The document discusses the physical principles behind signal generation in magnetic resonance imaging (MRI). It describes how MRI signals are generated through the transverse relaxation time (T2) of protons' spins after excitation by radio frequency pulses in the presence of magnetic fields. T2 relates to how quickly the signal decays and is affected by magnetic inhomogeneities, with T2* representing a faster decay time than T2. Refocusing pulses are used to recover signal loss and measure T2.
40. nuclei, protons and spins longitudinal relaxation T1 transverse relaxation T2 excitation and relaxation relation between T1 and T2 signal generation
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THIS IS THE
Transverse relaxation is also called Spin-Spin relaxation. The speed of rotation is influenced by the neighbouring protons. Compare with walking in a shopping mall on a Saturday afternoon. While walking you have to change your speed and direction constantly to avoid collisions whith other shoppers. The only difference is that protons do collide.
When we look at one single proton, we can see that it makes one 360 degree rotation in a certain time...
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... But in the same time the red proton has covered more than 360 degrees, and the blue proton less.
After the next rotation these differences are bigger.
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Back to the first rotation and look at the signals that belong to the protons..
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Now we add the summed signal, and you can see that the signal is getting weaker the more the protons are pointing in different directions.
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The signal shift relative to each other, cancelling each other out. This is called dephasing.
The signal that is generated after the 90 degree pulse is called FID or T2*.
And is different for the various tissues. The time untill 63% of the signal has ´´died´´ is called the T2 time.
And is different for the various tissues. The time untill 63% of the signal has ´´died´´ is called the T2 time.
At the dawn of MR this signal could not be measured, because the hardware did not allow the fast switching. So we had to find a trick to overcome this problem.
For that we need the help of the transmit coil again.
When we transmit a signal for so long time that the protons are flipped over 180 degrees, look what happens..
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The slow protons will be overtaken by the faster ones, thus generating a new strong signal.
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In other words. The signal is restored with the help of a 180 degree pulse, resulting in a strong echo.
Thats why we call it a refocussing pulse.
As you saw, after the rephasing of the protons they will dephase again. And again we can apply a 180 degree pulse in order to rephase them, resuting in new signal or echo...
The curve that we can draw through the tops of the echos is called the T2-envelope.
