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MRI Physics- What is MRI? – Deconstructing MRI Physics Test Review.

What is MRI?

 

What is MRI?

MRI or Magnetic Resonance Imaging is a rapidly growing imaging modality that uses an external magnetic field, radio frequencies, gradients, computer hardware/software, and numerous other components to obtain diagnostic images. Unlike CT, MRI does not use harmful ionizing radiation which is used in X-ray but instead utilizes non-ionizing radiation to obtain highly detailed images of soft tissue, the chemical metabolism of tissue or tumors, the patency of vasculature and many other structures and functions within the body. This guide is intended for future or current MRI Technologists to either review for their registry exam, be it ARRT or ARMRIT, or further their knowledge within the field of MRI Physics. While this first segment may seem technical, even to those familiar with the concepts each area of interest and importance to the registry exam will be covered in easily understandable language and analogies as the guide progresses.

In MRI, a patient who has been screened for unsafe metals and implanted devices is introduced to an external magnetic field, and the patient’s anatomical area of interest is placed as close to the center (or middle) of the bore of the magnet as possible, known as isocenter. The hydrogen nuclei within the body, which consists of a single proton are spinning around throughout much of the bodies tissue with random orientation. This spin is referred to in MRI as a Magnetic Moment.

In the presence of this external magnetic field, however, hydrogen nuclei lose their random orientation and align themselves in excess toward, or parallel to this external magnetic field. This is to say that while some nuclei align themselves against, or anti-parallel to the external magnetic field, more nuclei align themselves in the direction of the external magnetic field. This collection of excess magnetic moments (spinning nuclei) spinning in the parallel direction is referred to as a Net Magnetization Vector.

These hydrogen nuclei spin, or precess, around the external magnetic field at a certain frequency that is dependent on how powerful the external magnetic field is, which is measured in Gauss or Tesla.

A radiofrequency (RF) pulse of a similar frequency to that of the precessional frequency of the hydrogen nuclei is sent in and disrupts the excess nuclei parallel to the external magnetic field. This RF pulse flips these nuclei away from the external magnetic field and all the nuclei are temporarily in phase or precessing at the same rate in the same direction with the same orientation. As soon as this RF pulse dissipates, the hydrogen nuclei begin to lose phase and relax back to their natural state, back in the direction of the external magnetic field,.

This relaxation and loss of phase produces an analog signal within the receiver coils and is measured in several ways depending on the desired contrast weighting of the image. This signal is spatially encoded or simply honed into the area of interest (brain, foot, abdomen etc.), by gradient coils. These coils create small variations in the precessional frequency of the hydrogen nuclei, and in doing so are capable of only sampling hydrogen nuclei in the anatomical region of interest. A one-dimensional linear gradient can be thought of as follows.

Imagine that in lying on the table within the bore of the magnet the linear gradient can cause the hydrogen nuclei at the base of your feet to revolve or precess once per second. These nuclei would spin faster and faster as they progress further up your body (superior) until the nuclei at the top of your head revolve or precess 10 times per second. If you wanted images of your brain, you could selectively tell the machine to only collect data points from hydrogen atoms precessing between 9 and 10 times per second. Anything below 9, or the mid-neck down in this scenario, would not be sampled and data from that area would not be included, and only information from that anatomical area of interest would be sampled and collected to create the image.

Gradients are also the primary source of noise during an MRI due to rapidly changing electrical currents causing vibrations within the coils themselves. These rapidly changing electrical currents are what allow the gradient coils to create a gradient magnetic field, and the noise (while improvements are being made) is currently a necessity of this process.

In reviewing the fundamentals of Magnetic Resonance Imagine it is obvious that there are three major components that are necessary for obtaining and collecting a signal.

  1. The External Magnetic Field – To create a Net Magnetization Vector.
  2. A Radiofrequency Transmitter and Receiver- To disrupt the nuclei and collect data.
  3. A Magnetic Gradient – for spacial encoding.

 

This website is designed to educate future MRI Technologists and current professionals about the fundaments of MRI Physics. If you found this information helpful please follow our website for future updates and articles.

MRI Physics – Resonance – Deconstructing MRI Physics Test Review.

MRI Physics – Resonance – Deconstructing MRI Physics Test Review.

 

In our previous article we discussed atoms, their subatomic particles, as well as the three distinct spins occuring within the atom. We also covered the terms Magnetic Moment, Precession, Net Magnetization Vector, as well as defined the Larmor Equation and broke it down into easily digestible steps and terms.  In this article we will discuss the concept of Resonance, and its relation and importance in MRI.

 

In physics, resonance is a process in which a vibrating object or external force causes another system to vibrate with greater amplitude at specific frequencies. While this definition may sound somewhat dense, as is the case with many things in MRI, it can be broken down into simpler terms. This can be thought of as follows: if a force is vibrating at a certain frequency and it comes into contact with a seperate force vibrating at a similar frequence, that second force will become excited and gain energy.

 

Such excitation is on of the fundamental aspects of MRI.

 

In MRI the “external force” causing all of the excitement discussed above is a radiofrequency (RF) pulse and the “other system” gaining energy, becoming excited, is the precessing hydrogen atoms nucleus. If we recall from the previous article, hydrogen is precessing in accordance to the Larmor Equation. For every 1T of external magnetic field strength, hydrogen precesses at 42.57MHz or millions of times per second. If we send in a vibrating object, in this case a radiofrequency pulse, at a similar frequency to this precession, resonance should, and does occur. If, however, radiofrequency is sent in at a frequency different to the precessional frequency of the hydrogen atom, resonance and excitation will not occur. It is also important to note that 42.57MHz/T is the precessional frequency of hydrogen, but other elements have their own individual precessional frequencies, and as such when RF is sent in at the precessional frequency of hydrogen, these atoms do not achieve resonance, or excitation. Conversely, RF sent in at the precessional frequency of these other atoms will have no effect on hydrogen.

 

You may also recall from the previous article that the Net Magnetization Vector is a byproduct of an excess of low energy (parallel, spin up) hydrogen atoms. In using the analogy of a lazy river at a waterpark, we discussed how there will be more low energy, lazy patrons who are willing to be pulled by the current of the river than there are patrons attempting to swim against the current. This excess of low energy patrons, or in the case of MRI, precessing hydrogen atoms, are what compose the net magnetization vector. This NMV is, prior to excitation, is pointing north, or upward Y axis of an XY axis graph.

 

The introduction of RF at the resonant frequency of hydrogen causes excitation, and some of the hydrogen atoms that were previously spin up, north oriented, low energy prior to excitation, gain energy and transition into a spin down, high energy state. The introduction of RF at hydrogens resonant frequency and the excitation caused can be thought of as analogous to introducing energy drinks to the patrons in the lazy river. While many of the patrons will continue to lazily be pulled by the flow of the river, a few will find motivation from the new found energy and decide to swim against the current. This shift in hydrogen nuclei (patrons) will cause a shift in the Net Magnetization Vector, tilting it away from north on the Y axis of our XY axis graph. The tilt of this vector away from north on the Y axis can vary in its degrees of flip and is known as the Flip Angle. Flip Angle’s vary by pulse sequence which will be discussed in a future article, but for a point of reference common flip angles for Gradient Echo Sequences typically range between 1 and 89 degrees. Common values for Spin Echo Sequences are 90 and 180 degrees.

 

The XY graph described above, when pertaining to MRI, has unique names for the X and Y axis. The Y axis, which the Net Magnetization Vector is typically oriented toward in the presence of an external magnetic field, prior to excitation by an RF pulse, is known as the longitudinal plane. The X axis in MRI is referred to as the Transverse plane. Upon receiving excitation from an RF pulse the Net Magnetization Vector flips itself toward the transverse plane, and away from the longitudinal plane.

 

It is also important to note that the Radio Frequency pulse causing excitation and resonance must come in perpendicular to the angle of the Net Magnetization Vector. Again reinforcing our lazy river analogy, an energy drink thrown from behind cannot be caught by the lazy river patron. One being thrown from the front could potentially come in too fast and hit the patron in the head. An energy drink gently tossed from the side however would be easily caught and cause excitation, and resonance.

 

One final item of importance regarding excitation and resonance in MRI is the concept of Phase. When the net magnetization vector becomes excited, its protons begin to precess in-phase. This can be thought of as follows: prior to excitation from their energy drinks all of our patrons in the lazy river were pointed either directly with, or directly against the current of the lazy river. These patrons were all spinning, or precessing, at the same rate in their inner tubes, but were spinning independently from one another. While patrons were spinning at the same speed, one could be facing to the right while another could be facing to the left. Patrons who are spinning at the same rate, but have different orientation are out-of-phase from one another. In the presence of an RF pulse, which produces excitation or resonance, however, these patrons or independent magnetic moments would all align themselves with one another and spin at the same frequency, and have the same orientation. This means that all patrons would be spinning at the same rate in the lazy river, and would all be facing the same directing at the same time. This level of choreography from our patrons is short lived, however, and as soon as the RF pulse or energy drinks are removed our patrons begin to lose phase, and transition back into their natural out-of-phase state.       

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MRI Physics – The Atom – Deconstructing MRI Physics Test Review.

MRI Physics – The Atom

 

MRI Physics – The Atom

While a complete understanding of the Periodic Table of the Elements is useful, and a fundamental building block of many scientific fields, for our purposes as MRI Technologists we can hone our studies to focus on the basic properties that all elements share, and more importantly how these properties relate to magnetism and the particular Atom of interest in MRI, hydrogen-1 (protium), atomic symbol (H).

While there are three isotopes of hydrogen, hydrogen-1 or protium is by far the most naturally abundant comprising 99.985% of all hydrogen isotopes, and is the isotope relevant to MRI. From this point forward we will simply use “hydrogen” in place of hydrogen-1 (protium) to save space and your sanity.

 

Hydrogen is of such importance in MRI due to its relative abundance in the body, found in both water and fat and due to Hydrogen’s magnetic susceptibility due to its isotopic quality. Before delving further into these concepts, however, we must first understand the Hydrogen atom itself and the fundamental properties of atoms.

 

All atoms are made up of sub atomic particles that include, protons, neutrons, and electrons. Protons and neutrons comprise the nucleus (which can be thought of as a central core), while electrons inhabit an outer electron shell, which surrounds the nucleus.

Protons have a net positive charge, which can be remembered due to the fact that both proton and positive begin with the letter P.

Neutrons have no net charge, which can be remembered due to neutrons being neutral.

Electrons have a net negative charge; an easy memory device would be to remind yourself that electrocution would be a very negative experience.

 

Hydrogen is an atom with a single proton, no neutrons, and a single electron.  This results in an atom with an atomic number, which is simply the number of protons in an atom, of 1. The atomic weight of Hydrogen, which is defined by the average number of protons and neutrons within an atom, is 1.00794 AMU or atomic mass units. Electrons are excluded from atomic weight due to their relative lack of size, and while protons and neutrons are similar in mass they are both individually around 1,840 times as massive as an individual electron.

In having one proton but no neutron, hydrogen is an Isotope, which is an atom with an unbalanced or unequal number of protons to neutrons.  In having one positively charged proton, and one negatively charged electron, however, hydrogen does have a balanced charge and as such is not an Ion. An Ion is an atom with an unequal number of protons to electrons.

 

 Spin

 

The subatomic particles mentioned, protons, neutrons, and electrons are not motionless within the atom itself or within your body. These particles spin in three distinct ways within the atom in the absence of an external magnetic field.

  1. The protons and neutrons, which form the nucleus, spin together as the nucleus on its own axis.
  2. The electrons, as they are their own entity, spin on their own axis.
  3. The electrons also spin “around” the nucleus.

In non-isotopic atoms, in which there are an equal number of protons to neutrons, the spin of the nucleus is canceled out. Hydrogen, however, having an unequal number of protons to neutrons, is an isotope and as such has spin. This spin creates a magnetic moment vector with a north and south-pole within the atom and is the reason hydrogen is a viable option in MRI. Simply put, the spin within the nucleus causes hydrogen to act as a tiny magnet itself.

 

Without the presence of an external magnet the protons which comprise the hydrogen nuclei are spinning randomly and without uniform orientation to one another. When introduced to an external magnetic field, however, the orientation of the all nuclei switches course, and either align themselves toward or against the external magnetic field. There is logical due to the proton’s spin causing the nucleus to be a tiny magnet itself and would behave the way any magnet would in the presence of another magnetic force. There are many interchangeable terms to describe the orientation of the nuclei in this state, and while it is unfortunate that there are numerous terms to describe the same basic principle, all of the terms are relatively self-explanatory once the concepts become more clear.

 

Imagine the external magnetic field as a lazy river, and you, laying in your inner tube, are the nucleus. By simply allowing yourself to be taken by the lazy river you are expending very little energy. Similarly, the nuclei that simply align themselves with or toward the external magnetic field are considered in a low-energy state. Conversely, if you decided to swim against the lazy river’s current, you would expend significant energy fighting the current. Similarly, nuclei that align themselves against the external magnetic field are considered high-energy nuclei.

 

Now the interchangeable terms come into play.  Nuclei that are low energy are also referred to as parallel or spinup nuclei.  One visual device would be to imagine the lazy river pointed north or up, and in going parallel with the current your direction is north. Nuclei that are high energy, on the other hand, are referred to as Anti-Parallel or spin-down. Using the same visual device as above all three of these terms are easily memorable, and the concept itself is easily understandable.

 

Continuing with our analogy, there will naturally be more people inclined to take the low-energy route and allow the lazy river to continue pulling them in the direction of the current than those who will fight against the current and use up their energy. The same is true with atomic nuclei, and more nuclei are in a low-energy state than in a high-energy state when exposed to an external magnetic field. This excess of low energy nuclei is referred to as a net magnetization vector and will be discussed in depth throughout the text. One important detail to note is that this excess of “lazy” low energy nuclei is directly affected by the strength of the magnet. As the current in our metaphorical lazy river increases speed and strength, fewer people will be inclined to attempt to swim against it. Similarly, fewer nuclei are able to reach a high-energy state when the strength of the external magnetic field is increased. A real world example of this would be increasing magnetic field strength from 1.5 Tesla to 3 Tesla. With the increase in field strength, fewer nuclei will achieve a high-energy state and more nuclei will be in the low-energy, parallel, spin-up state.

 

This net magnetization vector, or excess hydrogen nuclei that align parallel to the external magnet, aligns itself in this fashion only in the presence of an external magnetic field. As stated before, in the absence of an external magnetic field, hydrogen nuclei are not in cohesion with the other hydrogen nuclei of the body and spin independently and randomly. This nucleic spin is known as a magnetic moment; magnetic, due to the spin of hydrogen exerting a small magnetic field of its own.

 

In the absence of an external magnetic field these randomly aligned magnetic moments can be thought of as random patrons at the waterpark wandering around as they please. Such patrons can walk in infinite directions independent of the orientation or direction of the other patrons in the park. Such is the case with magnetic moments. In the presence of an external magnetic field, however, the excess magnetic moments, or excess low energy patrons in the lazy river allowing themselves to be pulled in the direction of the current (or magnet) are what compose the net magnetization vector described above.

 

In discussing spin and its importance within MRI there is one additional topic that must be discussed to truly understand the fundamentals of magnetism on an atomic level. That is the concept of Precession. Precession is the spin of the magnetic moment in the presence of an external magnetic field and creates a circular pathway around the external magnetic field. This can be easily visualized using your arm. Extend your arm straight out in front of your body and aim your fingertips at a particular object in space. This object can be thought of as the external magnetic field, and your arm as the magnetic moment. In slowly rotating your arm around the object, such as in an arm circle exercise, you are illustrating the general pathway that a precessing atom would take. It is important to note that this rotation or spin is not of the atom itself, but is of the magnetism generated by the proton in the hydrogen nuclei under the influence of an external magnetic field.

The speed of this precession is variable and is dependent on the strength of the external magnetic field and the gyromagnetic ratio of the atom at hand. The formula to calculate Precessional Frequency is known as the Larmor equation, and while this formula looks intimidating initially, upon closer examination you will find that is no more difficult than the simplest of algebra problems. The Larmor Equation is as follows.

ωo = Bo x λ

Where ωo is the precessional frequency

Bo is the strength of the external magnetic field

And λ is the gyromagnetic ratio

Again, these complex terms and symbols may seem initially intimidating but we will break this equation down into very easily understandable terms.

We will start with Precessional frequency ωo. This can be understood easily using our outstretched arm circle exercise described above. Precessional Frequency is simply how many times your arm circles in a given time frame. In Magnetic Resonance, we are describing how many times the magnetic moment of hydrogen spins around the external magnetic field in one second. This value is described with the unit of measurement MHz or Megahertz which is millions of spins per second.

Next is external magnetic field strength Bo. As MRI Technologists this is simply how powerful the external magnetic field of your machine is, which is measured in Tesla. Common high field system strengths are 1.5 T and 3.0 T.

Finally, we will discuss gyromagnetic ratio λ. Gyromagnetic ratio is a constant number that is associated with a given atom containing an MRI active nuclei. It illustrates the effect of magnetism over precessional frequency. As the external magnetic field increases in strength, for instance from 1.5T to 3.0T MRI Active nuclei will increase their speed of precession. Therefore Gyromagnetic ratio is expressed as spins per field strength or MHz/T (Megahertz per Tesla). While there are numerous MRI Active Nuclei with their own Gyromagnetic Ratio the registry will not be expecting you to have these numbers memorized and will provide you with each Gyromagnetic Ratio when appropriate, with one exception. Any Guesses?

HYDROGEN!

The Gyromagnetic Ratio of hydrogen is 42.57 MHz/T or 42,570,000 spins per second per Tesla.

Given this number and these principles test questions regarding the Larmor equation become simple.

In given any two numbers you can solve for the third using basic algebra. For instance, say you are given an external field strength of 3.0T and are asked to solve for the precessional frequency of Hydrogen, which we know has a gyromagnetic ratio of 42.57MHz/T. Using the above equation we would plug in our known figures.

ωo = Bo x λ

We know that the external magnetic field strength is 3.0T

ωo = 3.0T x λ

We also know that the gyromagnetic ratio of hydrogen is 42.57MHz/T

ωo = 3.0T x 42.57MHz/T

At this point, we have broken the equation down into a very simple algebra problem as promised. We simply multiply 3.0×42.57 to get 127.71 and simplify units. The Tesla denominator in the gyromagnetic ratio cancels out the Tesla numerator in the external magnetic field leaving us with only MHz. (If these mathematical terms are dense or too far in your distant memory we understand, simply knowing that the value you end up with for precessional frequency is MHz will suffice). In simplifying we end up with a precessional frequency ωo of 127.71 MHz.

What does this value tell us about the principles of magnetism and how is this information useful?

The Larmor equation illustrates very clearly that external magnetic field strength has a direct effect on precessional frequency. The precessional frequency at a given field strength is of great importance in MRI as Resonance relies on a disturbance, in MRI a radiofrequency pulse, unsettling MRI active nuclei at a similar frequency to that which they are precessing.

 

The above concepts are extremely dense, particularly to those who are unfamiliar to them. Before continuing on to the next section we strongly recommend that you review the above concepts and analogies until they become clear, as MRI Physics largely relies on scaffolding and your future comprehension of the topic largely relies on your understanding of these fundamental principles.