Episode 158      27 min 13 sec
X-ray renaissance: The potential and promise of coherent X-ray optics

Professor Keith Nugent explains the physics behind X-rays and crystallography, and how new research into the development of X-ray lasers could provide medical scientists with radically new insights into the structure of proteins. With Science host Dr Shane Huntington.

"Seeing atoms doing their work in the biological context -- that's going to happen and it will be really exciting when it does." -- Prof Keith Nugent




           



Prof Keith Nugent
Professor Keith Nugent

Keith Nugent is Professor of Physics at the University of Melbourne and Research Director of the Australian Research Council Centre of Excellence for Coherent X-ray Science, a multi-institutional collaboration devoted to the application of X-rays to problems in biology.

Professor Nugent has made a number of contributions to X-ray science with a particular emphasis on new approaches to imaging. In 1989, in collaboration with Dr. Stephen Wilkins of CSIRO he pioneered a form of X-ray optics based on the capillary structure of lobster eyes. Nugent and Wilkins used the so-called lobster-eye optics to design telescopes with a 360 degree view of the sky.

In 2001, Professor Nugent was made an Australian Research Council Federation Fellow, a position that was renewed in 2006. Nugent is also a Fellow of the Australian Academy of Science (FAA) and the Australian Institute of Physics (FAIP). In 2011, Professor Nugent was appointed Director of the Australian Synchrotron.

Professor Nugent is the recipient of numerous awards in recognition of his contributions to science, including two R&D100 awards, the 2004 Victorian Prize for innovation and the Boas Medal from the Australian Institute of Physics.

Credits

Host: Dr Shane Huntington
Producers: Kelvin Param, Eric van Bemmel
Audio Engineer: Gavin Nebauer
Episode Research: Dr Dyani Lewis
Voiceover: Nerissa Hannink
Series Creators: Eric van Bemmel and Kelvin Param

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VOICEOVER 
Welcome to Up Close, the research, opinion and analysis podcast from the University of Melbourne, Australia. 

SHANE HUNTINGTON 
I'm Shane Huntington. Thanks for joining us.  Medical X-rays have been used for decades to diagnose problems in the body, such as broken bones.  Although valuable in the clinical setting, these technologies are simplistic from a physics perspective, especially compared to other, more recent forms of medical imaging. Nevertheless, X-rays have properties that make them ideal for many applications where other types of light cannot be used.In recent years, pioneers in the field of X-ray imaging have expanded our knowledge of how to control and utilise X-rays to their full potential and apply them to new materials on much smaller scales.  One of these pioneers is our guest today on Up Close.  Professor Keith Nugent is the head of the Australian Research Council's Centre for Coherent X-ray Science here at the University of Melbourne.  Keith is a recipient of the R&D 100 award and is here to tell us about his exciting work on X-ray imaging.Welcome to Up Close Keith.

KEITH NUGENT
Good morning.

SHANE HUNTINGTON
Keith first of all, X-rays are a form of electromagnetic radiation, just as visible light and microwaves are.  Can you give us an idea of where they sit on that spectrum in terms of size and how they compare to the other forms of light?

KEITH NUGENT
So light is obviously the form of electromagnetic radiation that we're most familiar with and as you go from red light to blue light, the size of the light, the wavelength as we call it, gets smaller and smaller.  Once you go beyond the blue light, you move into ultraviolet light, the sort of light that gives us sunburn and that’s shorter wavelength again.  If you go beyond that, you get into the X-ray region.  So what X-rays are, are a form of light which is very, very small in size essentially and the beauty of that is because they're smaller, they tend to have more energy and that means they can penetrate matter more and that's why you can see broken bones with X-rays and also because they're smaller you can see very small things like atoms.  So they're useful for probing the atomic structure of molecules and that's where the real value is for many aspects of science.

SHANE HUNTINGTON
Our listeners would be very familiar where they find many of those other forms of electromagnetic radiation in nature, but what about X-rays?  Do we see X-rays in nature?

KEITH NUGENT
We do.  When X-rays were first discovered by Röntgen in the late 19th century where he took a very famous photograph of his hand, that was from the emissions from a radioactive substance.  So you do get X-rays from radioactive substances, obviously that's relatively low, but still a natural occurrence of them. You get X-rays in space.  A lot of stars, for example, or the sun, emit great quantities of X-rays and there's a whole field of astronomy where X-rays are used.  It's called X-ray astronomy.  The key about X-ray astronomy, of course, is that the atmosphere protects us from the X-rays and so we don’t die from the exposure to them because they can be quite damaging.  So X-ray astronomy is performed using spacecraft.

SHANE HUNTINGTON
In terms of medical X-rays, many of our listeners would be very aware, they've probably had a medical X-ray at some stage in their life.  How exactly are these images generated and what's happening in terms of X-rays going through the body at that point?

KEITH NUGENT
Well X-rays, as I said before, are really strongly penetrating, so they go through things because they're very energetic and of course they go through your body. So what happens with a medical X-ray is the piece of your body that's being X-rayed, let's say your arm, if you have a broken arm, are passed through your body and they're transmitted through it.But the thing about X-rays is they get absorbed as they go through the material and the absorption depends on how dense the material is, how heavy it is, if you like.  Bones are much denser than the flesh around them and so you see the bones as a dark outline in the X-ray picture and that's how you diagnose, for example, broken bones.  But it's a density, if you like – the X-rays are a sense of the density of the material.

SHANE HUNTINGTON
What sort of other materials, other than human flesh, do X-rays actually pass through?  You mentioned the atmosphere that they don't get through, but what sort of things do they get through?

KEITH NUGENT
Well whenever you go through the airport scanner to catch an aeroplane, your luggage is being X-rayed.  They're really used extensively for what you might call non-destructive testing, for looking inside material so you can understand how they work, whether they're working properly and so on.  Clearly if you want to examine the material in an aircraft and you [[don’t (sic)]]want to do it non-destructively, then you can put it into an X-ray machine, see inside it, see if there are defects developing there and repair it if there's a problem.

SHANE HUNTINGTON
Now of course to get any sort of image in this system, there must be some absorption of at least some of the X-rays.

KEITH NUGENT
Yes.

SHANE HUNTINGTON
What's occurring on the physical level there and is that damaging?

KEITH NUGENT
Well it is. That's why X-rays can be so damaging.  Because, as I said before, they have a very short wavelength, they're very small, they can see very small things, they also have a high, what we call photon energy.   So they carry a lot of energy.  When a photon with a lot of energy, an X-ray, strikes an atom, it kicks an electron out of that atom, so it ionises it.  Sometimes X-rays are called ionising radiation for that reason.  What that means is that it breaks up the molecule, if you like, in the body and that can then create dangerous chemicals and create genetic defects and so on.  So it's the energy of the photon that causes the damage that we see from X-rays.  It's what causes sunburn, for example, because of the ultraviolet radiation.  It's also quite energetic.So what happens when an X-ray gets absorbed as it passes through your bone is that it kicks an electron out of the atom, that absorbs energy from the X-ray so it stops and the electron is ejected, but you see a dark patch in the photograph.

SHANE HUNTINGTON
In terms of X-rays in medical imaging, these are unfocused, so there’s just sort of a fairly broad exposure that we get to these X-rays.  Why are these not focused in such a way that you can zoom in, as you would with an optical microscope, on the area of interest in the body?

KEITH NUGENT
That’s an interesting point.  Very recently and by that I mean probably 10 or 15 years ago, it was realised that you could essentially focus X-rays.  So very recently it's been possible to make an X-ray lens.  The problem is it's a very, very weak effect.  Because again these X-rays are so energetic, you can put them in a material, they can be transmitted through it and they can be refracted.  What happens in a lens and we're all familiar from high school science about the refraction of light when it passes through water or glass or your spectacle lenses and so on, that also does happen with X-rays but it's just very, very weak.So it is possible to focus X-rays with a lens, but it's really quite impossible to produce an effective microscope for, for example, medical X-rays.  So you can't do it that way.

SHANE HUNTINGTON
So how do you go about focusing X-rays?

KEITH NUGENT
Well often you don't.  For a medical X-ray you just put a piece of film next to the body and you see the projection through it.  However, for a lot of the work that I do, which is what we call soft X-ray science which is soft meaning lower energy so the wavelengths of the X-rays are larger than they are from medical X-rays, so it's between ultraviolet and a medical X-ray, for them you can produce what's called a zone plate which uses diffraction.  Again, if you think back to your high school science, if you have, say, water waves impinging on a wall in the water, the waves tend to bend around the edge of that wall.  That's a process called diffraction.  You can make special diffracting objects called zone plates which actually do focus these soft X-rays.  So you can make a lens, you can make an X-ray microscope if the X-ray wavelength is long enough.

SHANE HUNTINGTON
This is Up Close, coming to you from the University of Melbourne, Australia.  I'm Shane Huntington.  Our guest today is Professor Keith Nugent and we're talking about new techniques and applications using X-rays.Now Keith, why are X-rays so particularly suited to imaging things on the molecular scale, in particular given the damage you mentioned that they can do to those particular objects?

KEITH NUGENT
Again, everything about X-rays comes down to the fact that they have a very short wavelength, they're very small.  So the value for looking at individual molecules is because they are so small that they can scatter off the atoms within a molecule.  For example, if I were to look at something with really big, fat, fluffy waves and I'll try to get a picture of something, then you really can't do it because the waves are too big.  If, on the other hand, you use a wave which is very small and very tight, then you can get, if you like, into all the nooks and crannies and they can bounce around and come out, when you see the pattern of the X-rays coming out of that object, you can determine its structure and that's a very important process for studying molecular structures.

SHANE HUNTINGTON
One of the terms that we often hear in the same phrasing as X-rays is around crystallography.  Can you describe what is involved in crystallography and what the X-rays are doing in that particular process?

KEITH NUGENT
Crystallography, as the name would suggest, involves looking at things in a crystal.  If you can imagine that you form a crystal of something, what that means is if you take, in the case of, for example, salt, with salt crystals, what you have in there is an arrangement of salt, sodium chloride molecules, organised in a very regular array.  That's what makes it a crystal.If you shine X-rays at a salt crystal and this was the first experiment that was done by the Australian Bragg, or the Bragg father and son team, what you found is that when you shine the X-rays at the salt crystal, you get a bunch of dots in the pattern where the X-ray's come out.  So what you do is you put a crystal in front of a beam of X-rays, you put a piece of film behind the salt crystal and what you see coming from that crystal is a bunch of dots, just points of X-ray light coming out.If you're very clever, then you can look at that arrangement of spots, their positions and their brightness and you can deduce the shape, the precise shape, of the molecules making up that, in this case, salt crystal.  So you could look at the shape of sodium chloride, salt, in this way.  That's the essence of crystallography.  It's taking something that you're interested in, turning it into a crystal, shining X-rays at it and then using the spot pattern that results to deduce the shape of the molecules that made up that crystal.

SHANE HUNTINGTON
Presumably – and this is where your work comes into it – this has great application in the area of protein crystallography, where we're looking at far more complex molecules than, say, salt.  How do you go about turning something like a biological specimen like a protein into a crystal?  This is not something we'd normally expect to find in nature, like salt, in a crystal form.

KEITH NUGENT
But think about sugar.  I mean sugar is a complex molecule, a carbohydrate molecule made of carbon and hydrogen and basically that's it.  That does, even though that's a complex molecule, it does form a crystal.  We've all seen sugar crystals.  So what protein crystallography is about is taking that to the next step. So you take something really complicated like a complex protein and you find precisely the right conditions that enable that protein to arrange itself into a crystal and then once you've successfully done that and that's a very, very tough thing to do, you can then put that in an X-ray beam and look at the spots that I described earlier and deduce the shape of that protein molecule.  So that's protein crystallography.The key issue of course for many of this work is you can't actually do that for all proteins, so there are certain key proteins that simply don't seem to want to form crystals.

SHANE HUNTINGTON
When it comes to the sort of protein structures we're talking about, how many atoms are in these structures typically and how do you go about backing out from that spot pattern this incredibly complex structure?

KEITH NUGENT
The molecules themselves can have thousands of atoms, tens of thousands of atoms, so they can be really huge, really complicated things.  So the problem of deducing the molecular shape from the diffraction pattern and the diffraction patterns are correspondingly complicated, there are thousands of spots in there, deducing the molecular structure from that pattern of spots is really quite a complex process.  There was a very famous problem around it called the phase problem which meant that you're lacking certain key information to back out the molecular structure from that diffraction pattern.So you have to solve the phase problem; that was many, many years of work doing that and there are a complicated set of procedures to do that, including substituting one atom for another using chemical techniques and looking at similar molecules that you know are similar and then comparing them.  When you put all that together, you can then back out the molecular structure of these proteins; you can see the individual atoms, essentially, in these proteins and in many ways you can see them interacting with each other.

SHANE HUNTINGTON
Now Keith, you lead the Centre for Excellence in Coherent X-ray Science.  In physics, usually when we hear the term coherent, we're talking about lasers.  What does this mean in the X-ray context?

KEITH NUGENT
Well it means exactly the same thing.  What we mean by coherence in the context of optics and lasers and X-rays is that you form a wave of X-rays.  So as I said before, X-rays are a form of light, light is a wave, electromagnetic wave.  In the case of a laser, what gives laser light its special properties, which means it's good for using as a laser pointer, for example, is that the waves are all in step with each other.  So it's like a bunch of soldiers marching all in step with each other. They all have the same frequencies, the same colour that amounts to in the laser, that's why laser has a very pure colour and like a troop of soldiers, they're all going in the same direction which is why you can use the laser pointer.  That's the key to it.  So that's what we mean by coherence.In the case of most X-ray sources, until very recently, they were not coherent, they were like a light bulb, not a laser pointer.  So we had these very, very bright X-ray sources, X-ray light bulbs, for example, the synchrotron is one example which is a large X-ray light bulb, really, and those facilities, because of the application of protein crystallography, are all over the world, including Australia.  But the light coming from them is not coherent.  It isn't all in step in the way that I've just described. So an X-ray laser is a system – a machine – that can produce X-rays that are all in step with each other.

SHANE HUNTINGTON
Now when I think of a laser, my mind conjures up images of something that might sit on a bench top.  You mentioned the synchrotron.  This is an enormous device that produces non-coherent X-rays.

KEITH NUGENT
Yes.

SHANE HUNTINGTON
What sort of device are we talking about as an X-ray laser to produce that coherent form of X-ray light?

KEITH NUGENT
The synchrotron, as you said, is a very large device in which electrons are going around a circle to produce the X-ray light that's used.  What happens when electrons go around in a circle is they're accelerating, centrifugal acceleration which you're all familiar with, for example, on a roundabout, that produces light which then produces the X-rays.With a free electron laser, it's not longer circular.  These devices are linear, so they've come from experiments in particle physics using linear accelerators.  So the acceleration in this case is created by the electrons jiggling backwards and forwards using a magnetic structure so that you have a very energetic beam of electrons, it's linear and it goes into something which is called an undulator.  It's called an undulator because it undulates the electrons as it goes through it.  That is acceleration that produces the light, the coherent light that we're seeing.  It's exactly the same as the electrons oscillating backwards and forwards in the antenna in your radio transmitter or your radio receiver or your mobile phone, but in this case they're jiggling backwards a very small distance very quickly and that produces X-rays.  If you get the conditions just right, it produces coherent X-rays like a laser.

SHANE HUNTINGTON
Keith, how big physically are these X-ray lasers, these linear lasers that we're talking about?

KEITH NUGENT
Typically, I would say, they are between 500 metres and a kilometre long, so they're substantial pieces of infrastructure.

SHANE HUNTINGTON
I'm Shane Huntington and my guest today is Professor Keith Nugent.  We're talking about new ideas in X-ray imaging here on Up Close, coming to you from the University of Melbourne Australia.Keith, with the advent of an X-ray laser, do you still actually need to crystallise these materials that you're working on, or can you actually look at them without that process.

KEITH NUGENT
That of course is the key.  One of the really exciting things about having an X-ray laser is that possibly – and this is the dream – that we can determine, measure, the structure of proteins without having to form a crystal.  One of the key questions in structural biology, the biology of where you're looking at structure of proteins and molecules and so on, is how do you determine the structure of membrane proteins.  Now these are the building blocks of the cell wall.  So when you have a medical treatment and you take a drug, obviously the drug has to get inside the cell.  For it to be effective, it has to get across the cell wall which is transported across using these membrane proteins.  Membranes are two dimensional things, they're surfaces, they're not a chunk, they're not a crystal, they're a surface.  They're a two dimensional thing like a piece of Glad Wrap rather than a chunk of butter, if you like.  So what that means is that they're chemically designed, just through evolution, to not produce three dimensional crystal structures.  So it's almost impossible to get the membrane proteins to form crystals.  So you can't do crystallography with them.So how do you deal with that?  The X-ray laser is a solution for that, we hope.  Again, thinking back to a laser pointer.  The quantity of energy per second from a laser pointer is of the order of 1 milliwatt, 1/1000th of a watt.  You compare that with a light bulb in your room, that's about 100 watts.  So that's 100,000 times more powerful.  But, by the same token, you can look at a light bulb without damaging your eye.  Laser pointers are restricted devices because you can't look at them and you can't look directly down the beam, even though they're 100,000 times lower in power. The reason for that is that when the coherent light from a laser pointer enters your eye, because it's coherent, it gets focused to a very, very tiny spot which can damage your retina.  That's why these lasers are used in eye surgery and so on.  So it's not to do with the power, it's to do with the coherence of the source.In the case of an X-ray laser, exactly the same physics apply, so in this case we would take a beam, a coherent beam of X-rays from the X-ray laser and we would be able to focus it down to a tiny spot and we'll be able to put a single molecule in there and it will be so bright that we'll be able to measure diffraction from a single molecule.  From that, we hope to be able to determine the structure of that protein.  So it's a way of getting rid of the need to form crystals and that's a really exciting prospect because this is one of the great frontiers, if you like, of structural biology.

SHANE HUNTINGTON
When you talk about the coherence and the power of these X-ray lasers, are both required?  I guess what I'm asking is, this sounds like a destructive process, so you presumably have to do the imaging very, very quickly.  Is that one of the requirements of these new sources, to do it in a non-crystalline way?

KEITH NUGENT
Absolutely.  The power – and power, again, these are technical terms – but the key thing for imaging single molecules is you need a lot of light, a lot of X-rays, onto a single molecule to get significant scattering from.  These are tiny things and you need to have a lot of photons to scatter from them.  That, inevitably, destroys that molecule.  It's a huge amount of power, so the molecule just falls apart in a very, very tiny period of time, absolutely miniscule: 10 to the minus 15 of a second; very, very short period of time.  So you need to make sure that the pulse has entered, if you like, the molecule and diffracted from it before it falls apart. So they need to be very, very short pulses.The beauty of doing this with crystallography is that the molecules are identical, so you can drop an identical molecule into the beam repeatedly and get lots of measurements of it and from that, build up a picture.

SHANE HUNTINGTON
Keith we've generally constrained our conversation to brightness and intensity with regards to the X-rays with a little bit on the coherence, but one of the other important aspects of these light sources is the phase.  Can you talk us through what is meant by the phase of one of these light sources and how we can use that?

KEITH NUGENT
The phase is an interesting concept in this context.  If we go back to our previous metaphor of a band of soldiers walking along the road and marching in step, that's a coherent beam, if you like, of soldiers and the period of the footsteps is the frequency, which terms the energy of the photons and the fact that they're in step means that they're coherent.  However you could take another set of soldiers marching alongside the other bunch, so we have two groups of soldiers, the same frequency, they're marching at the same speed and they're marching in the same direction, but their footsteps aren't landing with each other.  So you could have one set of footsteps from the band number one and then band number two could be a couple of seconds later, or a fraction of a second later.  They have a different phase.  So that's what we mean by phase; it's a third quantity of light.

SHANE HUNTINGTON
So if we were to look at a typical medical X-ray to return to that particular item everyone knows so much about, I assume we're looking at intensity maps.  What would we get if were to look at a phase map of the body in the same way with X-rays?

KEITH NUGENT
Some of the work that I was doing some years ago now and we continue to do this, is looking at the idea of how can you measure the phase of an X-ray, so in fact you can do it.  Thinking again a little bit about phase, if you think about what does a lens do to light, an analogy I often like is, let's say, looking at a swimming pool on a sunny day and you look at the bottom of the swimming pool and you will all have seen the characteristic bright stripes along the bottom of that swimming pool.  What is that? It's not changing the colour of the light. The water is transparent, so it's not changing the intensity.  What it is doing is changing the phase of the light as it comes through the surface of the swimming pool.  Essentially it's the same as refraction in this context, but it's often thought of in terms of phase; the two are closely related concepts.It turns out you can do that with X-rays as well.  So if you were to shine a beam of X-rays at a medical patient, for example, then as I said before, there is a small effect of refraction as the X-rays go through that was, if you remember we were talking about the possibility of building an X-ray lens, so there is a small effect there.  It was discovered, again about 10 or 15 years ago that with modern synchrotronics resources, you can measure the phase effect of the X-rays going through the sample.  It opens up a whole new way of doing X-ray imaging. It's really been quite exciting.

SHANE HUNTINGTON
What sort of information does it actually give you?  I mean medical X-rays seem to be a fairly broad brushstroke as an image.  It's either light or it's dark.  What would phase actually tell you about the body if you could accurately measure the phase of the X-rays going through?

KEITH NUGENT
Well one of the beauties of it is that you get to see much more subtle effects.  One of the things that's been really very interesting for us to see with phase imaging is the ability, for example, to see airways.  There's some beautiful work being done by colleagues at Monash University where they look at the aeration of a newborn rabbit.  In other words, if you have a baby, let's say, that's been born, obviously its lungs are full of amniotic fluid, how does it go through the process of expelling that amniotic fluid on birth?  It has to get air into its lungs.  So with phase contrast imaging, we can see the air going into the lungs, so you can understand that process much more deeply than was previously possible.  You can't do that with conventional absorption X-ray imaging, but you can do it with what we've called phase contrast imaging.Another example, which I think is really exciting, is looking at fossils embedded in amber.  Again, you have a relatively small change between the amber and a fossilised insect that might be in there.  Amber in its natural state is not transparent, so you can't see it, but if you use phase contrast imaging, you can see the insects in the amber and in fact you can reconstruct them in three dimensions and you get these spectacular images of these insects buried in this rock.  It's opened up a whole new perspective on palaeontology.  So this is really a new window in the way that you can see with X-rays.

SHANE HUNTINGTON
Keith just to finish up, we've talked about X-rays in general, we've talked about crystallography, we've talked about X-ray lasers and how to take the coherent elements of that and then use those in phase imaging and the like.  With all of this together, what sort of big things do you see on the horizon for X-ray imaging coming up in the next five to 10 years?

KEITH NUGENT
I think the really exciting work is going to be what I would call dynamics, that is, we now believe that we can see, or will shortly be able to see molecules in their natural state, non-crystalline, at high resolution.  But we're doing this with very, very short pulses; so the pulses of X-rays in the order of 10 to the minus 15 of a second; very, very short.  It should be possible, with those time scales and with that level of precision at atomic level imaging to see molecules working, seeing chemical reactions happening, maybe looking at proteins, docking with other proteins, transporting across a cell wall.So I think the really exciting work over the next five to 10 years will be that – seeing atoms doing their work in the biological context.  That's going to happen and it will be really exciting when it does.

SHANE HUNTINGTON
Professor Keith Nugent from the University of Melbourne, thank you for being our guest on Up Close today and giving us an understanding of what's called coherent X-ray imaging.

KEITH NUGENT
A pleasure, thank you.

SHANE HUNTINGTON
Relevant links, a full transcript and more info on this episode can be found at our website at upclose.unimelb.edu.au.  Up Close is a production of the University of Melbourne, Australia.  This episode was recorded on the 18th of August 2011.  Our producers for this episode were Kelvin Param and Eric van Bemmel; audio engineering by Gavin Nebauer; background research by Dyani Lewis.  Up Close is created by Eric van Bemmel and Kelvin Param.  I'm Shane Huntington, until next time, good bye.

VOICEOVER
You've been listening to Up Close. For more information, visit upclose.unimelb.edu.au. © 2011 The University of Melbourne.

 

 


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