Lecture 3: MRI...How a gyrating hydrogen proton changed the world

While the world is being scared to bits with the invasion of our peace by corona virus, a discussion on MRI could be a valuable distraction. The last time we discussed, we looked at a few things in quantum physics. It is very important that we understand the basic behaviour of atoms and this will help us understand their role in forming the images generated during an MR scan. I managed to explain that hydrogen as a proton is an active MR nuclei and that if you put these active nuclei in an external magnetic field and apply a radiofrequency pulse, there could be interactions. These interactions are a prerequisite for image formation in MRI. Please note that an external magnetic field (in an MRI discussion) simply means the magnetic field that the MRI scanner produces. In this post, I have 2 main objective

1.       To explain how hydrogen is selectively utilized in MRI away from other active MRI nuclei in the body. Remember I mentioned that other active nuclei are (or could be) present in the body and can also interact with the external magnetic field and radiofrequency in likewise manner.

2.       To explain how the hydrogen active MR nuclei interacts with the external magnetic field and radio frequency.

Now let us look at what it is about hydrogen that makes it very important and useful in MRI. Aside the physical properties of hydrogen that makes it an active MR nuclei, hydrogen is very abundant in the body and is found in both water(H₂O) and fat (COOH). And as I explained earlier in my first post, every tissue in the human can literally either be water or fat. This is so say that the tissues either predominately contains water or fat. For the purpose of this discussion, it is enough to just know that every tissue in the human body will contain hydrogen either in the form of water or fat or a mix of both. This also applies to diseases where there are significant changes in the water and/ or fat composition or content and this is mostly the basis of diagnoses. You might be wondering how MRI differentiates between hydrogen proton (active MR nuclei) in water and fat in making images. To be able to explain how hydrogen active MR nuclei are selectively used in MRI, I will have to use what is called “Larmor Equation”. Please forgive me in advance but this discussion will get technical at some point but I promise to keep it very simple. Just as simple as I can.

ω = γB_o

where

ω is the Larmor frequency in MHz,

γ is the gyromagnetic ratio in MHz/tesla

B_o is the strength of the external magnetic field in tesla

This equation is very important if not the most important in MRI. You will understand this equation when we discus the interaction of active MR nuclei with external magnetic field. But I mentioned it now just to pick out the gyromagnetic ratio. Gyromagnetic ratio is the only factor that uniquely separate hydrogen MR active nuclei from other and you will soon learn why. Now what is this ratio? Gyromagnetic ratio is of a particle or atom (in this case hydrogen) is the ratio of its magnetic moment to its angular momentum. Now this is getting very technical but let me explain it in simple terms. Remember we talked about how hydrogen protons possess a charge and motion (called spin) and hence have a magnetic field or simply exist as magnets. Now magnetic moment is simply how strong the magnets of the hydrogen proton are (measured in Tesla) and angular momentum is how fast the proton(s) are moving in a known circular axis/path (measured in radians per sec or MegaHertz). If you divide the angular momentum with the magnetic moment, you will get gyromagnetic ratio measured in radians per second per Tesla or MegaHertz per Tesla . Gyromagnetic ratio are different for different active MR nuclei and Hydrogen has a unique gyromagnetic ratio (42.58 MHz/T) that no other active MR nuclei can have. This will make more sense when I explain the interaction of active MR nuclei with magnetic field and radiofrequency.

Now let’s look at the interaction of hydrogen active MR nuclei with external magnetic field. Remember we already described the hydrogen atom as an active MR nuclei spinning in their axis and existing as individual magnets. Magnetic moment is a term used to describe the strength of those magnets. The magnetic moments of the active MR nuclei are positioned randomly when outside of the magnetic field. But once inside the magnetic field, the magnetic moments of the protons (active MR nuclei)align with the external magnetic field.  We mentioned earlier that because protons which have a positive charge spin (move around in a circular path), they behave like magnets and how strong these magnets are, is described as magnetic moments.  Remember there are many hydrogen protons in the human body and Its logical to assume that these protons spin at different velocities and if the magnetic moments resulting from those spins are dependent on velocity, this means that the magnetic moments of the protons have different energies. When aligning with the external magnetic field, the magnetic moments with energies that are higher than the external magnetic field are the high energy spin-down or antiparallel nuclei and the ones with lower energy are the low  energy spin-up or parallel nuclei. Conventionally, there are fewer high energy spin down nuclei than low energy spin up nuclei and when you minus the spin down nuclei from the spin up nuclei you get a net magnetic moment called net magnetic vector (NMV). The NMV is an important quantity in MRI. Please remember how we arrived at it. You probably have been told that a 3Tesla MRI scanner is better that a 1.5Tesla scanner. The reason is simple. A 3Tesla will mean that there are a lot fewer high energy spin down magnetic moments aligning antiparallel to the 3Tesla external magnetic field than the low energy spin up magnetic moments. Don’t forget, a magnetic moment is high energy if its energy is more than that of the external field. This means a higher field strength (3 Tesla) will have lesser high energy antiparallel magnetic moments than a 1.5 Tesla scanner. And logically, if you have fewer antiparallel magnetic moments and more parallel magnetic moments, you will have a higher net magnetic vector when you minus the antiparallel from the parallel. A higher NMV gives more signals than a lower NMV and you will learn why and how as we progress.

Another phenomenon that happens as a result of interaction of active MR nuclei with external magnetic field is precession. Remember we said the active MR nuclei spin around their axes. In addition to this spinning, when put in an external magnetic field, the nuclei also spin or precess around the axis of the external magnetic field. This extra spin is called precession and it’s a very important phenomenon in MRI. So there is now two spinning movements; one around the axis of the nucleus and the other (precession) around the axis of the external magnetic field

Let’s look at this equation  again

ω = γB_o

And remember that ω is the Larmor frequency in MHz; γ is the gyromagnetic ratio in MHz/tesla  and B_o is the strength of the external magnetic field in tesla. The equation implies that the frequency at which the protons (active MR nuclei) precess is proportional to the external magnetic field strength and the gyromagnetic ratio. We already said gyromagnetic ratio is a constant quantity unique to each proton and this means that a stronger magnetic field will make the protons precess faster. You will learn the significance of this as you progress in MRI  

Larmor frequency can also be called precessional frequency and is a measure of how fast the nuclei spin around the external magnetic field. From the equation, the frequency is a product of the gyromagnetic ratio and external field strength. Throughout your encounter with MRI, you will hear a lot of mention of Larmor (precessional) frequency and the reason will become obvious as you progress but it is very important to note that while Larmor frequency can change depending on the magnetic field strength, the gyromagnetic ratio is a constant for different nuclei and is the basis for selectively using hydrogen nuclei in MRI.

To have an interaction with a radiofrequency, the radiofrequency must match the Larmor frequency of hydrogen. When this condition is met, another phenomenon called Resonance occurs. Depending on what the external magnetic field strength is, the Larmor frequency of hydrogen varies. This resonance is very important for an MR image to be formed. The Larmor frequency of hydrogen in a known external magnetic field strength can be calculated since the gyromagnetic ratio of hydrogen is a constant and unique only to hydrogen. This also means that using this specific Larmor frequency, only the net magnetic vector of hydrogen is utilized.

Let recap all we have learnt so far. For images to form in MRI, there must be an interaction between the magnetic moments of the hydrogen protons (active MR nuclei) and the external magnetic field and a radiofrequency pulse. The external magnetic field creates an energy level difference resulting to a net magnetic vector and precession. On applying a radiofrequency matching the Larmor frequency of hydrogen resonance occurs. Resonance is so important in MRI that it deserves a dedicated lecture. And hence, I will leave this discussion here with a promise to talk about resonance and how the images in MRI are formed in my next post.

I hope you have enjoyed reading this. If you have any questions or contributions or suggestions, please leave a comment in the comment box.

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.  

Lecture1 MRI...You are either fat or you are water!!!

The most important reason why I am an MRI Radiographer today is that the first time I met an MRI scanner, it “wowed” me. I have been a cross sectional radiographer for about 8 years now and I must tell you that MRI still “wows” me. Throughout your career long encounter with MRI, it promises to keep "wowing" you because MRI is like a river of fascination that never runs dry.

I like to learn and share. And that has proven to be the best way to learn more and be better. I am not an expert in MRI. But I do know a few things that I hope could benefit those are new to MRI or struggling to get their feet in the practice. My blogs will be mostly beneficial to newbies in MRI or persons with growing interest in the practice. However, more experienced radiographers are not immune to learning from my blogs but I bet they will find my simple way of explaining what goes on with and in MRI very interesting.

It is often very difficult to explain MRI in very simple terms. I will try.  In doing so, I may dilute the information that I pass across. You will find this helpful as you read more, do more and learn more. My blog will be very helpful in understanding the basic physics behind MRI before you can appreciate it clinical application.


There are two very essential questions whose answers are very important in understanding the physics of MRI as a newbie.

1    1.    What are the compositions of the body that makes MRI possible?

2    2.    What are the interactions possible in MRI?

To answer the first question, the only two  compositions of the body that make MRI possible are fat and water. At this level, it is very important to note that anything you see on an MRI image is either fat or water or  a composition of both or none of the two. From basic biology, cells make up tissues which make up organs. For the purpose of your discussion, we can look at components of the body at the tissue level. So in essence, every tissue has either fat or water or a combination of both in unique proportions. Understanding MRI physics with its clinical application is very easy if we narrow down our dealings to tissues that are fat or water or a make of both or none at all. This is important because the primary essence of imaging is to differentiate tissues from one another. The very interesting thing about fat and water is that they both contain hydrogen particles and these hydrogen particles have a different physical property in fat or water that is exploited for the benefit of MRI . I won’t discuss the properties of hydrogen in this blog as this is an introductory one but in subsequent blogs, we will discuss those properties and it will become clearer how hydrogen play a major role in MR image formation. But it important to note that hydrogen is perhaps the single most important element in MRI.

To recap what we have said so far, every tissue has a composition of either fat or water or a combination of both that are unique to them and makes them different from other tissues on an MRI image. This also applies to diseases. A disease is differentiated from another or normal tissues on the basis of its unique composition of fat or water or a combination of both. As we go deeper into our learning, we will learn about MR signals and how they are generated and only then will we appreciate the significance of our narrowing this down to fat and water.


Now lets look at question number two. The interactions possible in MRI are magnetization and transfer of energy in the form of radiofrequency. This is possible by having a strong external magnetic field and a device capable of transmitting and receiving radiofrequencies. We are not going into this now. But also, it is important to note that these two interaction are only useful with the hydrogen particles in the body. We will be looking deeper into these interaction in subsequent blogs. But again, it is important to note that the interaction of hydrogen in water and fat with magnetization and radiofrequencies are different and this forms the basis of what I refer to as differential MR signal formation.


As a brief summary, when you make a strong magnetic field, put the body under the magnetic field, the hydrogen in the tissues in the body interact with the magnetic field. When radiofrequency pulses are applied, another form of interaction is possible. Depending on certain conditions which we will look at later, the hydrogen particles in fat and water are able to take up energy and return these energies as signals that are used to form the images we see from an MRI scan.


I have tried not to introduce any technical terms which sadly are many in MRI. The reason being that I want you to appreciate that MRI is all about differentiating fat and water. And that hydrogen is the most valuable player in the game.

Thank you for reading and I hope to see you in my next blog