2. nuclei, protons and spins signal generation longitudinal relaxation T1 transverse relaxation T2 excitation and relaxation relation between T1 and T2
3. nuclei, protons and spins signal generation longitudinal relaxation T1 transverse relaxation T2 exitation and relaxation relation between T1 and T2
4. ATOM nucleus proton positive charge neutron no charge electron negative charge hydrogen atom proton signal generation nuclei, protons and spins
39. nuclei, protons and spins signal generation longitudinal relaxation T1 transverse relaxation T2 excitation and relaxation relation between T1 and T2
Notes de l'éditeur
THIS IS THE
I will discuss these topics with you.
First...
The core of an atom is called the nucleus. The nucleus exists of neutrons and protons. Neutrons do not have an electrical charge, but protons do. Around this nucleus electrons are spinning. Electrons posses a negative electrical charge. In MRI we use the water atom as the source of our signals. First of all because it is the most abundant element in the Human Body, and, secondly, because it has only one electron around the nucleus. The more electrons around the nucleus, the more difficult it is to excite the nucleus or, in other words, the more the nucleus is ´´shielded´´.
The smallest part of the water atom is the proton The proton has a positive electrical charge....
...and it rotates around it‘s own axis. electric charge and rotation generate magnetism. Compare with electromotor. Electricity and magnets generate rotation!
So we can state that protons behave like little magnets.
In the body there are many protons. When we put those protons in a magnetic field....
...they will orient themselves into the direction of this magnetic field. When we look at the coordinate system to the right, we can say that the summed magnetisation is building up in the Z-direction.
...
...
However... If the protons are at a temperature close to the absolute 0 point, all protons will align.
But the reality is that we work at room temperature (or body temperature). In that case at a fieldstrenghth of 0.5 T only 5 protons per million will align. At 1.5 T, 15 protons will align. This explains why a higher fieldstrength has a better signal to noise ratio than a lower fieldstrength..
All these protons together can be represented as...
..one big proton.
This rotation is called precession or Larmor frequency, after professor Larmor, who lived from....till..... and described...
This simple formula (don‘t worry, it‘s the only one I‘m showing) shows that the frequency of rotation of the protons depend on the fieldstrength we are working with. The higher the fieldstrength, the higher the frequency.
In this table the frequencies for the different elements are shown, relative to the applied fieldstrength. As you can see it‘s a linear function.
When we look at the relative sensitivity of the different elements, and take hydrogen as 100%, we can see that other elements are less sensitive to magnetisation. Another reason why we use the water atom as our source for imaging.
