Advance makes MRI scans more than seven times faster

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In a paper that appeared Dec. 20 in the journalPLoS ONE,a University of California, Berkeley, physicist and colleagues from the University of Minnesota and Oxford University in the United Kingdom describe two improvements that allow full three-dimensional brain scans in less than half a second, instead of the typical 2 to 3 seconds.
"When we made the first images, it was unbelievable how fast we were going,"said first author David Feinberg, a physicist and adjunct professor in UC Berkeley's Helen Wills Neuroscience Institute and president of the company Advanced MRI Technologies in Sebastopol, Calif."It was like stepping out of a prop plane into a jet plane. It was that magnitude of difference."

For neuroscience, in particular, fast scans are critical for capturing the dynamic activity in the brain.

"When a functional MRI study of the brain is performed, about 30 to 60 images covering the entire 3-D brain are repeated hundreds of times like the frames of a movie but, with fMRI, a 3-D movie,"Feinberg said."By multiplexing theimage acquisitionfor higher speed, a higher frame rate is achieved for more information in a shorter period of time."

"The brain is a moving target, so the more refined you can sample this activity, the better understanding we will have of the real dynamics of what's going on here,"added Dr. Marc Raichle, a professor of radiology, neurology, neurobiology, biomedical engineering and psychology at Washington University in St. Louis who has followed Feinberg's work.

In addition to broadly advancing the field of neural-imaging, the discovery will have an immediate impact on the Human Connectome Project, funded last year by the National Institutes of Health (NIH) to map the connections of the human brain through functional MRI (fMRI) and structural MRI scans of 1,200 healthy adults.
"At the time we submitted our grant proposal for the Human Connectome Project, we had aspirations of acquiring better quality data from our study participants, so this discovery is a tremendous step in helping us accomplish the goals of the project,"said Dr. David Van Essen, a neurobiologist at Washington University and co-leader of the project."It's vital that we get the highest quality imaging data possible, so we can infer accurately the brain's circuitry– how connections are established, and how they perform."

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The new technique accelerates diffusion MRI as well. The colored tracks show the direction of nerve fiber bundles, providing a 3-D image of the axonal pathways in the white matter (cortex) of a resting human brain. A normal structural cross sectional image of the brain bisects the diffusion 3-D fibertrack image. The entire 3-D image was scanned in 8.5 minutes instead of 30 minutes. (David Feinberg)

The faster scans are made possible by combining two technical improvements invented in the past decade that separately boosted scanning speeds two to four times over what was already the fastest MRI technique, echo planar imaging (EPI). Physical limitations of each method prevented further speed improvements,"but together their image accelerations are multiplied,"Feinberg said. The team can now obtain brain scans substantially faster than the time reductions reported in their paper and many times faster than the capabilities of today's machines.

Probing the brain with radio waves

Magnetic resonance imagingworks by using a magnetic field and radio waves to probe the environment of hydrogen atoms in water molecules in the body. Because hydrogen atoms in blood, for example, respond differently than atoms in bone or tissue, computers can reconstruct the body's interior landscape without the use of penetrating X-rays.

Nearly 20 years ago, however, a new type of MRI called functional MRI (fMRI) was developed to highlight areas of the brain using oxygen, and thus presumably engaged in neuronal activity, such as thinking .Using echo planar imaging (EPI), fMRI vividly distinguishes oxygenated blood funneling into working areas of the brain from deoxygenated blood in less active areas.

As with standard MRI, fMRI machines create magnetic fields that vary slightly throughout the brain, providing a different magnetic environment for hydrogen atoms in different areas. The differing magnetic field strengths make the spin of each hydrogen atom precess at different rates, so that when a pulse of radio waves is focused on the head, the atoms respond differently depending on location and on their particular environment. Those that absorb radio energy and then release the energy are detected by magnetic coils surrounding the head, and these signals, or"echoes,"are used to produce an image of the brain.

With EPI, a single pulse of radio waves is used to excite the hydrogen atoms, but the magnetic fields are rapidly reversed several times to elicit about 50 to 100 echoes before the atoms settle down. The multiple echoes provide a high-resolution picture of the brain.

In 2002, Feinberg proposed using a sequence of two radio pulses to obtain two times the information in the same amount of time. Dubbed simultaneous image refocusing (SIR) EPI, it has proved useful in fMRI and for 3-D imaging of neuronal axonal fiber tracks, though the improvement in scanning speed is limited because with a train of more than four times as many echoes, the signal decays and the image resolution drops.

Another acceleration improvement, multiband excitation of several slices using multiple coil detection, was developed in the U.K. at about the same time by David Larkmann for spinal imaging. The technique was recently pioneered for fMRI by Steen Moeller and colleagues at the University of Minnesota. This technique, too, had limitations, primarily because the multiple coils are relatively widely spaced and cannot differentiate very closely spaced images.

In collaboration with Essa Yacoub, senior author on the paper, and Kamil Ugurbil, director of the University of Minnesota's Center for Magnetic Resonance Research and co-leader of the Human Connectome Project, Feinberg combined these techniques to get significantly greater acceleration than either technique alone while maintaining the same image resolution.

"With the two methods multiplexed, 10, 12 or 16 images the product of their two acceleration factors were read out in one echo train instead of one image,"Feinberg said.

fMRI moving closer to speed of EEG

The ability to scan the brain in under 400 milliseconds moves fMRI closer to electroencephalography (EEG) for capturing very rapid sequences of events in the brain.

"Other techniques which capture signals derived from neuronal activity, EEG or MEG, have much higher temporal resolution; hundred microsecond neuronal changes. But MRI has always been very slow, with 2 second temporal resolution,"Feinberg said."Now MRI is getting down to a few hundred milliseconds to scan the entire brain, and we are beginning to see neuronal network dynamics with the high spatial resolution of MRI."

The development will impact general fMRI as well as diffusion imaging of axonal fibers in the brain, both of which are needed to achieve the main goal of the Human Connectome Project. Diffusion imaging reveals the axonal fiber networks that are the main nerve connections between areas of the brain, while fMRI shows which areas of the brain are functionally connected, that is, which areas are active together or sequentially during various activities.

"While it simply is not possible to show the billions of synaptic connections in the live human brain, the hope is that understanding patterns of how the normal brain is functionally interacting and structurally connected will lead to insights about diseases that involve miswiring in the brain,"Feinberg said.

"We suspect several neurologic and psychiatric disorders, such as autism and schizophrenia, could be brain connectivity disorders, but we don't know what normal connectivity is,"Feinberg added."Although the fMRI and neuronal fiber images do not have the resolution of an electron microscope, the MRI derived Connectome reveals the live human brain and can be combined with genetic and environmental information to identify individual differences in brain circuitry."

Raichle, a collaborator in the NIH Human Connectome project, is one of the pioneers of"resting state"MRI, in which brain scans are taken of patients not involved in any specific task. He believes that the ongoing spontaneous activity discovered during such scans will tell us about how the brain remains flexible and maintains a degree of homeostatis so that"you know who you are."

"Being able to sample this ongoing activity at increasing temporal fidelity and precision becomes really important for understanding how thebrainis doing this,"Raichle said."David is superclever at this kind of technical stuff, and I have been cheering him along, saying that the faster we can go, the better we can understand the brain's spontaneous activity."

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Scientists make holograms of atoms using electrons

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The scientists, Ymkje Huismans from the FOM-Institute AMOLF in Amsterdam, The Netherlands, and an international research team have published their study in a recent issue ofScience Express.

“What we have experimentally demonstrated is that it is possible to make holograms by taking an electron out of a molecule and, using a laser field, redirect the electron toward the molecule,” coauthor Marc Vrakking, of the FOM-Institute AMOLF and the Max Born Institute in Berlin, toldPhysOrg.com.

In their experiments, the scientists beamed an intense infrared laser light at an atom or molecule, which resulted in the atom or molecule becoming ionized and releasing an electron. The laser field causes the liberated electron to oscillate away from and toward the ion. Sometimes, an electron and ion collide, releasing a very short burst of radiation.

Because theelectron motionis fully coherent, meaning it always has the same phase, the scientists realized that they could apply holographic techniques to record information about the ion and electron. The key to holographic electron imaging is to observe the interference between a reference wave (which is emitted by the electron and doesn’t interact with the ion) and a signal wave (which scatters off the ion and encodes its structure). When the reference wave and signal wave interfere on a detector, the encoded information about the electron and ion is stored and can be viewed in the future. As the scientists explained, the result is a hologram of an atom produced by its own electrons.

The researchers also developed theoretical models to simulate their measurements, confirming that the hologram had stored spatial and temporal information about theelectronsand ions. By using the holographic structures to develop a new kind of ultra-fastphotoelectron spectroscopy, researchers could be able to directly measure electron andionmovements on the attosecond timescale. This ability would be useful for understanding chemical reactions at the most basic level, particularly inmoleculesthat cannot be easily studied by other methods.

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New ways to tune electrical conductivity revealed by electron interaction

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The difference between anelectrical conductorand an insulator is thatelectronsin the latter cannot move freely through the crystal. This is because insulators have a gap in their energy spectrum that electrons cannot overcome. Hiroshi Watanabe, Tomonori Shirakawa and Seiji Yunoki from the RIKEN Advanced Science Institute in Wako and the Japan Science and Technology Agency have now uncovered how the electronic gap in Sr2IrO4arises. Other RIKEN scientists had shown previously that the compound is aninsulator.

Sr₂IrO₄is a member of the oxygen-containing compounds based on transition metals that have high atomic numbers. In these transition metals, the electrons of elements such as nickel, copper or cobalt strongly interact with each other, which results in effects such assuperconductivityor magnetism.

In compounds made from the heavier transition metals, the outermost electrons circle the atoms in the so-called‘5d electron shell’, which is relatively distant from the core. For electrons that occupy this shell there is an unusually strong interaction between their magnetic property, called spin, and the orbital motion around the atomic nucleus. The energy of this spin–orbit interaction is as large as the electron’s energy of motion or the energy arising from the electrostatic interaction between the electrons. This has dramatic consequences on their electronic properties, according to Yunoki, who led the research team.“Literally anything can happen in 5d electron systems because of the subtle balance of those three fundamental energy scales.”

How this energetic interplay modifies the electron conducting behavior in Sr2IrO4 became evident from the researchers’ calculations. The strong spin–orbit interaction in Sr₂IrO₄shifts some of the electronic states to higher energies, which is sufficiently strong to create an energy gap in the electronic states.

Furthermore, the calculations reveal an intriguing connection to the family of high-temperature superconductors that have a similar gap in their electronic states. In these compounds, superconductivity is achieved through a small addition of atoms introducing an electron surplus. The researchers are now investigating the possibility that this could also be the case here.“It would have an enormous impact if one can make Sr₂IrO₄superconducting,” says Yunoki.“We hope that our theoretical calculations will be of help to experimentalists.”

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The 'mad' Egyptian scholar who proved Aristotle wrong

January'sPhysics Worldfeatures a fanciful re-imagining of the 10-year period in the life of the medieval Muslim polymath, written by Los Angeles-based science writer Jennifer Ouellette.

The feature covers the time when al-Haytham -- banished from society and deprived of books -- came up with his revolutionary theories about the form and passage of light.

Ouellette brings detail to the skeletal plot of al-Haytham's life, from the awe and intimidation felt when he was summoned by the Caliph to use his engineering prowess to overcome the annual flooding of the Nile, to his fear of punishment when he realised he had failed in his task.

Al-Haytham was only able to escape a death sentence from the notoriously brutal Caliph by pretending he had gone mad. The Caliph instead incarcerated Al-Haytham, imprisoning him under house arrest to a cell. Confined and alone, it was here that Al-Haytham carried out the work that was to make him famous.

In 11th-century Egypt, Aristotle's ancient thought that visible objects and our own eyes emit rays of light to enable our vision still held.

Ouellette imagines al-Haytham lying alone in his darkened room questioning why the objects in the room are not emitting light and asking 'Is it possible that the ancients were mistaken?'

The question providing the crux, al-Haytham was spurred into experimental action with the candles and copper in his bare room to conclude that there is no mysterious"form"that all objects emit; rather there are sources of primary light that are reflected by other objects.

As Ouellette writes,"This is a work of fiction– a fanciful re-imagining of a 10-year period in the life of Ibn al-Haytham, considered by many historians to be the father of modern optics. Living at the height of the golden age of Arabic science, al-Haytham developed an early version of the scientific method 200 years before scholars in Western Europe."

Released from prison after the Caliph's death, Al-Haytham (AD 965-1040) went on to make contributions to astronomy, mathematics, engineering and medicine, as well as physics. But it his seven-volume Book of Optics, which he wrote while imprisoned, that remain his most famous contributions to science, covering visual perception, psychology and physical optics.

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Newly developed cloak hides underwater objects from sonar

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Led by mechanical science and engineering professor Nicholas Fang, Illinois researchers have demonstrated an acoustic cloak, a technology that renders underwater objects invisible to sonar and other ultrasound waves.

"We are not talking about science fiction. We are talking about controlling sound waves by bending and twisting them in a designer space,"said Fang, who also is affiliated with the Beckman Institute for Advanced Science and Technology."This is certainly not some trick Harry Potter is playing with."

While materials that can wrap sound around an object rather than reflecting or absorbing it have been theoretically possible for a few years, realization of the concept has been a challenge. In a paper accepted for publication in the journalPhysical Review Letters, Fang's team describe their working prototype, capable of hiding an object from a broad range of sound waves.

The cloak is made of metamaterial, a class ofartificial materialsthat have enhanced properties as a result of their carefully engineered structure. Fang's team designed a two-dimensional cylindrical cloak made of 16 concentric rings of acoustic circuits structured to guide sound waves. Each ring has a differentindex of refraction, meaning that sound waves vary their speed from the outer rings to the inner ones.

"Basically what you are looking at is an array of cavities that are connected by channels. The sound is going to propagate inside those channels, and the cavities are designed to slow the waves down,"Fang said."As you go further inside the rings, sound waves gain faster and faster speed."

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Illinois professor Nick Fang developed a two-dimensional acoustic cloak that makes objects in the center invisible to sonar and other ultrasound waves. Credit: L. Brian Stauffer

Since speeding up requires energy, the sound waves instead propagate around the cloak's outer rings, guided by the channels in the circuits. The specially structured acoustic circuits actually bend thesound wavesto wrap them around the outer layers of the cloak.

The researchers tested their cloak's ability to hide a steel cylinder. They submerged the cylinder in a tank with an ultrasound source on one side and a sensor array on the other, then placed the cylinder inside the cloak and watched it disappear from their sonar.

Curious to see if the hidden object's structure played a role in the cloaking phenomenon, the researchers conducted trials with other objects of various shapes and densities.

"The structure of what you're trying to hide doesn't matter,"Fang said."The effect is similar. After we placed the cloaked structure around the object we wanted to hide, the scattering or shadow effect was greatly reduced."

An advantage of the acoustic cloak is its ability to cover a broad range of sound wavelengths. The cloak offers acoustic invisibility toultrasound wavesfrom 40 to 80 KHz, although with modification could theoretically be tuned to cover tens of megahertz.

"This is not just a single wavelength effect. You don't have an invisible cloak that's showing up just by switching the frequencies slightly,"Fang said."The geometry is not theoretically scaled with wavelengths. The nice thing about the circuit element approach is that you can scale the channels down while maintaining the same wave propagation technology."

Next, the researchers plan to explore how the cloaking technology could influence applications from military stealth to soundproofing to health care. For example, ultrasound and other acoustic imaging techniques are common in medical practice, but many things in the body can cause interference and mar the image. A metamaterial bandage or shield could effectively hide a troublesome area so the scanner could focus on the region of interest.

The cloaking technology also may affect nonlinear acoustic phenomena. One problem plaguing fast-moving underwater objects is cavitation, or the formation and implosion of bubbles. Fang and his group believe that they could harness their cloak's abilities to balance energy in cavitation-causing areas, such as the vortex around a propeller.

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Device controls, measures spin current injected into semiconductor material

In contrast to electronics technology which led industrial development in the 20th century and is based on the flow of electron charge (electric current), this technology is an achievement which leads the way in spintronics which is based on the other basic attribute of an electron, itsspin. This technology is expected to contribute to significant energy conservation and increased functionality in social infrastructure, quantum computing and new directions in scientific development. The results of this development will be published in the 24th December 2010 edition ofScience.

Since the development of the transistor in the 1940s, the operation of electronic devices which contributed to the advancement of the electronics industry have utilized physical principles to electrically manipulate and measure the charge of electrons (electric current). Meanwhile, on the other hand, the electron has another basic attribute, its elementary magnetic moment so-called spin. The application of spintronics based on the manipulation of the spin of an electron is highly expected to open the way to new low-power consuming electronics, hybrid electric-magnetic systems and devices with completely new functionalities. The theory of electrically controlling and measuring the spin of an electron was proposed 20 years ago in the area of spin-transistors. However, many fundamental and critical issues in spintronics such as spin-injection, generation of pure spin-current, spin-manipulation and spin observation needed to be achieved to verify this theory. Until the present time, there have been no demonstration to manipulate spin current in the same way as electrical current or the measurement thereof.

In response to this need,Hitachiand international research team measured separately an up and down spin (Spin-Hall Effect) at an extremely low temperature of -269°C in a gallium-arsenide semiconductor, a non-magnetic material in 2005. Further in 2009, using the same gallium arsenide semiconductor at a temperature of -53°C, the team measured the flow of spin polarized current over a distance of a few microns (Spin-injection Hall effect). In the current development, the up or down spin was controlled by a gate voltage, and the successful ON/OFF operation as a transistor have been verified. In this experiment, a circularly polarized light was used to generate pure spin current in the semiconductor. If we can develop spin-injection technology for ferromagnetic material, the spintronics device which was proposed as a theory by Supriyo Datta&Biswajit A. Das in 1990, will be realized. Further, realizing a solid device which can control and detect the polarization of the light, a new dimension of light polarization can be employed as information in future optical communication to open the way for even larger capacity information transmission systems, or in new analytical systems to which use the polarization of light to study the characteristics of biological or molecular material.

The device consists of a planar photodiode with a pn-junction diode and a n-type channel which forms the Hall Bar. By shining light on the diode, photo-excited electrons generated by the photovoltaic effect are injected into the device. The degree of circular polarization of the incident light is used to generate the spin-polarized electrons. The injected spin precede as a spin-current (Spin-injection Hall effect). At this point, if a p-type electrode is formed above the n-type channel and a voltage is applied, according to quantum relativistic effects, the precession of the spins are controlled by the input gate-electrode voltages. These effects are also responsible for the onset of transverse electrical voltages in the device, which represent the output signal, dependent on the local orientation of precessing electron spins.

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Potential 4-D imaging technique modeled by UNL physicists

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How electrons rearrange when atoms or molecules come together is the essence of chemistry, and the ability to manipulate those rearrangements is the goal of the emerging sphere of nanotechnology. A fuller understanding could lead to enormous scientific and technological breakthroughs.

Unfortunately, significant problems confront scientists and engineers in attacking the question. Not only are atoms and molecules very small, requiring highly specialized equipment to"see"them, everything at theatomic levelhappens very, very fast. For example, an electron goes around the nucleus of an atom once every 150 attoseconds -- that's around one 10 millionth of a billionth of a second, far too quick for the human eye or any existing equipment to detect.

Scientists have a good understanding for simple chemical combinations, but not for complex entities likebiological molecules. And nanotechnologists essentially operate in the blind in the sense that they are not yet able to watch while the nanostructures they construct are being assembled. They only know if they have succeeded after the fact.

Nothing exists to allow scientists and engineers to see those electronic processes, but University of Nebraska-Lincoln physics graduate student Hua-Chieh Shao and his adviser,theoretical physicistAnthony Starace, have modeled a four-dimensional imaging technique that could lead to a breakthrough. They report their findings this week in the online edition ofPhysical Review Letters. Their paper will appear in the Dec. 31 print edition of the journal. The research was supported in part by funding from the National Science Foundation and the Nebraska Research Initiative.

Starace said he and Shao were intrigued by research that his UNL colleagues Herman Batelaan and Kees Uiterwaal were doing with Nobel laureate Ahmed Zewail of Cal Tech on what Zewail terms four-dimensional imaging -- imaging electronic processes in both space and time with very energetic, but extremely short, pulses of electrons.

"We got excited about this idea and decided to do what I think are among the first calculations, if not the first, to show what you would see,"said Starace, University Professor and George Holmes Professor of Physics.

Deriving analytic formulas as far as possible and then writing computer programs to evaluate the formulas and analyze the results, Shao and Starace determined what would happen if electron pulses of 110 attoseconds were fired at targets excited by a laser field. Their first target was the hydrogen atom, with one proton and one electron. Their second target was the tritium molecule, a hydrogen isotope with one electron and two nuclei.

In both instances, they found patterns in the distribution of scattered electrons that could be used to track changes in the target charge distribution over time -- in other words, how the position of the electrons was changing.

"These are sort of benchmark results showing this new technology that Zewail has called four-dimensional imaging,"Starace said"We've shown among the first results in a realistic calculation that one could see for both an atom and a molecule. We made some assumptions (in the tritium calculations), but it's a fairly realistic picture of how electrons move in molecules."

It remains for experimentalists to develop the technique.

"Herman Batelaan and Kees Uiterwaal are working in collaboration with Zewail to develop the short electron pulses and our newest faculty member, Martin Centurion, has been talking with us about what experiments he may be able to do that would provide evidence of time-dependent electron motion. He does experiments with short pulses of electrons, but much longer than the pulses which we considered here,"Starace said.

"I hope this will give added incentive to pursue this technology because we've shown that it can provide information that no other technique can provide. So now it's up to experimentalists to develop the tools. Scientifically, there's nothing stopping the development of the tools except money and manpower, as usual."

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