MRI Fundamentals. Class II (MR Principles)

Transcription

1 MRI Fundamentals Class II (MR Principles) 1

2 Requirements for MRI Human body (Patient) Strong magnetic field (Magnet) External Radio Frequency source (RF Pulse) 2

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4 Body Composition The molecular composition of the Human body is: Water Fat Proteins 4

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6 Atom? MRI works at the atomic level more specific MRI will works with the atomic nucleus of an atom that meets certain requirements. 6

7 Human Body The atom we are looking for has to meet two prerequisites to be use in MRI. 1. It must be an stable atomic nucleolus with an odd mass number. 2. It must be abundant throughout the body 7

8 Water in Human Body Most of the human body is made up of water (H2O), with cells consisting of 65-90% water by weight. It is not surprising that Oxygen and Hydrogen are some of the most abundant atoms in human body. H2O 8

9 Water Water can be broken down into the following Compartments: Intracellular fluid (2/3 of body water). Extracellular fluid (1/3 of body water). Plasma (1/5 of extracellular fluid). Interstitial fluid (4/5 of extracellular fluid) Transcellular fluid (Contained inside organs, such as the gastrointestinal, cerebrospinal, peritoneal, and ocular fluids). 9

10 Intracellular Water 10

11 Extracellular Water 11

12 Transcellular Water 12

13 Water 13

14 Fat Another very abundant element of the body is adipose tissue (Lipids/Fat). Fat can be found in different forms: Phospholipids Glycolipids Cholesterol 14

15 Phospholipids Phospholipids consist of two fatty acid chains, a phosphate group and a glycerol group. They are the main molecule found in cellular membranes. 15

16 Glycolipids Glycolipids are lipid molecules that contain a sugar unit, such as glucose or galactose. 16

17 Cholesterol Cholesterol is a lipid that consists of four hydrocarbon rings attached to a hydroxyl--hydrogen bound to oxygen-- group. 17

18 Proteins Proteins are large biological molecules, or macromolecules, consisting of one or more long chains of amino acid residues. 18

19 Proteins The body's primary building block for muscle, bone, skin, hair, and many other tissues is protein. Over 10,000 different proteins are found and needed in the body for maintaining life. In fact, after water has been excluded 75 percent of your body weight consists of protein. Proteins play many important roles in the body including the structure of enzymes, transport proteins, hormones, neurotransmitters, etc. 19

20 Proteins Protein consists of building blocks called amino acids, which are linked together in different patterns to form specific proteins with different characteristics. There are twenty different amino acids of which eight are considered essential because they cannot be generated by the body but are necessary for survival. N-acetil aspartate Creatine 20

21 Human Body The elemental composition of the human body can be looked at from the point of view of either mass composition, or atomic composition. To illustrate both views, the adult male human body is approximately 75% water. 21

22 Human Body The most abundant elements in the body by mass are: Oxygen 16 (65%) Carbon 14 (18%) Hydrogen 1 (10%) Oxygen Carbon Hydrogen 22

23 Human Body The most abundant elements in the body by amount (quantity) are: Hydrogen 1 (65%) Oxygen 16 (20%) Carbon 14 (10%) Oxygen Carbon Hydrogen 23

24 Oxygen Isotopes Oxygen isotopes are: Oxygen 16 Oxygen 17 Oxygen 18

25 Oxygen in Human Body Oxygen found in water is characterized by having 8 protons and 8 neutrons in the nucleus. As a result its mass number is 16. Since oxygen mass number is an even number, Oxygen is not consider to be a MR active nuclei. 25

26 Carbon Isotopes Oxygen isotopes are: Carbon 12 Carbon 13 Carbon 14 26

27 Carbon in Human Body Carbon found in fat is characterized by having 6 protons and 8 neutrons in the nucleus. As a result its mass number is 14. Since Carbon mass number is an even number, Carbon is not consider to be a MR active nuclei. 27

28 Hydrogen Isotopes Hydrogen Isotopes: Hydrogen 1 Hydrogen 2 Hydrogen 3 28

29 Hydrogen in Human Body Hydrogen found in human water, fat, and proteins is characterized by having 1 proton and no neutrons in the nucleus. As a result its mass number is 1. Since Hydrogen mass number is an odd number, Hydrogen is a MR active nuclei. 29

30 Hydrogen Water is ~11% hydrogen by mass but ~ 67% hydrogen by atomic percent. 30

31 Hydrogen in Water One 1ml water contains approximately: = 669,100,000,000,000,000,000,000,00 atoms of hydrogen 31

32 Atoms There are three types of motion within the atom: Electrons (-) spinning on its own axis Electron (-) orbiting the nucleus The nucleus of some elements spinning itself about its own axis

33 Magnetism The degree of magnetism exhibited by a substance is due to a property known as the atomic magnetic dipole (or magnetic moment). These dipoles are generated in an atom by the movement of electrons.

34 Magnetism The law of electromagnetic induction indicate that whenever a charge particle exhibits motion, a corresponding magnetic field is induced. Atom Magnetic bar Vector

35 Magnetism Electrons present in the energy shells of atoms can be descried as spin up or spin down depending on the direction where they spin.

36 Magnetism Typically there are equal numbers of each type in a fully filled electron shell. e e e e e e e e

37 Magnetism Electrons on the same element creates equal but opposite magnetic field strength. The opposite polarities of these electrons will cancel each other leaving not magnetic moment on a particular atom. e e e e e e

38 Magnetism Certain atoms having partially filled shells there will be unpaired electrons, the presence of which will create a net magnetic effect in the atom. e e e e e Magnetic moment

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40 Magnetism Some of these electron will migrate from one shell to the next level (ions) and give more or less magnetic characteristics to a particular substance. Migrating electrons e e e e e e Magnetic moment

41 Magnetism The magnetic behavior of an atom is therefore dictated by the configuration of the orbiting electrons. Changes in electron configuration between elements lead to them being classified as: Diamagnetism Paramagnetism Superparamagnetism Ferromagnetism

42 Diamagnetism Diamagnetic substances show a small magnetic moment that opposes an applied magnetic field. Substances of this type are therefore not attracted to, but are slightly repelled by, the magnetic field. Example of diamagnetic substances are: Gold Titanium Platinum Silver Cooper Inert gases Sodium chloride Sulfur e e e e

43 Paramagnetism Paramagnetic substances are the result of unpaired electrons within the atom, they have an small magnetic moment. In a presence of an external magnetic field, paramagnetic substances align with the direction of the field and so the magnetic moments add together. Therefore paramagnetic substances are attracted by the an external magnetic field. Examples of paramagnetic substances are: Oxygen Gadolinium chelates. e e e

44 Superparamagnetism Superparamagnetic materials have positive magnetic susceptibility that are greater than paramagnetic materials and less than those ferromagnetic materials. An example of superparamagnetic material is iron oxide (Fe2O3) particles. e e e e

45 Ferromagnetism Ferromagnetic substances have a greater amount of unpaired electrons within the atom. They are strongly attracted by an external magnetic field. They become dangerous projectiles when they are exposed to an external magnetic field. They maintain their magnetization even when the external magnetic filed has been removed. Examples of ferromagnetic substances are: Iron Steel Cobalt e e e e e Nickel

46 Atomic Nucleus Spinning of the nucleus comes from individual spins of protons and neutrons within the nucleus.

47 Atomic Nucleus Atoms with the same # of protons and neutrons (even mass number) in the nucleus do not spin. If the nucleus of the atom does not spin they will be will electromagnetically neutral (not electro magnetic forces will be present on the nucleus). P N

48 Atomic Nucleus Atoms with different # of protons and neutrons (odd mass number) in the nucleus will spin. Atomic nucleus that spin will pose electromagnetic characteristics and it will make the atom to spin in one direction This atoms will be considered MR Active nuclei

49 Atoms The electrons of an atom are bound to the nucleus by the electromagnetic forces. Likewise, a group of atoms can remain bound to each other by chemical bonds based on the same force, forming a molecule.

50 Atom Atoms with an odd mass number will result in spin of the nucleus. These are known as MR active nuclei. 50

51 Atom Nuclei that have a net charge and are spinning, automatically acquire magnetic moment and align with an external magnetic field. These nuclei are called MR Active Nuclei. Examples of MR active nuclei: Hydrogen 1 Carbon 13 Nitrogen 15 Oxygen 17 Fluoride 19 Sodium 23 Phosphorus 31 51

52 Hydrogen Most of these hydrogen atoms can be found in different categories Free water (Cerebrospinal Fluid) Non free water (Intracellular and Interstitial) Bound to large molecules (proteins) Within fat (Glycolipids) 52

53 Hydrogen Atom The hydrogen atom is the most abundant atom in the human body. In addition to this the atom of hydrogen has and odd mass number in (protium) making the atom of hydrogen the element of choice to be use in MRI. 53

54 Hydrogen As mentioned before the hydrogen atom is characterize by having only one proton in the nucleus, for this particular reason in MRI we will refer to the hydrogen atom as a Proton. Protons have a particular characteristic which is Spin. Spin of different protons may be added together, resulting in an overall spin, which is the spin of the nucleus, then, an atomic nucleus can thus be thought of as a tiny spinning top. 54

55 Spin, Charge, and Magnetism The law of electromagnetism refers to three individual forces motion, magnetism and charge, and states that if two of these are present the third is automatically induced. Magnetic Field 55

56 As a spinning charged particle, a proton is also associated with magnetic force. Each proton generates a tiny magnetic field called a magnetic moment. 56

57 Hydrogen The magnet of each hydrogen nucleus has a north and a south pole of equal strength. The magnetic moment of each nucleus has vector properties (magnitude and direction) and is denoted by an arrow (vector). Hydrogen Magnetic bar Vector (indicate forces in an specific directions) 57

58 Hydrogen In the absence of an applied magnetic field, the magnetic moments of the hydrogen nuclei are randomly oriented 58

59 Hydrogen From now on become used to refer to the hydrogen atom as: Proton (because it has only one proton on the nucleus) or Spin (because it spins when is within the external magnetic field) 59

60 Magnets In MRI a very powerful magnets are used. Between.35 to 8 Tesla. 60

61 Types of Magnets There are two main types of magnets: Permanent magnets Electromagnets

62 A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. Permanent Magnet

63 Permanent Magnet 63

64 Electromagnet An electromagnet is a type of magnet in which the magnetic field is produced by electric current. The magnetic field disappears when the current is turned off.

65 Electromagnet 65

66 Electromagnet 66

67 Vectors In MRI the external static magnetic field (magnet) is represented by a vector. The vector use to represent external static magnetic field is named Bo. Bo Bo is measured in Tesla (T) or Gauss (G) 1T = G 67

68 Earth Magnetic Field The earth magnetic field is equivalent to 0.25 G ( T) to 0.65 G ( T). EARTH MRI 0.25 to 0.65 G 3500 G to 80,000 G T to T 0.35 T to 8 T 68

69 Hydrogen Atom When placed in a strong static external magnetic field (Bo), the magnetic moments of the hydrogen will align with this magnetic field. Bo 69

70 Hydrogen Alignment Bo 70

71 Radiofrequency Source In MRI a radio frequency is needed in order to disturb the hydrogen protons within the external magnetic field. 71

72 Radiofrequency Source In order for the RF Pulse to disturb the hydrogen protons it must meet two requisites: It must match the precessional frequency of the hydrogen protons It must be applied perpendicular to the magnetic field 72

73 Radiofrequency Source If RF Pulses matches the oscillating frequency of the nucleus of hydrogen they will resonate. 73

74 Radiofrequency Source As they resonate, the will gain energy and move away from its position 74

75 Radiofrequency Source One RF stops protons will start to loose the RF energy and they will go back to it original state. 75

76 History of MRI Sir Joseph Larmor ( ) developed the equation demonstrating that the angular frequency of precession of the nuclear spins if proportional to the strength of the magnetic field. Bo ώo=bo x ג

77 History of MRI Felix Bloch (Stanford University) and Edward Purcell (Harvard University) developed instruments, which could measure the magnetic resonance in bulk material such as liquids and solids. (Both honored with the Nobel Prize for Physics in 1952.) [The birth of the NMR spectroscopy] Felix Blosh developed a complicated equation for predicting the MR signal intensity from a tissue subject to a spin-echo sequence.

78 History of MRI In the early 70's, Raymond Damadian (State University of New York) demonstrated with his NMR device, that there are different T1 relaxation times between normal and abnormal tissues of the same type, as well as between different types of normal tissues.

79 History of MRI In 1973, Paul Lauterbur (State University of New York) described a new imaging technique that he termed Zeugmatography. By utilizing gradients in the magnetic field, this technique was able to produce a twodimensional image (backprojection).

80 History of MRI Peter Mansfield further developed the utilization of gradients in the magnetic field and the mathematically analysis of these signals for a more useful imaging technique. Paul C Lauterbur and Peter Mansfield were awarded with the 2003 Nobel Prize in Medicine.

81 History of MRI Jean-Baptiste-Joseph Fourier described the mathematical method for converting a timevarying signal into its frequency components, which are used to obtain positional information from the MR signal. Fourier Transform

82 History of MRI In 1975, Richard Ernst introduced 2D NMR using phase and frequency encoding, and the Fourier Transform. Instead of Paul Lauterbur's back-projection, he timely switched magnetic field gradients ('NMR Fourier Zeugmatography'). This basic reconstruction method is the basis of current MRI techniques.

83 History of MRI 1977/78: First images could be presented. A cross section through a finger by Peter Mansfield and Andrew A.Maudsley. Peter Mansfield also could present the first image through the abdomen and brain.

84 History of MRI In 1977, Raymond Damadian completed (after 7 years) the first MR scanner (Indomitable). 84

85 History of MRI In 1978, Damadian founded the FONAR Corporation, which manufactured the first commercial MRI scanner in Fonar went public In 1981: Schering submitted a patent application for Gd- DTPA dimeglumine (Gadolinium based IV contrast agent). 1982: The first 'magnetization-transfer' imaging by Robert N. Muller. In 1983, Toshiba obtained approval from the Ministry of Health and Welfare in Japan for the first commercial MRI system. In 1984, FONAR Corporation receives FDA approval for its first MRI scanner.

86 History of MRI 1986: Jürgen Hennig, A. Nauerth, and Hartmut Friedburg (University of Freiburg) introduced RARE (rapid acquisition with relaxation enhancement) imaging. Axel Haase, Jens Frahm, Dieter Matthaei, Wolfgang Haenicke, and Dietmar K. Merboldt (Max-Planck- Institute, Göttingen) developed the FLASH (fast low angle shot) sequence. 1988: Schering's MAGNEVIST gets its first approval by the FDA.

87 History of MRI In 1991, fmri was developed independently by the University of Minnesota s Center for Magnetic Resonance Research (CMRR) and Massachusetts General Hospital s (MGH) MR Center. From 1992 to 1997 Fonar was paid for the infringement of it's patents from 'nearly every one of its competitors in the MRI industry including giant multi-nationals as Toshiba, Siemens, Shimadzu, Philips and GE.