locations is a lack of transmission lines, since it is difficult and expensive to get new transmission lines approved and built.

Despite these problems, many people think a massive scale-up of the industry is imminent. Among them is Mr Woolard, who believes that solar-thermal power could regain its historical lead over the solar-photovoltaic approach. Competition from photovoltaic systems for large-scale power generation should not be underestimated, however. According to Mr Bullard, thin-film solar-cell modules are rapidly falling in price, and can generate electricity more cheaply than solar-thermal power in some situations. But no matter which approach comes out on top, competition between the two technologies is sure to foster continued innovation, and a growing supply of clean electricity, in the years to come.