Science In Real Life

Last time, on a very special Science In Real Life …

“But doctor! What will we do?”
“Stand back! I’m going to magnetically resonate his nuclei!”

And now, the conclusion … wherein we discuss two methods of imaging the brain that I didn’t have room for in the previous essay. Welcome to SiRL, where we have more surprise twins than a fertility clinic. Now, some of you will remember all the way back to the very first thing that was ever posted to this blog. The very first Science Fact of the Day: The electrical activity in your brain, i.e. your thoughts, constantly produces measurable magnetic fields outside your head. This phenomenon is, dare I say it, the brainchild behind imaging methods known as MEG and EEG, Magneto- and ElectroEncephaloGraph, respectively.

Let’s go back to basics. What is the brain? Putting aside for now the much broader question of how the brain does all those wonderful things it does, the relevant description for our purposes is a big messy jumble of very tiny wires that each carry pulses of electrical current. Like a telephone switchboard with thoughts, ideas, and concepts whizzing every which way. So now we’ve got memories flying everywhere, passing each other. The electric field is like AAAAAHH – which is to say it’s complicated. This is no exaggeration. You have neurons firing signals in all directions. You might then think that measuring the brain’s activity is hopeless, since one neuron’s signal is far too miniscule to measure. Keep that defeatist attitude out of the lab, buddy. We’re trying to do science here.

It so happens that, given the tens of billions of neurons in your brain, and hundreds of trillions of connections between them, that groups of signals heading in more or less the same direction spontaneously form like mobs of children heading for the ice cream truck. This analogy becomes especially apt when one considers that different regions of the brain correspond to different processing subroutines at our disposal. For example, if the electrical impulses currently comprising your consciousness all decide that it’s a really good idea to pack the kids into the car, stop the mail delivery, and head for scenic Brodmann area 4, it’s a good bet you’re about to move some part of your body, since that’s the motor cortex. So what the EEG does is detect these (relatively) massive throngs of collected neural impulses.

How? It’s all about fields. The business end of the EEG device is a collection of metal electrodes attached by wires to an amplifier and a computer. One of the electrodes is placed apart from the others and used as a base reference value. The sensor watches for a difference between any of the other electrodes and the reference, like you might watch for a flash of green and brown while playing Duck Hunt. The amplifier boosts the signal and you’re in business. The advantage to using an EEG is its amazing temporal accuracy, but what you gain there you lose in spatial accuracy. Since a massive convocation of neurons is required for a signal, the EEG cannot pinpoint exactly where it is happening.

Another disadvantage is a directional dependence. Imagine we divide all the neurons into two groups: those that aim signals along the surface of your skull (tangential), and those that aim signals outwards or inwards (radial). Like a biased reporter, EEG is much better at detecting the latter than the former. Enter MEG. By measuring the magnetic fields resulting from firing neurons, we can focus on the tangential signals. In addition, due to the properties of skull and skin, MEG measurements are less distorted and more spatially localized. So what we have are Jack Sprat and his wife – by using them simultaneously we can lick the plate clean and measure both kinds of signals with increased precision.

Now, the really cool part is that in order to design an instrument sensitive enough to detect the brain’s magnetic fields, we have to use a SQUID. Put the tentacles away – SQUID stands for Superconducting QUantum Interference Device. Instead of plain vanilla electrodes, we now have paired loops of superconductors (superconductivity is an inherently quantum mechanical process). When one of your idle musings sends out a magnetic field, it disrupts the balance between the two loops, producing an electrical voltage. The reason this is measurable is due to the dramatic difference between superconducting and not superconducting. The tiniest push across that critical threshold is enough to reorganize the entire electromagnetic structure of the sensor loop, like a librarian who finally sees one book too many out of place.

Of course some of the imaging methods I discussed in the previous essay can also be used to measure the brain, certain types of MRI in particular. Often an EEG and specialized MRI scan are performed simultaneously to better suck the patient’s brain dry of tasty, tasty information. Yum.

Someone who was probably not Albert Einstein once said, “Only two things are infinite, the Universe and human stupidity, and I’m not sure about the former.” To that list I would like to add human curiosity. We have a compulsion to look inside things and see what is going on. It has resulted in every exploration from highest mountain to deepest sea trench, it has resulted in all of modern science, it has resulted in this blog, but most saliently when turned on ourselves it has resulted in a group of technologies collectively known as medical imaging. As recently as 150 years ago the only way to view the inner workings of a human body was to cut one open. Now we have x-ray imagers, optical tomography, ultrasound, MRIs, and PETs, to name a few.

Late in the 19th century, on several fateful days in various laboratories scattered across Europe and America like so many chocolate chips in the dough of civilization, X-rays were first observed as high energy emissions from vacuum tubes. They are, in fact, ordinary light rays that happen to have a rather high energy compared to the light we can see. It is this property that renders most ordinary matter transparent to them, including the organic tissue of which we are all composed. Within months of Röntgen publishing the first detailed paper, scientists at Dartmouth College made the first medical x-ray image. The reason it works is that different tissues have different densities, and therefore block more or less of the x-ray light. Bone shows up very well because it blocks far more than surrounding tissue, but variations in soft tissue can also be used for diagnostic purposes. Unfortunately we still cannot detect susceptibility to showtunes.

Now, we’ve all heard of a CAT scan, even if only in the context of that joke about a veterinarian who charges $500 for visually examining a feline. Also known as a CT scan, the C and T stand for Computed and Tomography (the A, if present, stands for Axial). This is an x-ray image on steroids. A movable x-ray scanner creates many 2-D images of the patient at multiple angles, and computer analysis reconstructs this into a 3-D image. This can also be done with visible light, in a pulsed-light process so far into the red end of the spectrum the inside of your body resembles the bridge of the Enterprise when the Klingons are attacking. Optical tomography has found uses studying, as one article put it, Boobs, Brains, and Blood. How poetic.

The physics of ultrasound imaging is beautiful and complex, and properly ought to be given an entire book, not a paragraph in an essay; that will have to wait until I have a deal with a publisher. For now, the basic idea is the same principle the police use to catch you speeding – waves reflect at boundaries. A transducer sends a high frequency sound wave into the body and listens for echoes. Every echo indicates a boundary layer between two different tissues. From the time elapsed the depth is determined, from the strength their relative densities. Furthermore, by measuring the frequency shift in the sound wave, motion can be measured as well. By concatenating one- and two-dimensional images, real-time three-dimensional images of a fetus or a beating heart can be displayed in sonic glory for all to see. Hallelujah!

MRI, I think, has one of the highest usage to explanation ratios of all the acronyms in our culture. Based as it is, and thoroughly so, on quantum mechanics, this is hardly surprising. MRI means Magnetic Resonance Imaging. The things doing the magnetic resonating are the nuclei of atoms in your own body. Here’s how it goes down – you slide into that giant magnetic donut and instantly the protons in your hydrogen atoms (the human body is mostly water, remember) align with the massive magnetic field. Now, the magnetic field is like a political idealist – the protons have different energies depending on whether they aligned with or against it.

It so happens that for hydrogen, the energy required to flip them into the high energy state corresponds to a radio wave of some frequency. This is the resonance part – by providing an electromagnetic field oscillating just right, we bump the protons up. Then, when they relax again, they emit radio waves. The exact frequency at which they emit depends on the chemical properties of the molecules the protons are sitting in. So by observing how the protons across the body relax, we can form an image of how they spend their disposable income. Or, even better, the precise layout of the inside of your body. As a diagnostic tool, the MRI is extremely flexible – it is able to observe a wide range of tissue types with astounding precision; it has therefore become extremely common.

All of these methods so far have relied on sending some signal inside and observing what comes out. What if the human body could directly emit something that would provide an image? Well, it can, as long as you ingest a radioactive tracer substance. There are certain radioactive isotopes that emit positrons – that’s right, antimatter. When one of those bad boys gets loose in your colon (or wherever) it doesn’t get very far before it annihilates with an electron, sending light in two directions. Cameras pick up the light and they know where the tracer is. All you have to do is drink it down and you’re ready for your closeup. Welcome to Positron Emission Tomography. One of the attractive features of PET is that the tracer can be attached to a biologically specific molecule, for example something that might only bind to cancer cells, or red blood cells. This allows doctors to observe specific processes with uncanny precision.

If this were a lecture I would already be over time. That, in a nutshell, are the most common diagnostic imaging tools used by doctors today – second to, of course, a good old look-see. Tune in next time for a couple of brainy twins named MEG and EEG.