MRI MADE EASY PDF DEUTSCH
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From here on we will also illustrate the protons as vectors as little arrows. So we can stop drawing the external magnet in all other illustrations. Maybe you remember: The result is very by others. It is easy to only look at the four unopposed precessing anti-parallel protons 6b. But as we. A picture taken exactly in the opposite direc. So in effect it is sufficient and 5 protons pointing down.
So we are left. There "ice cream cone" more than Fig. The five protons. The pre. In this direction. The magnetic force of proton A.
B and B' for example This means that the opposing magnetic forces of the re- maining protons cancel each other out in these directions. The same holds true magnetization.
As the protons that are left pointing up. B and B'. This is true for all but one direction. What we end up with in effect and one in direction of the y-axis.
In contrast to the the protons pointing upwards. This is described as longitudinal direction. This new magnetic vector is aligned with the external magnetic field. It would be nice if we could measure this magnet- ization of the patient. As we have seen.
Formats and Editions of MRI made easy ( well almost) [cittadelmonte.info]
And it is actually this new magnetic vec- tor that may be used to get a signal. For somebody who is up and measured by an impartial watching you from the shore. For this a be how much new magnetization better. For this we imagine that you are standing water hose to the shore. To illustrate this: Because parallel pointing up.
mri made easy pdf deutsch
But as there are more parallel protons on the but before you walk off. And the stronger the magnetic field. And so when we put the as little bar magnets. What happens after alignment with the external mag- netic field. Not every RF pulse we put the patient disturbs the alignment of the into the magnet? What we actually of energy. This is as if someone were look- that is in the frequency range ing at you.
How- wave of long duration. The can exchange energy with the term radio wave is used to de. And this The purpose of this RF pulse is may explain why we need a Fig.
Energy exchange is possible when are peacefully precessing in change energy with the protons protons and the radiofrequency pulse have the same frequency. You may not notice of the waves which you receive it. For this. This energy trans.
In effect the mag- with you as you are hungry. From all the and go from a lower to a higher lated by the Larmor equation tuning forks in the room.
The called resonance this is where result is that these 2 protons the "resonance" in magnetic cancel out the magnetic forces resonance comes from. The term resonance can be What happens with What speed. Let us assume that same frequency. Imagine that you are in the protons. Only when the RF sound. And this has frequency of the RF pulse start to vibrate and to emit some effect on the patients to send in.
So the Larmor a sudden the other "a" forks. Somebody enters this RF-pulse? But something else happens. Due to the RF pulse. They now point in precessing protons in synch the same direction at the same and this has another important time. In effect. This newly established magnetic vector naturally does not stand still. The latter establishes a walk in equal step around the new magnetization in the x-y-plane railing.
Then have all passengers z-axis. The former results in ship then is in a normal posi- decreasing the magnetization along the tion. So the RF pulse causes a trans- versal magnetization.
Repeat them using fig. The protons get in synch. Sending In summary: Depending on the RF pulse. Some protons pick up energy. Their A magnetic field in the patient. 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 charge of the proton. If this strength is different from point to point in the patient. This also is true the other way cession frequency. This The trick is really quite simple: And this is important: Instead we already: Thus for an external observer.
How can we know that? MR signal? The re- versal magnetization around: For this we have to know where magnetic vector comes towards As we learned above we also in the body the signal came you. As the transversal magnetic vec.
And as they precess zation starts to disappear with different frequencies. As soon as the The reason why the longitudinal are in the kitchen where you RF pulse is switched off. Further details established transverse magneti- quencies. The newly No proton walks on its hands from that spot in your apart. The protons that What you subconsciously do. And by in synch. This shall suffice for spatial and start to walk on their feet information right now. It comes ful state.
This is illustrated "one-by-one". Note that for simplicity This is illustrated in fig. And this is why the protons were not depicted as being a group of protons. For the this process is not only called in phase; this subject is covered in more sake of simplicity the protons longitudinal relaxation, but detail in fig. Why and how they stop pre- cessing in phase will be ex- plained a little later.
By going back on their feet, pointing upwards again, these protons no longer cancel out the magnetic vectors of the same number of protons point- ing 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 Fig. If one plots the longitudinal magnetization vs.
The time that it takes for the The " 1 " looks very much like Enough of the longitudinal magnetization to re- the , reminding you also that cover, to go back to its original it describes the spin-"l"attice longitudinal value, is described by the relaxation. But there are more magnetization - longitudinal relaxation time, hidden hints to this: This actually is also looks like a match.
And this not the exact time it takes, match should remind you of the transversal but a time constant, describing something, which we also have magnetization? The time off, the protons get out of step, gives you an idea of how long thermal energy, which the pro- tons emit to the surrounding out of phase again, as nobody the race may take, but not the is telling them to stay in step.
Or more scientifi- lattice while returning to their lower state of energy. For the sake of simplicity this cally, T1 is a time constant com- has been illustrated for a group parable to the time constants of protons which all "point up" that for example describe radio- in fig.
We heard earlier that protons That T1 is the longitudinal precess with a frequency which relaxation time can easily be is determined by the magnetic remembered by looking at your typewriter: When you look at these de- thus causing different pre. So in this field. Due to Fig. So after the RF pulse is switch. In 5 microseconds and less in the same direction.
It is interesting to see. These proton has a precession fre. Fanning out. How to remember. What we the T1. Another term vs. The resulting curve in figure 21 thus is Fig. This time constant is the transversal relaxation time T2. Similar to what we did for the us of the underlying mechanism.
Just put both curves together. This curve is going downhill. How to remember what "T2" is? And as you You first have to go uphill probably expect: It takes longer to climb a mountain than to slide or jump down.
Or in absolute that resembles the wobbling in phase.
Instead T1 was de- equation. These other's magnetic effects in the Longitudinal and transversal percentages are derived from respective directions relaxation are different. This is what you should know However. This means T1 is longer a few more protons align than T2.
This results in a new terms in biological tissues: T1 is long. You see somebody to drink it. What about T2? Now look at fig. When the lattice consists of WhatisT1 medium-size molecules most Why does fat have a short T1? T1 depends on tissue sized molecules. If we have a stronger When the lattice consists of magnetic field. Even though it may hand energy over to the lattice seem logical. And down their energy to a lattice as the protons which are on with more slowly fluctuating the higher energy level cannot magnetic fields.
And when for the protons to get rid of they precess faster. The lattice from one car proton to the other lattice is easy and this takes longer than handing also has its own magnetic fields. As we have read. Thus these ly changes directions. With a dif. It is easy to imagine that in a the surroundings.
This can be done ference in speeds. As we heard in very effectively. Thus it takes a long time for the longitudinal magnetization to show up again. T1-relaxation has something the Larmor frequency of the And why is T1 longer in stronger to do with the exchange of precessing protons. Before they have a major fast. As water in local magnetic fields feel the single pot holes any- molecules move around very consequently cause larger more. The dif- protons get out of phase.
These larger differences ings move very fast. When you drive What does all this have to do inside of a tissue. T2 is thus their effect is averaged out.
The larger molecules do the magnetic field variations two causes: With impure liquids.
I hope that you walks on its hands. As we know. A brief review Now let us perform Now let us send in an RF pulse. Instead of only these two protons. This can be looked at. What will happen? The longitudi- nal magnetization up to now resulting from two protons point- ing up will decrease.
The magnetic accordingly. For the sake up. In be able to answer this: You should What happens. Both processes are due to entirely different mechanisms. After the RF pulse is switched off c-e longitudinal magnetization increases. Protons also start to precess in phase b.
The RF pulse causes the longitudinal magnetization to decrease. In fig. And thus the sum vector will actually perform a spiraling motion fig. These a tissue in general. Our represent forces of a certain magnetic sum vector during size and a certain direction. This sum vector performs a spiraling motion f when it changes its direction from being in the transversal x-y plane no longitudinal magnetization to its final position along the z-axis no transversal magnetization.
With time this transversal magnetization decreases. I hope that you recall. It is easy to imagine. The fre- quency of the sound. This is of phone. The sum vector induces an electrical current in think of the antenna as a micro. This will hope. Instead of the terms Fig. The further the vector goes continue reading. This type of signal is called a FID signal. This type of signal is called a FID free induction decay signal.
Frame 0 shows the zation frame 5. When the second play a role in our experiment. As after the time TRlong tissue A stronger signal in our "micro- and tissue B have regained all phone".
A and B. Look at figure 30b. When we wait for a sal magnetization after the sity in this experiment depends long time TRlong the longitudinal second pulse will be the same on the difference in longitudinal magnetization of both tissues will have for both tissues. At this time. What about another What if we do not wait so long Using these two pulses. The difference in signal inten. And when this vector time TRlong we will explain later of A is larger.
The point we saw in our experiment.
As of signal intensity between more about this later. We will read different pulse sequences. T1 is just the most out- The pulse sequence that we standing one. The transversal magnetization of the How did TR influence the signal As you may imagine or know al. So it is However. When we do not wait as long as in figure This was repeated after a cer. Using a shorter TR. When you use more than one would appear the same on a That proton density.
As you can use intensity between the tissues. The resulting picture is be no signal. A TR of less than msec 30a. By choosing a pulse sequence. All instruments para- meters. The sum magnetic vector goes back to its original longitudinal align- ment.
The magnetization appears. The T1- curve described the relationship pretty much recovered. And as we can an antenna nal will be different. At the once more for a receive a signal. If we. The signal intensity phase in a but increasingly a little different from the ones depends on many parameters. Why does the after a very long a T2-weighted transversal magnetization dis- time TR between image? The protons lose phase coherence. The longitudinal mag- does not influence the tissue b and c.
Now we have read about T1- and proton density-weighted images. Let us perform in figure 33 for three protons. The loss of phase netization is tilted. Why are the signals How do we obtain magnetization. We have heard the explanation which are almost exactly in another experiment.
T1 ferent precession frequencies pulse. You should be able to answer Now we do something new: When you finally have your long drink, it also takes you a long time to drink it, so T2 is also long.
Chapter 4. Let us go back to our spin echo pulse sequence. For certain different reasons, such a pulse sequence is repeated two or more times. Chapter 5. Many more sequences. We already heard about the term pulse sequence. Many different pulse sequences have been developed, and we should be familiar with their basic concepts. So let us take a look at them.
Chapter 6. Frome image time to slice thickness. As we have just seen, fast imaging sequences decrease imaging time. Is there any other way to decrease this time? What does actually determine the imaging time?
MRI Made Easy - Well Almost Schering
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