实例介绍
【实例简介】
作者用了很多比喻和漫画，帮助理解mri成像的物理含意和原理
eface This book is dedicated to anyone, who tries to teach medicine instead ing medical facts(like my anatomy teacher, Prof. Dr.R. Bock, who is a master of this art) and to anyone, whose stumbling feet find the MRI path difficult The book was written the hope rather than the belief that they m find some help from it) (modified from Alastair G. Smith, Surgeons Hall Edinburgh, October 1939) H.H. Schild 0 about this book This book was written as an introduction to magnetic reson ance imaging(MRI. It is dedi cated 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 ex tremely 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 under stand, 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 magneti Resonance imaging, but rather to serve as an appetizer for further readil Let us start with a genera overview o of mri The single steps of an MR examination can be described quite simply the patient is placed in a magnet e a radio wave is sent in, the radio wave is turned off. the patient emits a signal, which is received and used for reconstruction of the picture Let's take a look at these steps in detail What happens, when we put a patient into the magnet of an MR machine? To understand this it is necessary to at least know some very basic physics -even though this may seem to be borin 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 a are analogous to little planets Like earth, they are constantly This can be demonstrated very turning, or spinning around an easily. Take a rusty nail and axis(fig. 1); or-as one says approach an electrical outlet protons possess a spin. The closer, closer. Do you feel it positive electrical charge, being being repelled by the magnetic attached to the proton, naturally force so you do not put the nail spins around with it. and what into the outlet? is a moving electrical charge? It is an electrical current 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 S Protons possess a positive charge Like the earth they are constantly turnin around an axis and have their own magnetic field Let's review what What happens to in the direction of the external we have read the protons field, parallel to it. Or they may point exactly in the complete when we put them opposite direction, anti-parallel a proton has a spin, and thus the electrical charge of the into an external These types of alignment proton also moves. a moving magnetic field on different energy leve are electrical charge is ar To explain this a man can alig electrical current and this is himself parallel to the magnetic accompanied by a magnetic The protons- being little field of the earth i e, walk on magnets -align themselves in field. Thus, the proton has his feet or he can align himself its own magnetic field and it the external magnetic field like anti-parallel, in the opposite a compass needle in the magnet can be seen as a little bar direction both states are on dif- magnet (fig. Ic) ic field of the earth. However, fe erent energy levels, i.e. need there is an important difference. For the compass different amounts of energy needle there is only one way Walking on one's feet is to align itself with the undoubtedly less exhausting magnetic field, for the protons takes less energy than walking however. there are two (fig. 2). on one's hands. (In the figures The protons may align with this will be illustrated as their South and north pol pointing up or down, see fig. 2) Fig 2 Normally protons are aligned in a random fashion This however changes when they are exposed to a strong external magnetic field. Then they are aligned in only two ways, either parallel or anti parallel to the external magnetic field Naturally the preferred state of It may be obvious at this point alignment is the one that needs already, that for mri the When there are two possible states less energy. So more protons mobile protons are important of alignment the one that takes less are on the lower energy level,(which are a subset of all pr energy, is on a lower energy level is preferred parallel to the external magnetic tons that are in the body 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 pro tons walking on their hands there are about 10 000 007 walking on their feet"(the difference 007" is probably easy to remember) Let us take What type of For reasons we will learn below a closer look at it is important to know how movement Is fast the protons precess. This these protons recession "? speed can be measured as precession frequency, that is We will see that the protons Just imagine a spinning top how many times the protons do not just lay there, aligned When you hit it, it starts precess per second. This pre parallel or anti-parallel to the o wobble or tumble around cession frequency is not magnetic field lines. Instead, It does not, however, fall over. constant. It depends upon the they move around in a certain During this precession, the axis strength of the magnetic field way. The type of movement of the spinning top circles (for magnetic field strength is called precession(fig. 4) forming a cone shape (fig. 4) see page 96), in which the otons are placed It is hard to draw such a pre- The stronger the magnetic field cessing proton, as this is a very the faster the precession rate fast movement as we will see and the higher the pre cession below. For the sake of quency simplicity, we will just make freeze framepictures, as if This is like a violin string we were taking a fast flash the stronger the force exerted light photograph of the situation upon the string, the higher its at a specific moment in time frequency top, which is hit, perf f motion Protons in tic field also show this type of motion, which is called precession It is possible and necessary to Time to take a The precession frequency can precisely calculate this fre be calculated by the larmor quency. This is done by using break equation, and is higher in strong an equation called larmor er magnetic fields. Why is equation however, let us briefly review this pi recession frequency what we have read up to now important? 0=YB0 It has something to do with the ● Protons have a positive resonance in magnetic reson electrical charge which Wo is the precession frequency ance imaging. But to under- is constantly moving, because stand this will take a few (in Hz or mhz), the protons possess a spin more minutes Bo is the strength of the This moving electrical charge After the break you should go external magnetic field which is nothing more than an over the last summary again is given in Tesla electrical current, and the latter and then continue (see page 96), and always induces a magnetic field Y is the So-called gyro- So every proton has its little magnetic ratio magnetic field, and can thus be The equation states that the seen as a little bar magnet precession frequency becomes When we put a patient in the higher when the magnetic field MR magnet, the protons, being strength increases. The exact little magnets align with the relationship is determined external magnetic field. They do by the gy ro-magne tic ratio y this in two ways: parallel and This gyro-magnetic ratio is dif- anti-parallel. The state that ferent for different materials needs less energy is preferred (e. g. the value for protons is and so there are a few more 42.5 MHZ/T). It can be protons walking on their feet compared to an exchange rate than " on their hands"(fig 3) which is different for different currencies The protons precess along the field lines of the magnetic field, just like a spinning top that precesses along the field lines of the magnetic field of earth 【实例截图】
【核心代码】
作者用了很多比喻和漫画，帮助理解mri成像的物理含意和原理
eface This book is dedicated to anyone, who tries to teach medicine instead ing medical facts(like my anatomy teacher, Prof. Dr.R. Bock, who is a master of this art) and to anyone, whose stumbling feet find the MRI path difficult The book was written the hope rather than the belief that they m find some help from it) (modified from Alastair G. Smith, Surgeons Hall Edinburgh, October 1939) H.H. Schild 0 about this book This book was written as an introduction to magnetic reson ance imaging(MRI. It is dedi cated 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 ex tremely 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 under stand, 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 magneti Resonance imaging, but rather to serve as an appetizer for further readil Let us start with a genera overview o of mri The single steps of an MR examination can be described quite simply the patient is placed in a magnet e a radio wave is sent in, the radio wave is turned off. the patient emits a signal, which is received and used for reconstruction of the picture Let's take a look at these steps in detail What happens, when we put a patient into the magnet of an MR machine? To understand this it is necessary to at least know some very basic physics -even though this may seem to be borin 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 a are analogous to little planets Like earth, they are constantly This can be demonstrated very turning, or spinning around an easily. Take a rusty nail and axis(fig. 1); or-as one says approach an electrical outlet protons possess a spin. The closer, closer. Do you feel it positive electrical charge, being being repelled by the magnetic attached to the proton, naturally force so you do not put the nail spins around with it. and what into the outlet? is a moving electrical charge? It is an electrical current 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 S Protons possess a positive charge Like the earth they are constantly turnin around an axis and have their own magnetic field Let's review what What happens to in the direction of the external we have read the protons field, parallel to it. Or they may point exactly in the complete when we put them opposite direction, anti-parallel a proton has a spin, and thus the electrical charge of the into an external These types of alignment proton also moves. a moving magnetic field on different energy leve are electrical charge is ar To explain this a man can alig electrical current and this is himself parallel to the magnetic accompanied by a magnetic The protons- being little field of the earth i e, walk on magnets -align themselves in field. Thus, the proton has his feet or he can align himself its own magnetic field and it the external magnetic field like anti-parallel, in the opposite a compass needle in the magnet can be seen as a little bar direction both states are on dif- magnet (fig. Ic) ic field of the earth. However, fe erent energy levels, i.e. need there is an important difference. For the compass different amounts of energy needle there is only one way Walking on one's feet is to align itself with the undoubtedly less exhausting magnetic field, for the protons takes less energy than walking however. there are two (fig. 2). on one's hands. (In the figures The protons may align with this will be illustrated as their South and north pol pointing up or down, see fig. 2) Fig 2 Normally protons are aligned in a random fashion This however changes when they are exposed to a strong external magnetic field. Then they are aligned in only two ways, either parallel or anti parallel to the external magnetic field Naturally the preferred state of It may be obvious at this point alignment is the one that needs already, that for mri the When there are two possible states less energy. So more protons mobile protons are important of alignment the one that takes less are on the lower energy level,(which are a subset of all pr energy, is on a lower energy level is preferred parallel to the external magnetic tons that are in the body 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 pro tons walking on their hands there are about 10 000 007 walking on their feet"(the difference 007" is probably easy to remember) Let us take What type of For reasons we will learn below a closer look at it is important to know how movement Is fast the protons precess. This these protons recession "? speed can be measured as precession frequency, that is We will see that the protons Just imagine a spinning top how many times the protons do not just lay there, aligned When you hit it, it starts precess per second. This pre parallel or anti-parallel to the o wobble or tumble around cession frequency is not magnetic field lines. Instead, It does not, however, fall over. constant. It depends upon the they move around in a certain During this precession, the axis strength of the magnetic field way. The type of movement of the spinning top circles (for magnetic field strength is called precession(fig. 4) forming a cone shape (fig. 4) see page 96), in which the otons are placed It is hard to draw such a pre- The stronger the magnetic field cessing proton, as this is a very the faster the precession rate fast movement as we will see and the higher the pre cession below. For the sake of quency simplicity, we will just make freeze framepictures, as if This is like a violin string we were taking a fast flash the stronger the force exerted light photograph of the situation upon the string, the higher its at a specific moment in time frequency top, which is hit, perf f motion Protons in tic field also show this type of motion, which is called precession It is possible and necessary to Time to take a The precession frequency can precisely calculate this fre be calculated by the larmor quency. This is done by using break equation, and is higher in strong an equation called larmor er magnetic fields. Why is equation however, let us briefly review this pi recession frequency what we have read up to now important? 0=YB0 It has something to do with the ● Protons have a positive resonance in magnetic reson electrical charge which Wo is the precession frequency ance imaging. But to under- is constantly moving, because stand this will take a few (in Hz or mhz), the protons possess a spin more minutes Bo is the strength of the This moving electrical charge After the break you should go external magnetic field which is nothing more than an over the last summary again is given in Tesla electrical current, and the latter and then continue (see page 96), and always induces a magnetic field Y is the So-called gyro- So every proton has its little magnetic ratio magnetic field, and can thus be The equation states that the seen as a little bar magnet precession frequency becomes When we put a patient in the higher when the magnetic field MR magnet, the protons, being strength increases. The exact little magnets align with the relationship is determined external magnetic field. They do by the gy ro-magne tic ratio y this in two ways: parallel and This gyro-magnetic ratio is dif- anti-parallel. The state that ferent for different materials needs less energy is preferred (e. g. the value for protons is and so there are a few more 42.5 MHZ/T). It can be protons walking on their feet compared to an exchange rate than " on their hands"(fig 3) which is different for different currencies The protons precess along the field lines of the magnetic field, just like a spinning top that precesses along the field lines of the magnetic field of earth 【实例截图】
【核心代码】
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