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Physics SectionSo how does that MRI machine work? There are plenty of books out there that will enlighten you on the theory behind MR image. Each scanner is a little bit different and each new sequence is a variation of one of few themes. Most of the MRI sequences are based on basic acquisitions such as spin echo or gradient echo techniques. There are also fancy new sequences, which exploit movement of protons in the blood stream and, at times, even artifacts to the advantage of highlighting a particular property of a given tissue. Below, we attempt to explain physics of MRI in the simplest possible terms. If you have ever taken college level physics and chemistry courses, you should be able to follow along with ease.
One fundamental principle required to understand MR imaging is the concept of electromagentic energy. Radio receiver in your car accepts radiofrequency (RF) waves, which represent one form of electromagnetic energy. Similarly, protons in living tissues are able to accept electromagentic energy in the form of RF pulse. This RF pulse "excites" the protons to a higher energy level. As the protons "relax", energy is emitted back to the antennas in MR machines.
Unfortunatelly, the amount of energy that is emitted back from the protons is very small. So a few tricks are performed to maximize signal to noise ratio.
Each proton in the body acts like a small magnet with its own magnetic field. While each proton can receive and emit back the RF energy, any sort of coherent signal generated by such experiment is promptly cancelled out by neighbouring protons. One analogy is a room full of people talking at the same time. While each individual person is transmitting meaningful information, the sound of entire room makes absolutely no sense.
So, the first trick is to get the protons to talk in unison with each other. When placed into external magnetic field, protons align parallel to it. There is almost an equal chance that a proton orients itself at 0 or at 180 degrees to the main magnetic field. The keyword here is "almost," because fortunatelly for us, a few more protons align with the magnetic field rather than 180 degrees to it. (Explanation for this phenomenon requires understading of quantum physics.) These extra few protons (actually quite a lot of them) create a net magnetic field that we can now manipulate. The number of protons aligned with the magnetic field varies with strength of the magnet. Utilization of a 3T magnet results in much better signal-to-noise ratio compared to a 0.5T magnet.
Next point is to get the protons to emit enough energy for the MR antennas to pick it up. Unlike static pictures on this page, each proton actually rotates around its axis (precesses) like a wobble. Turns out that the signal generated by each rotating proton is much stronger if we make the protons precess in unison with each other at 90 degrees to the main magnetic field. RF pulse does just that.
The signal generated by protons precessing in unison and at 90 degrees to main magnetic field is the strongest signal we get. However, 90 degree rotating frame is a high energy state, in which protons will not stay forever. There are two types of relaxation. First, there is relaxation to align back with the main magnetic field (T1 relaxation), Second, there is dephasing in the transverse plane (90 degree plane). Each individual proton precesses at slightly different speed. After a while, the signal from protons in transverse plane degenerates as protons start precessing out of phase with each other (T2 relaxation).
It takes different amount of time to loose strength of the signal from T1 and T2 relaxations. By manipulating manner and frequency in which we apply RF pulses (TR), and by changing time to start of signal acquisition after RF has been applied (TE), we are able to produce T1-weighted or T2-weighted images.
We can also manipulate the stregth of the magnetic field and properties of RF pulses to create predictable variations in the body so that complex signals returned to the antennas contain positional information.
We are not quite at the point where MRI can substitute for direct tissue evaluation. Slowly, but surely the resolution of the image is getting improved, however. Anatomical and functional information that is transformed into images is becoming crisper by the day, if not by the hour.
This overly abbreviated introduction most likely leaves you with more questions than answers. A simple internet search for other sites dealing with MRI education will produce a number of links to very good websites put together by physicists and others. Dr. Hornak's website is one such example. If you like question and answer type of approach to learning, check out ReviseMRI.com