MRI Made Easy (for beginners) (2nd ed.) by Govind B. Chavhan. Read online, or download in secure PDF format. MRI made Easy. This book is dedicated – to anyone, who tries to teach medicine instead of just reporting medical facts and to anyone, whose stumbling feet find. Clinical Experience of Revision of Metal on Metal Hip Arthroplasty for Aseptic Lymphocyte Dominated Vasculitis Associated Lesions (ALVAL) Weissman BNW, Sledge CB. Treatment of heterotopic ossification Book Review / Critiques de livres Book Review: MRI Made Easy (for Beginners.
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MRI Made Easy 2nd Ed - Govind B. Chavhan - Jaypee - Ebook download as PDF File .pdf), Text File .txt) or read book online. MRI. MRI Made Easy is an excellent portable pocket guide which can be consulted by the reader when monitoring scans on screen and during. Get this from a library! MRI made easy: for beginners. [Govind B Chavhan] -- The first edition of this introductory book was written when the author felt the need.
Start by pressing the button below! Hans H. Schild Lt. No part of this book may be reproduced by any means without the written permission of the publisher. Printed in Germany by Nationales Druckhaus Berlin. Bock, who is a master of this art. It is dedicated to anyone, who would like to know something about MRI without having to study physics for years.
If this applies to you, then read this text from front to back, though not at one sitting. While the subject matter is extremely complex, it is not by any means beyond comprehension. It does however, require some concentration and consideration. I have therefore on occasion suggested that you set the book down and take a break. Do so, it will help you to stick with the material, but don't forget to come back. Subjects, that in my experience are particularly difficult to understand, I have repeated once or even more times, so the reader will be able to understand and remember them by the end of the book.
Some valuable introductory texts helped with writing this book; they are cited in the references, and recommended for further information, as a text of this size cannot cover everything. Indeed, it is not the objective of this book to represent the 'be all and end all' of Magnetic Resonance Imaging, but rather to serve as an appetizer for further reading. Let us start with a general overview of MRI. To understand this, it is necessary to at least know some very basic physics - even though this may seem to be boring.
As we all know, atoms consist of a nucleus and a shell, which is made up of electrons. In the nucleus - besides other things there are protons, little particles, that have a positive electrical charge whatever that may actually be. These protons are analogous to little planets. Like earth, they are constantly turning, or spinning around an axis fig. The positive electrical charge, being attached to the proton, naturally spins around with it.
And what is a moving electrical charge? It is an electrical current. This can be demonstrated very easily. Take a rusty nail and approach an electrical outlet closer, closer. Do you feel it being repelled by the magnetic force so you do not put the nail into the outlet? Now, may be you remember from your physics at school that an electrical current induces, causes a magnetic force, or magnetic field.
So, where there is an electrical current, there is also a magnetic field. Protons possess a positive charge. Like the earth they are constantly turning around an axis and have their own magnetic field.
Let's review what we have read A proton has a spin, and thus the electrical charge of the proton also moves. A moving electrical charge is an electrical current, and this is accompanied by a magnetic field. Thus, the proton has its own magnetic field and it can be seen as a little bar magnet fig.
This, however, changes when they are exposed to a strong external magnetic field. Then they are aligned in only two ways, either parallel or antiparallel to the external magnetic field. What happens to the protons, when we put them into an external magnetic field?
Or they may point exactly in the complete opposite direction, anti-parallel. These types of alignment are on different energy levels. To explain this; a man can align himself parallel to the magnetic The protons - being little field of the earth, i.
Both states are on dific field of the earth. However, ferent energy levels, i. For the compass Walking on one's feet is needle there is only one way undoubtedly less exhausting, to align itself with the takes less energy than walking magnetic field, for the protons however, there are two fig. In the figures, this will be illustrated as The protons may align with pointing up or down, see fig.
So more protons are on the lower energy level, parallel to the external magnetic field walk on their feet. The difference in number is, however, very small and depends on the strength of the applied magnetic field. To get a rough idea: for about 10 million protons "walking on their hands", there are about 10 "walking on their feet" the difference "" is probably easy to remember.
It may be obvious at this point already, that for MRI the mobile protons are important which are a subset of all protons that are in the body.
Let us take a closer look at these protons We will see that the protons do not just lay there, aligned parallel or anti-parallel to the magnetic field lines. Instead, they move around in a certain way.
The type of movement is called precession fig. What type of movement is "precession"? Just imagine a spinning top. When you hit it, it starts to "wobble" or tumble around. It does not, however, fall over.
During this precession, the axis of the spinning top circles forming a cone shape fig. It is hard to draw such a precessing proton, as this is a very fast movement as we will see below. For the sake of simplicity, we will just make "freeze frame" pictures, as if we were taking a fast flash light photograph of the situation at a specific moment in time.
For reasons we will learn below, it is important to know how fast the protons precess.
This speed can be measured as precession frequency, that is, how many times the protons precess per second. This precession frequency is not constant. It depends upon the strength of the magnetic field for magnetic field strength see page 96 , in which the protons are placed. The stronger the magnetic field, the faster the precession rate and the higher the precession frequency. This is like a violin string: the stronger the force exerted upon the string, the higher its frequency.
Protons in a strong magnetic field also show this type of motion, which is called precession. It is possible and necessary to precisely calculate this frequency.
The equation states that the precession frequency becomes higher when the magnetic field strength increases. The exact relationship is determined by the gyro-magnetic ratio This gyro-magnetic ratio is different for different materials e. It can be compared to an exchange rate, which is different for different currencies. They do this in two ways: parallel and anti-parallel. The state that needs less energy is preferred, and so there are a few more protons "walking on their feet" than "on their hands" fig.
Why is this precession frequency important? It has something to do with the resonance in magnetic resonance imaging. But to understand this will take a few more minutes. After the break you should go over the last summary again, and then continue. Introducing the coordinate system To make communication and drawing of illustrations easier, let us start using a coordinate system like we know from school fig. As you see, the z-axis runs in the direction of the magnetic field lines, and thus can represent them.
So we can stop drawing the external magnet in all other illustrations. From here on we will also illustrate the protons as vectors as little arrows. Maybe you remember: a vector represents a certain force by its size that acts in a certain direction direction of the arrow.
The force that is represented by vectors in our illustrations, is the magnetic force. Now, look at figure 6. There we have 9 protons pointing up, precessing parallel to the external magnetic field lines, and 5 protons pointing down, precessing anti-parallel to the external magnetic field.
As we stated above, what we see in the figure is just a picture taken at a specific point in time. A picture taken just a little later would show the protons in different positions because they precess.
The precession actually goes very fast, the precession frequency for hydrogen protons is somewhere around 42 MHz in a magnetic field strength of 1 Tesla see page 96 ; this means that the protons precess around the "ice cream cone" more than 42 million times per second.
Now there are millions and millions of protons in your body precessing this fast. It is easy to imagine, that at a certain moment, there may be one proton A in the illustration pointing in one direction, and another proton A' pointing exactly in the opposite direction. The result is very important; the magnetic forces in the opposing directions cancel each other out, like two persons pulling at the opposite ends of a rope.
Finally, for every proton pointing down, there is one pointing up, cancelling its magnetic effects. But as we Fig.
So in effect it is sufficient to only look at the four unopposed protons 6b. So we are left - in effect - with some protons 4 in our example pointing up fig. However, not only magnetic forces pointing up and down can cancel or neutralize each other. As the protons that are left pointing up, precess, there may be one pointing to the right, when there is another one pointing to the left; or for one pointing to the front, there is one pointing backwards, and so on the corresponding protons in fig.
This is true for all but one direction, the direction of the z-axis, along the external magnetic field fig. In this direction, the single vectors, the single magnetic forces add up, like people pulling on the same end of a rope. What we end up with in effect is a magnetic vector in the direction of the external magnetic field the arrow on the z-axis in fig.
Now - what does this mean? This means that by placing a patient in the magnet of the MR unit or in any other strong magnetic field , the patient himself becomes a magnet, i. Because the vectors of the protons, that do not cancel each other out, add up fig. The component along the y-axis is cancelled out by proton A', the magnetic force of which also has a component along the y-axis, however, in the opposite direction.
The same holds true for other protons, e.
B and B', which cancel their respective magnetic vectors along the x-axis. In contrast to the magnetic vectors in the x-y-plane, which cancel each other out, the vectors along the z-axis point in the same direction, and thus add up to a new magnetic sum vector pointing up. This new magnetic vector is aligned with the external magnetic field.
As we have seen, the resulting new magnetic vector of the patient points in the direction of the external field, along its field lines. This is described as longitudinal direction. And it is actually this new magnetic vector that may be used to get a signal. It would be nice if we could measure this magnetization of the patient, but there is a problem: we cannot measure this magnetic force, as it is in the same direction, parallel to the external magnetic field figs.
To illustrate this: imagine that you are standing on a boat, floating down a river. You have a water hose in your hand and squirt water into the river. For somebody who is watching you from the shore, it is impossible to tell how much water you pour out this shall be how much new magnetization is added in the old direction.
However, when you point the water hose to the shore, change the direction of the new magnetic field, then the water may perhaps be directly picked up and measured by an impartial observer on the shore fig. What we should learn from this is: magnetization along or, better, longitudinal to the external magnetic field cannot be measured directly. For this we need a magnetization which is not longitudinal, but transversal to the external magnetic field. For this a magnetization transverse to the external magnetic field is necessary.
Time to take a break.
Download File MRI Made Easy for beginners 2nd Edition.pdf
And when you come back, start out with the summary again. Due to this, they have a magnetic field and can be seen as little bar magnets. And the stronger the magnetic field, the higher the precession frequency, a relationship that is mathematically described in the Larmor equation.
But as there are more parallel protons on the lower energy level "pointing up" , we are left with some protons, the magnetic forces of which are not cancelled. All of these protons pointing up, add up their forces in the direction of the external magnetic field. And so when we put the patient in the MR magnet, he has his own magnetic field, which is longitudinal to the external field of the MR machine's magnet figs.
Because it is longitudinal however, it cannot be measured directly. What happens after we put the patient into the magnet? We send in a radio wave. The term radio wave is used to describe an electromagnetic wave, that is in the frequency range of the waves which you receive in your radio.
What we actually send into the patient is not a wave of long duration, but a short burst of some electromagnetic wave, which is called a radio frequency RF pulse. The purpose of this RF pulse is to disturb the protons, which are peacefully precessing in Fig. Not every RF pulse disturbs the alignment of the protons.
For this, we need a special RF pulse, one that can exchange energy with the protons. This is as if someone were looking at you. However, if someone were to pound you in the stomach, exchange energy with you, your alignment would be disturbed. And this may explain why we need a certain RF pulse that can exchange energy with the protons to change their alignment. For this it must have the same frequency; the same "speed" as the protons.
Just imagine that you are driving down a race track in your car, and someone in the lane next to you wants to hand you a couple of sandwiches, i.
This energy trans- fer is possible when both cars have the same speed, move around the race track with the same frequency. In effect the magnetization along the z-axis decreases, as the protons which point down "neutralize" the same number of protons pointing up. What speed, or better, what frequency did the protons have?
They had their precession frequency which can be calculated by the Larmor equation see page So the Larmor equation gives us the necessary frequency of the RF pulse to send in.
Only when the RF pulse and the protons have the same frequency, can protons pick up some energy from the radio wave, a phenomenon called resonance this is where the "resonance" in magnetic resonance comes from. The term resonance can be What happens with illustrated by the use of tuning the protons, when forks.
Imagine that you are in a room with different kinds of they are exposed to tuning forks, tuned e. Somebody enters the room with a tuning fork with Some of them pick up energy, "a"-frequency, that was struck and go from a lower to a higher to emit sound. From all the energy level. Some, which were tuning forks in the room, all of walking on their feet, start walka sudden the other "a" forks, ing on their hands.
And this has and only those, pick up energy, some effect on the patients start to vibrate and to emit magnetization, as you can see in sound, they show a phenomenon figure Let us assume that called resonance. The result is that these 2 protons cancel out the magnetic forces of the same number of protons, that point up. In effect, then, the magnetization in longitudinal direction decreases from 6 to 2.
But something else happens. Do you remember what drawings of radio waves look like? Just look at fig. Due to the RF pulse, the protons do not point in random directions any more, but move in step, in synch - they are "in phase". They now point in the same direction at the same time, and thus their magnetic vectors add up in this direction. This results in a magnetic vector pointing to the side to which the precessing protons. This is why it is called transversal magnetization. The former results in decreasing the magnetization along the z-axis, the so-called longitudinal magnetization.
The situation can be compared to a ship: think about the passengers being distributed randomly all over the deck, the ship then is in a normal position. Then have all passengers walk in equal step around the railing; what happens? The ship is leaning towards the side where the people are, a new force is established, and becomes visible fig.
So the RF pulse causes a transversal magnetization. This newly established magnetic vector naturally does not stand still, but moves in line with the precessing protons, and thus with the precession frequency, fig. So - what were the new things that we have learned? Repeat them using fig. A magnetic field in the patient, longitudinal to the external field results fig. Sending in an RF pulse causes a new transversal magnetization while longitudinal magnetiation decreases b. Depending on the RF pulse, longitudinal magnetization may even totally disappear c.
Their vectors now also add up in a direction transverse to the external magnetic field, and thus a transversal magnetization is established. In summary: the RF pulse causes longitudinal magnetization to decrease, and establishes a new transversal magnetization figs. The reThis also is true the other way around: a moving magnetic field sulting MR signal therefore also has the precession causes an electrical current, frequency fig.
How can we know that? This The trick is really quite simple: And this is important: the can also induce an electrical we do not put the patient into magnetic vector, by constantly current in an antenna, which is a magnetic field which has the moving, constantly changing, the MRI signal. As the transversal magnetic vec- section of the patient, which We talked about the opposite tor moves around with the we want to examine.
Instead we already: the moving electrical precessing protons, it comes to- take a magnetic field, which has a different strength at each point of the patients cross section. What does this do? We heard that the precession frequency of a proton depends on the strength of the magnetic field as the frequency of a violin string depends on the strength with which you pull it. Let us have a look at that newly established transversal magnetization vector If this strength is different from point to point in the patient, then protons in different places Fig.
Thus for an external observer, transversal magnetization constantly changes its direction, and can induce a signal in an antenna. This, however, is not The reason why the longitudinal It is like with your TV: when you what happens.
As soon as the magnetization grows back to are in the kitchen where you RF pulse is switched off, the its normal size is easier to exprobably do not have a TV and whole system, which was displain, so let us start with that hear a sound from your favourite turbed by the RF pulse, goes TV show, you know where the back to its original quiet, peace- see fig.
It comes ful state, it relaxes. The newly No proton walks on its hands from that spot in your apartlonger than it has to - sort of a ment where the TV stands. The protons that What you subconsciously do, were lifted to a higher energy is connect a certain sound to level by the RF pulse go back a certain location in space. And as they precess with different frequencies, the resulting MR signal from different locations also has a different frequency.
And by the frequency we can assign a signal to a certain location. Further details about the MR signal Fig. This is illustrated "one-by-one". The effect is that longitudinal magnetization increases and grows back to its original value. Note that for simplicity the protons were not depicted as being in phase; this subject is covered in more detail in fig.
Not all protons do this at the very same time, instead it is a continuous process, as if one proton after the other goes back to its original state. This is illustrated in fig. For the sake of simplicity the protons are shown as being out of phase, which of course they aren't in the beginning. Why and how they stop precessing in phase will be explained a little later.
What happens to the energy which they had picked up from the RF pulse? This energy is just handed over to their surroundings, the socalled lattice. And this is why this process is not only called longitudinal relaxation, but also spin-lattice-relaxation. By going back on their feet, pointing upwards again, these protons no longer cancel out the magnetic vectors of the same number of protons pointing up, as they did before.
So, the magnetization in this direction, the longitudinal magnetization increases, and finally goes back to its original value fig. If you plot the time vs. This curve is also called a T1-curve. The time that it takes for the longitudinal magnetization to recover, to go back to its original value, is described by the longitudinal relaxation time, also called T1.
This actually is not the exact time it takes, but a time constant, describing how fast this process goes. This is like taking time for one round at a car race. The time gives you an idea of how long the race may take, but not the exact time. Or more scientifically, T1 is a time constant comparable to the time constants that for example describe radioactive decay.
That T1 is the longitudinal relaxation time can easily be remembered by looking at your typewriter: fig. The " 1 " looks very much like the , reminding you also that it describes the spin-"l"attice relaxation. But there are more hidden hints to this: the "1" also looks like a match.
And this match should remind you of something, which we also have mentioned already: longitudinal relaxation has something to do with exchange of energy, thermal energy, which the protons emit to the surrounding lattice while returning to their lower state of energy. Enough of the longitudinal magnetization what happens with the transversal magnetization? After the RF pulse is switched off, the protons get out of step, out of phase again, as nobody is telling them to stay in step.
For the sake of simplicity this has been illustrated for a group of protons which all "point up" in fig. We heard earlier that protons precess with a frequency which is determined by the magnetic field strength that they are in. And all the protons should experience the same magnetic field. These internal magnetic field variations are somehow characteristic for a tissue.
So after the RF pulse is switched off, the protons are no longer forced to stay in step; and as they have different precession frequencies, they will be soon out of phase. In 5 microseconds 0.
MRI Made Easy - Well Almost Schering
When you look at these dephasing proton ensembles from the top which is illustrated in the lower part of the figure , it becomes obvious, how they fan out. Fanning out, they point less and less in the same direction, and thus transversal magnetization decreases.
Similar to what we did for the longitudinal magnetization, we can plot transversal magnetization versus time. What we get is a curve like in figure This curve is going downhill, as transversal magnetization disappears with time. And as you probably expect: there is also a time constant, describing how fast transversal magnetization vanishes, goes downhill.
This time constant is the transversal relaxation time T2. How to remember what "T2" is? The resulting curve in figure 21 thus is called a T2-curve. Another term for transversal relaxation is spin-spin-relaxation, reminding us of the underlying mechanism, a spin-spin interaction.
How to remember, which one is the T1- and which the T2-curve? Just put both curves together, and you can see something like a mountain with a ski slope.
You first have to go uphill T1-curve , before you jump down T2-curve fig. It takes longer to climb a mountain than to slide or jump down, which helps to remember that T1 is normally longer than T2.
The stronger the magnetic field, the higher the precession frequency. Longitudinal and transversal relaxation are different, independent processes, and that is why we discussed them individually see figs. This is what you should know by now. How long is a relaxation time? It is probably easy and logical, that it takes you more time to get to the top of the mountain, than to go back down, to jump off.
This means T1 is longer than T2, and just to give you an idea: T1 is about times as long as T2. Or in absolute terms in biological tissues: T1 is about to msec, and T2 is about 30 to msec. It is difficult to pinpoint the end of the longitudinal and transversal relaxation exactly. Thus, T1 and T2 were not defined as the times when relaxation is completed. Previously it was believed that measuring the relaxation times, would give tissue characteristic results, and thus enable exact tissue typing.
This, however, proved to be wrong, as there is quite some overlap of time ranges; and also T1 is dependent on the magnetic field strength used for the examination see page Look at fig. You see somebody having a long drink, something liquid representing water. When you go to your favourite bar which naturally is crowded, as it is a popular place and order a long drink, you have to wait quite a while to get your drink - T1 is long.
When you finally have your long drink, it takes you also a long time to drink it, so T2 also is long. Now look at fig. These normally contain much fat, and shall represent fat for us. The hamburger is fast food, you get it fast, thus fat has a short T1. What about T2? It takes some time to eat fast food, fat; however, you normally spend more time with your long drink, so fat has a shorter T2 than water.
As water has a long T1 and a long T2, it is easy to imagine that "watery tissues", tissues with a high water content, can also have long relaxation times. WhatisT1 influenced by? Actually, T1 depends on tissue composition, structure and surroundings.
As we have read, T1-relaxation has something to do with the exchange of thermal energy, which is handed over from the protons to the surroundings, the lattice. The precessing protons have a magnetic field, that constantly changes directions, that constantly fluctuates with the Larmor frequency. The lattice also has its own magnetic fields. The protons now want to hand energy over to the lattice to relax. This can be done very effectively, when the fluctuations of the magnetic fields in the lattice occur with a frequency that is near the Larmor frequency.
And as the protons which are on the higher energy level cannot hand their energy over to the lattice quickly, they will only slowly go back to their lower energy level, their longitudinal alignment. When the lattice consists of medium-size molecules most body tissues can be looked at as liquids containing various sized molecules, kind of like a soup , that move and have fluctuating magnetic fields near the Larmor frequency of the precessing protons, energy can be transferred much faster, thus T1 is short.
This can again be illustrated by our sandwich and race car example: see page 17 handing over sandwiches i. With a difference in speeds, the energy transfer will be less efficient.
Why does fat have a short T1? The carbon bonds at the ends of the fatty acids have frequencies near the Larmor frequency, thus resulting in effective energy transfer. And why is T1 longer in stronger magnetic fields? It is easy to imagine that in a stronger magnetic field it takes more energy for the protons to align against it. Thus these protons have more energy to hand down to the lattice, and this takes longer than handing down just a small amout of energy.
Even though it may seem logical, this is the wrong explanation. As we heard in the beginning, the precession frequency depends on magnetic field strength, a relationship described by the Larmor equation. If we have a stronger magnetic field, then the protons precess faster. And when they precess faster, they have more problems handing down their energy to a lattice with more slowly fluctuating magnetic fields.
What influences T2? T2-relaxation comes about when protons get out of phase, which - as we already know - has two causes: inhomogeneities of the external magnetic field, and inhomogeneities of the local magnetic fields within the tissues see page As water molecules move around very fast, their local magnetic fields fluctuate fast, and thus kind of average each other out, so there are no big net differences in internal magnetic fields from place to place.
And if there are no big differences in magnetic field strength inside of a tissue, the protons stay in step for a long time, and so T2 is longer.
With impure liquids, e. The larger molecules do not move around as fast, so their local magnetic fields do not cancel each other out as much. These larger differences in local magnetic fields consequently cause larger differences in precession frequencies, thus protons get out of phase faster, T2 is shorter.
This can be illustrated by the following example: imagine that you drive down a street with many pot holes. When you drive slow which is equal to the surroundings moving slow and you standing still , you will be jumping up and down in your car with each pot hole. The differences in the surroundings the magnetic field variations influence you considerably.
When you drive very fast which is the same as if the surroundings move very fast , you do not feel the single pot holes anymore. Start on.
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MRI made easy : for beginners
No notes for slide. Mri made easy pdf 1. Chavhan 2. Anshan Publishing Release Date: Govind B. Chavhan Download Here http: MRI Made Easy is an excellent portable pocket guide which can be consulted by the reader when monitoring scans on screen and during interpretations of images.
The book is a simple overview of Magnetic Resonance Imaging, which explains the fundamentals in a clear, concise manner. It has been written to serve as an introduction to the subject, specifically useful for beginners to MRI, particularly radiology residents.
The book is divided into two sections: The second explains how MRI has advanced in recent years, and its applications across diverse disciplines. Basic Principles 2. T1, T2 Relaxation and Other Parameters 3.
Instrumentation 4. Sequences I: Basic Principles 5. Sequences II: Few Common Sequences 6. Artifacts 7. MR Safety 8.
MR Contrast Media 9.In ROPE also respiratory tracings are obtained either by tying a bellows or by navigator pre-pulse. On out-of-phase images. It is impossible to eliminate all artifacts though they can be reduced to acceptable level.
The lattice also has its own magnetic fields. Cross Excitation: Slice selection gradient Ch As soon as the magnetization grows back to are in the kitchen where you RF pulse is switched off, the its normal size is easier to exprobably do not have a TV and whole system, which was displain, so let us start with that hear a sound from your favourite turbed by the RF pulse, goes TV show, you know where the back to its original quiet, peace- see fig.
Only when the RF pulse and the protons have the same frequency, can protons pick up some energy from the radio wave, a phenomenon called resonance this is where the "resonance" in magnetic resonance comes from.
The difference in number is, however, very small and depends on the strength of the applied magnetic field.