String cannot be played or plucked when it’s coiled on the floor.
But when it’s tightly stretched between two points – like on this guitar, then it can be plucked and will sign with a certain note. The exact note will depend on the tension in the string, the thickness of the string and the length. On a guitar the length can be altered by moving your fingers along the fret board.
If you play two or more notes at once it’s called a chord. Working out which notes are in a chord could be tricky since they blend together. Fortunately there is a clever piece of maths called a Fourier Transform that will separate two notes for you.
It doesn’t matter if you’re not good at maths. Although a Fourier Transform looks scary, it turns out to be an easy thing to get a computer to do - so we never have to even understand the symbols. So, if we record the chord we will get a graph that looks like this:
The computer can then use the Fourier Transform to do some work we can get the individual notes back. By feeding the recorded note into a Fourrier Transform we find out that three notes are being played at the same time.
One is a low note (5 kHz), one is in the middle (10 kHz) and the third is right at the top end of the human hearing range (20 kHz).
If we know how thick the string is and how tightly it’s wound up, then we should know where the fingers were on the fret-board of the guitar. We should be able to do this for lots of strings, even if there was a whole choir of strings singing.
It turns out that some atoms are like strings. In MRI the most important atoms are hydrogen. There are two hydrogen atoms in water (the white spheres in the picture below) and your body is 70 % water. If water molecules are placed in a magnetic field the hydrogen atoms get wound up – like a string on a guitar. Just like a string, they can be played – not with your fingers of course. The atoms are played with a radio-pulse and will sing in response – just like a string, but they sing in radio-pulses - just like the local radio-station.
Remember we said that if you knew the thickness of the string and knew how tightly it was wound then we would know where the fingers were on the fret board? Well we know which atom is singing (how thick the string is) and we know how tightly we’ve wound it (the magnetic field strength). So – we should be able to work out something about the hydrogen atom’s environment (where the fingers are on the fret board). In fact the environment depends on which organ the water is in. So, by listening to the song the water molecules are singing, separating the notes and listening from lots of different angles, we can build a picture. A 3d picture of the water in your body. The image below left is an example. It looks pretty amazing, but if you zoom in on the stomach area you get the image below right.
You can see that although the liver is basically clear – there is not a lot of detail. The problem is that your body is 70% water, so different organs look similar. If we inject the patient with a small amount of a rare metal called gadolinium, then the metal changes the environment dramatically. The biggest change will be in the blood vessels. The image below shows the liver with the blood vessels containing gadolinium. You can see most of the big vessels quite clearly. This can be very useful in heart attacks or strokes, when a critical blood vessel is blocked. It can also be useful in cancer, where the blood vessels are often rather leaky.
Some of our scientists are very interested in MRI.
Now find out about the other imaging techniques.