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Physics: Magnetic Resonance Imaging Lecturers Lawrence Panych, PhD Lei Qin, PhD Bruno Madore, PhD Stephan Maier, MD, PhD Robert Mulkern, PhD Brigham and Women’s Hospital Harvard Medical School Avdelningen för radiologi Sahlgrenska Universitetssjukhuset (S. Maier) • • • • • • • • • • • • Magnetism, Magnetic Fields and Nuclear Magnetic Resonance MR Signal Properties - Manipulating the Magnetization MR Signal Properties – Relaxation, Diffusion, Dephasing Pulse Sequences – the Basics Pulse Sequences – Image Contrast Mechanisms Image Reconstruction – Basic Mathematics of MRI Image Characteristics and their Manipulation Special Acquisition Techniques Contrast Agents and their Use Image Artifacts MR Instrumentation Safety and Bioeffects of MR Studieböker MRI in Practice. Catherine Westbrook. “lättläst” MRI The Basics Ray H. Hashemi. “lite mer teori” Studieböker The Essential Physics of Medical Imaging. Jerrold T Bushberg. “Utförligt handbok om alla bildgivande metoder” Review of Radiologic Physics Walter Huda “kompendium för examens förberedning av radiologer i USA” Magnetism, Magnetic Fields and Nuclear Magnetic Resonance Main Areas Covered in Lecture • Magnetic susceptibility • Types of magnetic materials • Magnetic fields • Net magnetization due to Bo and field strength Study Questions • What is the basic source of the signal in MRI ? • How is a magnetic field generated ? • What is magnetic susceptibility ? Electric Charge Charge (positive or negative) is a fundamental property of certain elementary particles of matter (e.g. the electron). Opposite charges attract and like charges repel. An electric field is generated around a charge. Current is comprised of flowing electrical charge. Magnetic Field A property of space caused by the motion of an electric charge. A stationary charge will produce only an electric field in the surrounding space. If a charge is moving, a magnetic field is also produced. - Encyclopedia Britannica Magnetar 100 Billion Tesla 3T MRI = 30,000 Gauss 25 Tesla = 250,000 Gauss Magnetic Field Strength Types of Magnets in MRI Permanent Magnet Electro-Magnet Types of Magnets in MRI Permanent Magnet Super-conducting Electro-Magnet Strengths of Magnets in MRI 1 T (Tesla) = 10,000 G (Gauss) Ultra-Low Field < 0.2T (2000G) Low Field Midfield High Field Ultra-High Field 0.25T Permanent Magnet 0.5T 3.0T 7.0T + Super-conducting Electro-Magnet Magnetic Field H is the magnetic field intensity and is measured in units of amps/meter. .p iδL, incremental current element with charge moving along the path, δL rP Ar δ H at the point, p, is equal to: ( iδL X Ar ) / (4 π rP2) where Ar is a unit vector from the current element to the point, p. δ H is a vector orthogonal to vectors, δL and Ar δH δL Ar Magnetic Field Direction of the Magnetic Field Direction of the Current Element Magnetic Flux Density B is the magnetic flux density and is measured in units of Tesla. B=µH where µ is the magnetic permeability, a measure of the ability of a medium to support a magnetic field. In free space µ = µo where µo is the permeability of free space, a measure of the ability to form a magnetic field in a vacuum. Natural Sources of Magnetic Fields The orbital motion of the negatively charged electron around the nucleus as well as its spin are central to explaining the magnetic properties of materials. Nature of Magnetic Materials Types of material classified in terms of magnetic properties: 1.Diamagnetic 2.Paramagnetic 3.Ferromagnetic 4.Antiferromagnetic 5.Ferrimagnetic 6.Superparamagnetic 1. Diamagnetism In Diamagnetic atoms the small magnetic fields caused by the electron orbital motion are cancelled by the magnetic fields caused by the electron spin of the atom and in the absence of an external magnetic field the magnetic moment of each atom is zero. -M: magnetic moment due to electron spin +M: magnetic moment due to electron orbital motion 1. Diamagnetism In the presence of an external magnetic field the electron motion is altered and this slightly lowers the moment due to the electronic orbital motion and there will be a small net magnetic moment, δ, that is oriented opposite to the external magnetic field. -M: magnetic moment due to electron’ spin +M - δ: magnetic moment due to electron orbital motion External magnetic field 1. Diamagnetism Water, e.g. in living objects, is diamagnetic. In strong magnetic fields, the magnetic forces can cause levitation! 2. Paramagnetism In Paramagnetic atoms the small magnetic fields caused by the electron orbital motion are NOT cancelled and each atom has a small magnetic moment, ∆. -m: magnetic moment due to electron’ spin +m + ∆: magnetic moment due to electron orbital motion Net magnetic moment, ∆ 2. Paramagnetism In the absence of an external magnetic field the magnetic moments of the paramagnetic atoms are randomly aligned and the average effect is that there is no net magnetic moment of the atoms. 2. Paramagnetism In the presence of an external magnetic field the magnetic moments of the paramagnetic atoms tend to align and the average effect is that there is a net magnetic moment in the same direction as the external field. External Magnetic field 3. Ferromagnetism In Ferromagnetic atoms there is a relatively large magnetic dipole moment. These moments align over regions called domains. In the absence of an external field, however, the domains are randomly aligned so there is no net field. 3. Ferromagnetism In the presence of an external magnetic field, the domains tend to align in the direction of the external field. External Magnetic field When the field is removed, the random alignment of the domains is not immediately established and there is a residual magnetization of the material. 3. Ferromagnetism A permanent magnet is created from ferromagnetic material that is processed in a strong magnetic field. When the field is removed, the domains remain strongly aligned creating a permanent magnetization of the material. 4. Antiferromagnetism In antiferromagnetic materials, neighboring atoms align anti-parallel with each other, therefore, there is no net moment. Presence of an external field has little effect. 5. Ferrimagnetism As with antiferromagnetic materials, neighboring atoms align anti-parallel with each other, however, neighboring dipoles are unequal giving a net magnetic moment. The presence of an external field has a significant effect. 6. Superparamagnetism In superparamagnetism, ferromagnetic nanoparticles are randomly aligned in a non-ferromagnetic matrix. In a magnetic field the particles align with the field giving a relatively strong magnetic moment. When the field is removed there is no residual magnetization. Nature of Magnetic Materials Types of material classified in terms of magnetic properties: 1.Diamagnetic (most tissues in the body) 2.Paramagnetic (gadolinium) 3.Ferromagnetic (iron, nickel, cobalt) 4.Antiferromagnetic (nickel oxide - NiO) 5.Ferrimagnetic (ferrites) 6.Superparamagnetic (iron oxide nano-particles) Magnetic Susceptibility χ m is the magnetic susceptibilty and is a measure of the degree to which magnetization, M, of a material varies. M = χm H Ferromagnetic materials such as iron and nickel have very large magnetic susceptibilities. Magnetic Susceptibility Spectrum Pyrex Perspex Nuclear Magnetism and Nuclear Magnetic Resonance Source of the Signal in MRI: Net Spin of the Nucleus 12 P P (6 protons and 6 neutrons) N N N N P P N P C N P • Nuclei of all elements contain protons and neutrons • Protons and neutrons combine to give a net spin • If even number of protons and an even number of neutrons Then the net spin of the nucleus is zero: Source of the Signal in MRI: Net Spin of the Nucleus 13 P N P N N (6 protons and 7 neutrons) N N P P N P C N P • If there is an odd number of protons or neutrons Then the net spin of the nucleus is not zero: • Circulating charge creates a magnetic dipole moment. Some Nuclei with MR Signal Nucleus Abundance of Element in the Human Body Natural Abundance of Isotope Percent of All Atoms in the Human Body C 9.4% 1.11% 0.1% Na 0.04% 100% 0.04% P 0.24% 100% 0.24% H 63% 99.985% 63% 13 23 31 1 Some Nuclei with MR Signal Nucleus Abundance of Element in the Human Body Natural Abundance of Isotope Percent of All Atoms in the Human Body C 9.4% 1.11% 0.1% Na 0.04% 100% 0.04% P 0.24% 100% 0.24% H 63% 99.985% 63% 13 23 31 1 Some Nuclei with MR Signal Nucleus Abundance of Element in the Human Body Natural Abundance of Isotope Percent of All Atoms in the Human Body C 9.4% 1.11% 0.1% Na 0.04% 100% 0.04% P 0.24% 100% 0.24% H 63% 99.985% 63% 13 23 31 1 Some Nuclei with MR Signal Nucleus Abundance of Element in the Human Body Natural Abundance of Isotope Percent of All Atoms in the Human Body C 9.4% 1.11% 0.1% Na 0.04% 100% 0.04% P 0.24% 100% 0.24% H 63% 99.985% 63% 13 23 31 1 H is a very good candidate nucleus for MRI because (1) there is a lot of hydrogen in the body, (2) 1H is the most common isotope of hydrogen and (3) the magnetic moment is relatively strong compared to other nuclei. 1 “Proton” MRI 1 H P • In medical imaging we deal almost exclusively with the signal from the hydrogen atom, which is comprised of one proton. • This is why in MRI we often hear the term proton imaging. • Hydrogen atoms in the body are primarily in water and fat. Quantum States 1 Bo P H 1 H P • In a background magnetic field, Bo, the spin magnetic moments of the hydrogen atoms will tend to align with the field. • The possible spin states are quantized. • The magnetic moments must align with the field or against it. • The energy needed to change states depends on field strength. Alignment of Nuclear Spins Bo • The preferred, low-energy state is for nuclear magnetic moments to align with the external magnetic field. Alignment of Nuclear Spins Bo • Although the preferred, low-energy state is to align with the field, because of thermal fluctuations, some spin moments may occasionally align opposite to the field in the higher energy state. Distribution of States Bo • At high-temperature the moments constantly flip back and forth and only a small net magnetic moment of very many nuclei is aligned with the field. • At body temperature, given 1 million hydrogen spins at 1.5T, only 5 more spins are aligned with the field than against the field. Net number of protons aligned with a 1.5T field in 1 mm3 of water: 330 trillion Magnetization Vector mz Bo Net magnetic moment from billions of nuclear spins M my mx Small volume element << 1mm3 Most of MRI can be explained using a magnetization vector. The magnetization vector, M, represents the net moment. Next Lecture MRI Signal Properties Manipulating the Magnetization Vector mz M my mx Study Questions • What is the basic source of the signal in MRI ? • How is a magnetic field generated ? • What is magnetic susceptibility ? Study Questions • What is the basic source of the signal in MRI ? The magnetic moment of the hydrogen nuclei. • How is a magnetic field generated ? • What is magnetic susceptibility ? Study Questions • What is the basic source of the signal in MRI ? The magnetic moment of the hydrogen nuclei. • How is a magnetic field generated ? By moving electrical charge (current). • What is magnetic susceptibility ? Study Questions • What is the basic source of the signal in MRI ? Magnetic moment of the hydrogen nuclei (proton). • How is a magnetic field generated ? By moving electrical charge (current). • What is magnetic susceptibility ? A measure of the degree to which a material can be magnetized. Additional Study Questions • What materials in the body are the main sources of ‘proton’ MRI ? • What would be considered ‘high field’ and what kind of magnet is employed to generate it ? • What type of material has the highest magnetic susceptibility ? Additional Study Questions • What materials in the body are the main sources of ‘proton’ MRI ? Water and fat. • What would be considered ‘high field’ and what kind of magnet is employed to generate it ? 3 T (=30,000 G), super-conducting electro-magnet. • What type of material has the highest magnetic susceptibility ? Ferromagnetic material, e.g. iron. Home Study Questions • How many ‘states’ can the hydrogen nuclei be in when in an external magnetic field ? • Do all carbon nuclei have a net nuclear magnetic moment ? • What is the rule for determining if a nucleus has a net magnetic moment ? • What will increase the strength of the NMR signal ?