Lecture 2: MRI is a party that only active nuclei are invited!!!

In my last post, I talked about how important hydrogen is MRI.  I am tempted to go right into explaining this further. But let’s slow it down for now and talk about a more boring topic: Atoms. Some of us found secondary school physics/ chemistry very interesting but I remember being told in junior science class that atoms were the smaller indivisible particles until someone showed up in senior science class to say atoms were made of protons, electrons and neutrons. They even said stuffs like nucleons, isotopes, mass numbers, atomic number and all those boring stuffs. Some people are lucky they don’t have to worry about these stuffs for the rest of their adult lives. But for us who are luckier, we must understand these things, so we don’t get embarrassed in our professions.  

The truth is, understanding how MRI works is based on an understanding of the basic behaviours of atoms. I am going to try to explain this but again only as simply as I understand them because I am no expert in quantum physics myself.  Atoms are made up of protons, electrons and neutrons. In the actual sense of its seeming existence, atom is made up of a nucleus with an electron or electrons revolving around it. The nucleus consists of the nucleons (which is protons and neutrons). These are pretty much basic concepts in science but very important. Moving on; the nucleon determines the mass number of the atom which is essentially the number of protons and neutrons. The number is protons in the atom is the atomic number and this is another important concept in basic science. As a standard, the number of protons and neutrons are almost always the same number meaning mass numbers are almost always even. Nuclei with old mass numbers are the ones we need in MRI. These nuclei are isotopes. Let’s not go into discussing them in detail but you need to note that isotopes are variants of the same atom with same number of protons (atomic number) but with a different mass number ( the number of protons and neutrons are not the same).

Another important sub-particle of the atom is the electrons. But for the sake of avoiding ambiguity, we are going to pretend as if electrons don't exist but I will mention a few things that are unavoidable. For example, electrons are negatively charged and protons positively charged while neutrons have no charge or are neutral just as they sound. An atom is said to be electrically neutral if the number of electrons equals the number of protons as these charges cancel out making the atom neutral. Electrons are located more outermost and are easier to be knocked off. This gives you an ion which is only but a charged atom with more or less proton or electron. Now I guess you will be wondering why and how these relates to MRI. I promise you that I will get there maybe not sooner but definitely later and these will all make sense. But for now, pretend that this is a lecture on basic quantum physics.

The protons and neutrons in the nucleus turn around and around very quickly individually and in their own axis but in opposite directions. This “turn around” movement is called a spin in physics and axis is just an imaginary or real path that this movement occurs. So from now on, whenever I will use the words “spin” (and I will use a lot of it for as long as the discussion is about MRI), please don’t be confused, its just a turn around movement. As I mentioned before, protons and neutrons make up the nucleus and the spinning of the protons and neutrons determine the spinning of the nucleus in a way I am going to explain. The proton spins in one direction and the neutron spins in another direction. Where the number of protons equals the number of neutrons, their spins in opposite direction cancel out each other. With this, the nucleus will appear not to spin in any direction. Remember again that I mentioned earlier that the nuclei with old mass number are the ones we use in MRI. And you haven’t forgotten that the number of protons and neutrons make up the mass number. So to have an odd number, we will need to have different number of protons and neutrons. For example, if we have 2 protons and 2 neutrons, we are going to end up with a mass number of 4 or 3 protons and 3 neutrons will give a mass number of 6. So if you add the same numbers of protons and neutrons, you will get an even mass number. If you add 3 protons and 2 neutrons, you get a mass number of 5. That means if the number of protons and neutrons are different, you might get an odd mass number. This is as simple as it gets. Now remember I said protons and neutrons spin in different direction and if their numbers equal each other, they cancel out each other and the nucleus appears not to spin in any direction. Again, its like a thug of war between two groups that have exactly the same strength; they are stuck in a fixed position. No group moves towards the other. Imagine if one group has more strength, they pull the rival group to their direction. This is exactly what happens when you have an odd mass number where the number of protons is different from the number of the neutron. Their spins do not cancel out but rather, you minus them out and what remains creates what is called a net angular momentum (this simply means movement in a known direction). With this, the nucleus has a spin in a direction and is described as MR active nuclei.   Does this now make sense? Knowing how an MR active nuclei comes about is very important.

Now that we have established a nucleus that spins (don’t forget, spinning is a form of movement), let me remind us of faraday’s law of electromagnetic induction. That law relates to motion, charge and magnetism and states that if you have any of the 2 present, the third is induced. So if you have a charge and motion, magnetism is induced or if you have magnetism and charge, motion is induced or if you have magnetism and motion, charge is induced. Now back to our nucleus. Remember the proton has a positive charge and the neutron has no charge meaning the nucleus has a positive charge. Also remember that an MR active nucleus has motion (it spins). Now we have a charge, and motion and if we obey faraday’s law, we will have an induced magnetism. So MR active nuclei is a charged nucleus that spins in its own axis and has a magnetic field induced around it. This induced magnetic is called a magnetic moment as it has a direction. 

In my previous post, I mentioned that hydrogen interacts with magnetization. I am going to make a connection between an MR active nuclei and hydrogen. But can I ask you to put the next thing I will say in a corner of your brain where you won’t forget. When you place the MR active nuclei in an external magnetic field, it will interact with the external magnetic field because it is a magnet on its own. This interaction is called alignment or simply put, when the MR active is placed in an external magnetic field, it aligns (interacts) with the external magnetic field. This alignment is perhaps the most important phenomenon in MRI. I am sure at this point you are wondering “ How the heck does all these concern hydrogen?”. Now let get hydrogen involved.

When I started this post, I didn’t mention anything about elements. Does anyone still remember a thing or two in high school chemistry? If you do, then you will agree that an element is substance that contains only one type of atom. This means that an element is just one atom. This also means that hydrogen being an element is an atom. So everything we have described about an atom above applies to hydrogen. Remember that an atom is made up of a nucleus and electron(s). If we ignore the electron(s) and focus on the nucleus, that means hydrogen is a nucleus. But that doesn’t make it an MR active nuclei yet. We talked about how an MR active nucleus has to have an odd mass number. Does anyone still recall when I mentioned isotopes which are a variant of an atom with same number of proton but different number of neutrons. Isotopes are the only instance where an odd mass number is possible.  In the case of hydrogen, the isotope of hydrogen called protium has 1 proton and zero neutron and hence the mass number is 1. This is the most abundant form of hydrogen in the body. Now remember everything we said about the behaviour of nuclei with odd number and apply all of those to hydrogen. This makes hydrogen an MR active nucleus. By the way, nuclei is the plural form of nucleus. Hydrogen is not the only nucleus capable of being MR active. I have managed to explain how hydrogen is an MR active nucleus. If you understand this, you are going to find the next post easier to understand because I will discuss how hydrogen is isolated from other MR active nuclei in the body for MRI.