Mri made easy pdf free download

The first section describes the basic principles, instrumentation and interpretation of MRI, whilst the second section discusses the higher applications of the technique. Authored by Canadian radiologist Govind Chavhan, this second edition includes images and illustrations, as well as a photo CD, to assist trainees with learning.

Key points New edition introducing radiology trainees to principles, sequences and interpretation of MRI Authored by Canadian radiology specialist Features images and illustrations Includes photo CD First edition published in Beginning with an outline of some of the basic principles of MRI, the following chapters concentrate on the cardiac side of CMR with a later section on its more established vascular uses.

This is a unique book in which MRI interpretation of lumbar spine has been made very easy. Each image has a corresponding line diagram, followed by image interpretation and a brief comment about the disease.

Color diagrams are also used to enhance the understanding of the images. It is hoped that after reading this book physicians will become familiar with the MR images and the correlating imaging studies with clinical findings. This book is an effort to help the clinician in the visualization of the lumbar spine by defining normal and abnormal spinal anatomy and pathology in a clear concise manner.

This will be attempted by means of high quality images, abundant line diagrams some of them in color , image interpretation and appropriate comments. A general overview in certain conditions spine secondaries, disc herniations, etc. This pocket book has seventy seven MR images, ninety one line diagrams, fifteen color images and six tables. Almost each MR image has a corresponding line diagram for better interpretation and understanding. It gives a glimpse of the art of interpretation of lumbar spinal MR images needed for day to day practice.

This handy book would be most useful not only to general practitioners but also young radiologists and all physicians. Score: 5. This text is essential reading on undergraduate and postgraduate MRI courses. The book explains in clear terms the theory that underpins magnetic resonance so that the capabilities and operation of MRI systems can be fully appreciated and maximised. This fourth edition captures recent advances, and coverage includes: parallel imaging techniques and new sequences such as balanced gradient echo.

Building on the success of the first three editions, the fourth edition has been fully revised and updated. The book now comes with a companion website at www.

Thus, by changing the time between successive With a long TR we got similar weighted images, and so-called RF pulses, we can influence and modify signals from both tissues, both proton density -weighted magnetization and the signal intensity of tissues.

By choosing a pulse sequence, Fig. All instruments para- meters , however, always play some role in the final sound signal. At the once more for a receive a signal. The T1- curve described the relationship pretty much recovered.

The magnetization appears. If we, however, send in the will have recovered totally. The sum magnetic vector goes back to its original longitudinal align- ment, the signal disappears.

Why does the after a very long a T2-weighted transversal magnetization dis- time TR between image? The protons lose phase coherence, as we have heard pulses not identical? And this is illustrated This is a little more difficult to understand. Let us perform in figure 33 for three protons, We have heard the explanation which are almost exactly in another experiment, which is already. The signal intensity phase in a but increasingly a little different from the ones depends on many parameters.

The longitudinal mag- does not influence the tissue b and c. The loss of phase netization is tilted, we get a contrast any more, however, coherence results in decreasing transversal magnetization. Now we have read about T1- and proton density-weighted images. The result is that the faster precessing protons are now be- hind the slower ones. At that time the protons are nearly in phase again, which results in a stronger trans- versal magnetization, and thus in a stronger signal again. A little later however, the faster precessing protons will be ahead again, with the signal decreasing again.

This is why bit is ahead of the turtle. Thus it is possible to run with constant speed. We naturally obtain more than one signal, more than one spin echo.

A curve connecting the spin echo and another and another. If we did If we now plot time vs. A curve describing the signal intensity in that in fig. These time. In our example with the buses constant manner, and these are Or the difference in signal this would mean that we just the constant inhomogeneities intensities, the difference in record the signals as the buses of the external magnetic field.

The signals vanish Inconstant inhomogeneities from ferences of inherent properties due to extrinsic bus speed local magnetic fields inside of the two groups internal and intrinsic exhaustion of the of the tissue cannot be "evened inhomogeneities ; may be in one passengers properties under out", as they may influence bus, there are only the "party these circumstances see fig.

So some of the the other crowd. It is im- May be we should illustrate this portant to realize that with a buses will be back at the start- by an example: imagine two spin echo sequence, we cannot ing line. The signal intensity, buses full of people, e. They and proton density - weighted phone then depends only on are standing at a starting line pictures.

We will get to that a inherent properties, i. With two micro- little later. Then the buses inhomogeneities, the protons leave in the same direction. Due to this they will be Without having the buses come back out of phase faster, the trans- i. To distinguish this erties the differing shape of the shorter transversal relaxation bus passengers , or due to external influences, i. First we sent have become smaller. However, this The intensity of this echo is transversal magnetization dis- given by the T2-curve at the time appears, due to T2-effects.

How TE. And as we can transversal magnetization faster. As both T2-curves and only little static noise. So it might be reasonable to away, you may not be able With a short TE, however, there wait a very long TE; the result- to discern the music from the will be a problem fig.

And this Both T2-curves in this example heavily T2-weighted. If we But and there is always a there is always some noise in only wait a very short TE, the "but" if we wait longer, the the system, however, when the difference in signal intensity total signal intensity becomes signal is strong, this does not between tissue A and tissue B smaller and smaller.

The signal matter much. However, the is very small, both tissues may to noise ratio becomes smaller, smaller the signal, the harder hardly be distinguished as there the picture appears grainy. Consequence: with signal-to-noise problem: when a short TE, differences in T2 you receive a local radio station do not influence tissue contrast in. To figure out how much signal the starting point from which Fig.

So we just attach the T2- sequence by combining the T1-and the spin echo sequence, you actual- curve at this point. How much T2-curve for that tissue. There image, also depends on TE, the as it is "tilted" 90 degrees. This trans- we have the T1 and T2-curve of time that we wait after the versal magnetization immediately starts to disappear by a rate which is deter- a certain tissue. So we now only have mined by the transversal relaxation time, Which parameter determined to look for the signal intensity and thus by the T2-curve.

The signal the amount of longitudinal at the time TE on the T2-curve. That was TR. T1 to relax totally. With a long TR, differences depicted. When we only pulse in between. The resulting picture is thus neither T1- nor T2-weighted, ing longitudinal magnetization not really matter. When we but mostly determined by the proton into a transversal plane, also use a short TE, differences density of the tissues for this, ideally resulting in a transversal in signal intensity due to TE should be zero.

The more longi- differences in T2 have not had tudinal magnetization we have, enough time to become pro- the stronger the initial nounced yet. T2- weighted, but mainly in- As we read earlier, with a very fluenced by differences in pro- long TR, all tissues will have ton or spin density. The more recovered their longitudinal mag- protons, the more signal, if you netization totally; differences look at it in a simple manner in T1 of the tissues examined figure With the long TE however, dif- ferences in T2 become pro- nounced figure Thus the resulting picture is T2-weighted.

What if we use a shorter TR What if we use a very short Fig. TR and a very long TE? When we wait a shorter time TR, differences in T1 influence tissue con- With a short TR, tissues have This is only a theoretical trast to a larger extent, the picture is T1-weighted, especially when we also not recovered their longitudinal question. With a very short wait a short TE when signal differences magnetization, thus differences TR, there will be only very little due to differing T2s have not had time in T1 which determines, how longitudinal magnetization which to become pronounced.

And with a long TE regained will show up in form we even allow the small amount of signal intensity differences of transversal magnetization fig. When TE is short, resulting to disappear to a large differences in T2 cannot really extent.

The resulting signal will manifest themselves, so the be so small, of so little inten- resulting picture is still T1- sity, that it cannot be used to weighted there is a lower limit make a reasonable picture. If you have not been concentrating for the last minutes, you will probably be just short of giving up right now.

How to remem- ber this - even if you do not understand all of it which hopefully is not the case? Try looking at fig. What do you see?

A man with short TRousers. And considering the weather conditions, this makes only one person in the picture happy. This should remind you that a short TR ousers gives a T1- Fig. What do you see in figure 44? The same couple is having tea. Now having tea, which is usually served hot, always takes long. And in the illustration the long TEa makes two people happy. This should remind you that a long TE gives a T2-weighted image.

The CSF is black on the T1- weighted image. However, it has the strongest signal in the T2-weighted image. On the spin-density image it is of intermediate signal intensity. Some practical hints to image interpretation How can we tell from a picture, whether it is a T1- or a T2- weighted image, when imaging was done with a normal pulse sequence, not one of the fast sequences see below.

As a rule of thumb: if you see white fluid, e. CSF or urine, you are dealing with a T2- weighted image. If the fluid is darker than the solids, we have a T1- or a proton-density image. Look at fig. In b CSF still is dark, even though its signal intensity is slightly higher than in the T1- weighted image; contrast be- tween grey and white substance is reversed. In c CSF has a higher signal intensity than grey and white substance, the picture is T2- weighted.

These are rules-of-thumb only. Thus, you might be unlucky, Fig. The signal intensity of the tissues is reversed choosing a TE beyond the pictures taken with different meters that do not allow tissue crossing point TEC : before this cross- imaging parameters.

Look at figure You see that reason for performing two This means that image contrast is still in this example the T2-curves different examinations with dif- determined by differences in T1: the start at different "heights", and ferent T1- and T2-weightings. After the crossing point e. Think about the same signal intensity. Now we have already heard How does flow tion through which a vessel is about many parameters, that crossing.

The first experi- record a signal. At this time, ments on this subject were all the original blood in our carried out about 30 years ago. So there is no nomenon was used to measure signal coming out of the vessel; flow in the fuel pipes of satel- it appears black in the picture. The subject of how flow in- fluences the MR signal is rather Fig.

If we send in much more complicated. For things, e. If we wait some flow profile, and whether there mope time before we send in is laminar or turbulent flow. They will also give you more information on MRI angiography. In this technique the fact that flow influences the MRI signal is used in a beneficial man- ner by displaying the moving protons.

The respective T1- and T2- curves are shifted towards the left. In effect, that means that for a certain TR there is more, for a certain TE there is less signal. Chemically the substance is a Certain so-called paramagnetic rare earth. As Gadolinium is substances have small local toxic in its free state, it is magnetic fields which cause a bound to DTPA in a certain way shortening of the relaxation chelation , which solves the times of the surrounding pro- problem of toxicity.

This effect is named The effect of the contrast me- proton relaxation enhancement. Examples are degradation products of hemo- globin, e. The i. As the substance is not distrib- uted evenly throughout the Ready for a administered Gadolinium enters body, signals from different repetition? Vascularized tumor As we know by now, many para- curve is shifted to the left. The tissues are enhanced for ex- meters, e. T1, T2, proton den- result is that the signal from ample. It is also important that sity, pulse sequence, influence tissue A at time TR is stronger the Gadolinium does not go the appearance of tissues in than it was before and the through the intact, but rather an MR picture.

As loss of signal often is more Gadolinium entering into the the contrast medium Gado- difficult to appreciate than a tumor tissue shortens the T1, linium-DTPA, shorten T1 and T2 signal enhancement, T1-weighted thus making the tumor bright in of the surrounding protons. This images are the predominant a T1-weighted image, while the results in a signal increase in imaging technique used after surrounding edema may not be T1-weighted images and a signal contrast medium injection.

And because imaging time de- pends on TR, as we will see By choosing the pulse sequence later, imaging then may take and imaging parameters, like TR Fig. Many different to the left, as contrast agent entered similar to the ones of contrast pulse sequences have been de- tissue A but not tissue B. At the same media in conventional radiology; veloped and we should be famil- time TR there now is a much greater Gadolinium-DTPA however seems liar with their basic concepts.

So let us take a look at them. With a TRshort, the fig. We have already partial saturation protons have discussed them, but we did not relaxed the T1 becomes not give them a name. Look at figure 52 with the T1-curves going uphill!

If we Fig. With a shorter TR, the partial saturation sequence, the resulting image is T1-weighted. The tissue in the bottom row illustrated, this results in less trans- after the time TI. Inversion recovery water. To get a measurable signal, however, we need some sequence transversal magnetization. What happens?

The signal intensity up, now point down. This is in an inversion recovery image illustrated in fig. So we get a T1- shorter T1, is in the bottom weighted image, which is even row. If we do not do anything more T1-weighted than partial else, the longitudinal magneti- saturation recovery images. If you do not remember or even understand this by now, sequence echoes. The disadvantage is you should read pages 50 to 65 however, that the signal be- again.

We have talked about the spin comes weaker and weaker. MR signal in the spin echo At this time, you should be able sequences? After the time TE, we the TR do? They determined how the resulting image was weighted: TE was responsible for the T2- Fig. This is repeatedly illustrated as the spin echo sequence is so important. In tudinal magnetization to be this, go back to page 29 for a addition, there is some unavoid- tilted by the next pulse, yielding short repetition.

Due to these larger magnetic and heart beat. This same strength, but in opposite Gradient Recalled Acquisition means that an uneven magnetic direction. The faster moving at Steady State. However, magnetic field. This results we have talked about up to in some rephasing, and thus now. Here is a rough outline. After suming parameter of an imaging this echo the signal decreases sequence see also pages 58 again. It makes sense to shorten TR if we want to make imaging faster.

And this is done in the fast imaging sequences. Instead, there is al- figure With these fast scans it is possible to do imaging in a second or even less.

Time to repeat About imaging measurement, but to repeat the measurement several times. TR is relatively use fast sequences? Actually, what you relatively long with saturation imaging time? While saturation re- For MR imaging with normal signal-to-noise ratio. Naturally, covery yields proton spin pulse sequences this can be imaging time increases with density images, the images easily calculated; the acquisition every additional measurement.

What does that mean? This sequence can give proton density- weighted, T1-weighted, or T2- weighted images. This is deter- mined by the imaging parameters which are chosen TR, TE.

Image weighting is also determined by the type of sequence and the imaging para- meters chosen. To illustrate this: signal does. So all together you faster than if you have 25 Just imagine that you are sitting will have a better signal-to- rows to write.

However, you in a large audience, where many noise ratio which you would have more contents, more detail people make noise. As you know from cannot really understand him, other imaging methods or your because there is so much back- TV , pictures are made of pic- ground noise.

What you will ture elements, which altogether probably do, is ask him to re- make up the image matrix, e. You mentally add up the of picture elements pixels.

As this of rows in a matrix, like rows signal is always the same, it will in a letter. The more rows you increase by adding it up. The have, the more time it takes background noise, however, is for the image. Just think about not always the same. Instead it this as writing a letter: is random and fluctuates and if you have paper with 5 rows Repeated measurements result in does not add up the way the on a page, you will finish a page a better signal-to-noise ratio.

And why does TR influence acquisition time? If you choose a long time TR to repeat your pulse sequence, to perform additional signal measurements, imaging takes longer than with a short TR. However, there is a trick that can shorten imaging time. While we are waiting to repeat our imaging sequence in one slice, i. The longer the TR, the more slices we can excite in the meantime. So for just adding a little extra time, we will examine many slices instead of one, and imaging time per slice decreases substantially.

We perform so called multi- slice imaging. Another way to possibly reduce TR, and thus imaging time, is the use of a contrast medium: as we have read, Gadolinium shortens T1. And when T1 is Fig. So during the time TR, we actual- ly recorded signals for more than one image, though from different slices.

Let us review all How can we select the factors that a slice which we influence signal want to examine? This additional field is called a If you are not sure about one gradient field, and is produced of these, go back to the page by the so-called gradient coils. In case you feel familiar strength of the original magnet- with these facts, go on, and ic field.

In figure 57, magnetic read about some important field strength increases for things in MR imaging, that we different cross sections from have not talked about yet. Consequently, the protons in the different slices experience different magnetic fields, and thus have different precession frequencies. So the RF pulses which disturb the protons in the different slices must have different frequencies as well.

As gradient fields can be super- imposed in any direction, it is possible to define not only transversal slices, but all kinds of different imaging planes without moving the patient. The gradient field that enables us to examine a specific slice is also called slice selecting gradient.

In the illustration the resulting magnetic field strength is increasing from 1. By selecting a certain RF pulse frequency we deter- mine the location of the slice which we examine. How can we determine or select a certain slice thickness? We can select a different slice thickness in two ways fig.

This has been illustrated in figure If we use an RF pulse with fre- quencies from 64 to 65 mHz, we will get a slice thickness S1 fig. If, however, we only use frequencies from 64 to If we have a steeper gradient field, i. The slice thickness in 58c with the steeper gradient field is, however, smaller than in a. The result is that the protons Fig. The first is to use an RF pulse signal come from?

The gradient but a certain range of frequencies, Now we have selected position applied is thus also called the a so-called bandwidth. If, for example, we send in an RF pulse, which contains and thickness of our slice.

But frequency encoding gradient. As quencies between 64 mHz and When there is more difference The trick is similar to the slice thing else. Theoretically, we in magnetic field strength between the selecting gradient which is could use the same trick with level of the feet and the head, i. This, how- sulting slice will be thinner, even though tion of the RF pulse.

After the RF pulse is sent in, difficulties e. The problem is solved quencies are 56 to 72 mHz in c vs. Using the same protons in the slice selected, in a different way this time. RF pulse containing frequencies from 64 precessing all with the same fre- to 65 mHz results in imaging of a thinner quency. We now apply another slice 3 in c than in a. So the precession fre- quency of the protons will also decrease from left to right in our example the precession frequencies are 65, 64 and 63 mHz, respectively.

The protons in the three To determine where in a certain slice rows now experience different magnetic a signal comes from we use a magnetic fields, and thus give off their signals gradient field. In a nine protons in with different frequencies e. They pre- and 63 mHz. The corresponding cess in phase with the same frequency magnetic gradient is called the frequency after the RF pulse is sent in.

We now can tell A magnetic gradient field is then super- from which row a signal comes from, but imposed on the external field, which still cannot pinpoint the exact place in b decreases in strength from left of origin.

Look at figure 60, where the strength of the magnetic same precession frequency. In the example fig. Formerly the pro- 65 mHz column. The protons 60b the increase in speed is tons and their signals were are in phase after the RF pulse less from top to bottom in in phase. Now the protons and "whipping". Now we apply the column. When this short their signals still have the same a magnetic gradient along this gradient is switched off, all frequency, but they are out of column for a short time.

This the protons of the column ex- phase this can be viewed as if causes the protons to speed up perience the same magnetic their magnetic vectors come by their precession according to field again, and thus have the the antenna at different times. In a the row with the precession frequency of 65 mHz from figure 61 is depicted.

We now switch on a gradient field, which is stronger at the top than at the bottom of the row b for a very short time. The proton at the top thus precesses faster than the one in the middle, which in turn precesses faster than the proton at the bottom. This difference in precessing frequency only lasts for a very short time. How- ever, when the gradient is switched off, all protons experience the same magnetic field again, thus have the same 65 mHz precession frequency again c.

However, now we have a little difference among these protons: even though they precess with the same frequency again, they are a little out of phase, and consequently give off signals of the same frequency, which, however, are dif- ferent in phase, and because of this can be differentiated.

The corresponding gradient is called the phase encoding gradient. As the gradient which we used phases, all according to their causes protons to precess in location. By means of a math- different phases, it is called ematical process called Fourier the phase encoding gradient. As these These have different fre- signals can be assigned to a cer- quencies, and signals with the tain location in the slice, we same frequency have different now can reconstruct our image.

As they have different the y-axis. This results in dif- quency or certain phase comes precession frequencies, we ferent precession frequencies from. And as the Fourier trans- can send in an RF pulse that con- along the y-axis, and thus formation gave us the corres- tains only those frequencies, different frequencies of the ponding signal intensities, which excite the protons in the corresponding signals we can now assign a certain slice which we want to image.

During this short time, band width of the RF pulse, the protons along the x-axis or by modifying the steepness precess with different fre- of the gradient field. When this gradient is D the slice selecting gradient switched off, they go back to is only turned on during the their former precession fre- RF pulse quency, which was the same for all of them. Due to this phase encoding gradient, how- ever, the protons and their signals are now out of phase, which can be detected.

Note: which gradient is applied in what direction y-axis, x-axis can be varied! The result: their magnetic A few more basics Can we use every moments would cancel each By now we have discussed just other nucleus for other out. If we have a nucleus about every important aspect imaging? But: why did we e. We can only still cling together and neutral- only?

What about the nuclei? An exception is ment. Nuclei with odd numbers and neutrons, but this will go the hydrogen nucleus, which of protons thus have a magnetic into too much physics, so we only consists of one proton. And when we talk about the pro- used for MRI. The hydrogen their electrical charge was nucleus is best for MR imaging also spinning, moving.

And the as hydrogen occurs in large moving electrical charge was abundance throughout the body. If not for the spin, there nuclei in the same magnetic would be no magnetic field.

All of the routine requirement, the odd number? However, a little bar magnet. If you have lots of research is being done a nucleus with two or any on the use of other nuclei.

Let us have a look Magnets used for imaging mostly have field strengths somewhere Permanent magnets: at some hardware between. It The most important part of the ic field is between 0. The strength of magnet is always magnetic Their magnetic field has to be and does not use any energy for of a magnet is given in Tesla or very homogeneous, as it direct- Gauss, where work, which are its advantages.

The homogeneity is thermal instability, its limited quoted in terms as ppm, part field strength, and its weight Gauss was a German math- per million, in a defined volume a magnet of 0. He was a peculiar How detrimental even rather fellow, having refused to share small inhomogeneities and thus the Nobel prize with the differences in precession fre- inventor Thomas Edison in the quency can be, was illustrated early s. Homogeneity of the magnetic field can be im- proved by making some electri- cal or mechanical adjustments, a process called shimming.

In MRI different types of mag- nets are used. Resistive magnets: Superconducting Advantages of superconducting magnets are high magnetic field In a resistive magnet, an elec- magnets: strength and excellent magnetic trical current is passed through field homogeneity.

This is in Superconducting magnets are the order of ppm over a loop of wire and generates the ones most widely used in a region 45 cm in diameter.

Resistive mag- MR machines at the present Disadvantages of the supercon- nets are therefore also called time. They also make use of ducting magnets are high costs, electromagnets. They are only electricity, but they have a and use of rather expensive magnetic as long as there is special current carrying con- cryogens.

This is cooled down to through them. Thus, they use superconducting temperature electrical energy. At a resistance to the flow of the this temperature, the current electricity through the wire, conducting material loses its these magnets get warm when resistance for electricity.

So if in operation, and have to be you send in an electrical current cooled. So called cryo- field strength. Resistive magnets gens helium, nitrogen are are not very practical with very used for cooling of these mag- high field strengths because nets, and have to be refilled they create lots of heat that once in a while. When for some reason the The relatively new iron core temperature rises above the hybrid resistive magnets superconducting temperature have features of permanent and in these magnets, there will "normal" resistive magnets, be a loss of superconductivity combining some of their advan- so-called quench , and tages.

This results in rapid heat production, which causes cryogens to boil off rapidly these leave the system via the so-called quench lines. What is all this talk about the magnetic field strength? Another piece of the Volume coils Which is the ideal field strength? These completely sur- the ideal horsepower for a car. These volume and cons: to receive the resulting signal. The body have a better spatial reso- be used for transmission of coil is a permanent part of the lution and may be used for the RF pulse and receiving the scanner, and surrounds the spectroscopy signal.

A variety of coils are patient. It is important, as it is in use. It also receives contrast, are cheaper in price the signal when larger parts and in operating costs.