MR Physics - Generating and Acquiring the MR Signal - USA

Upon successful completion of this course, you will be able to: Describe the importance of an external magnetic field in MRI Explain why the human body is susceptible to magnetic fields Define nuclear spin and spin precession Discuss the physics principles that govern magnetization Explain how MR signals are generated Congratulations. You have completed the MR Physics - Generating and Acquiring the MR Signal Online Training course. Listed below are the key points that you have learned about the physics principles that govern MR imaging. Take time to review the material before you proceed to the final quiz. Describe the importance of an external magnetic field in MR Magnetic fields exert a force on magnetizable particles (i.e. hydrogen atoms) MR scanners have a stronger magnetic field than the Earth's magnetic field   Explain why the human body is susceptible to magnetic fields The human consists of 70% water and therefore large amounts of hydrogen protons Hydrogen nuclei show the most sensitive magnetic effect in the body and give off the strongest magnetic resonance signal Protons interact with an externally applied magnetic field and align parallel with the field   Define nuclear spin and spin precession All fundamental particles have an individual spin Nuclear spin remains constant and never stops; it only varies by changing the direction of the axis Spin of the hydrogen proton moves in the shape of a cone about the direction of gravity Larmor frequency tells us how frequently the protons precess around the external magnetic field Precessional frequency of the spins is proportional to the field strength (gyromagnetic ratio) Discuss the physics principles that govern magnetization Magnetic moment is a proton quantity that determines the magnitude and direction of the force that this elementary magnet can exert   Explain how MR signals are generated Magnetic resonance is necessary to generate MR signals Resonance occurs when the frequency of the RF pulse and Larmor frequency of spins are equal Applied RF pulse generates measurable transverse magnetization MR signals are picked up by the receiving antennas or coils Select this link to download the Glossary of MR terms. MRI uses magnetic fields and radio waves X-ray and CT scanning uses ionizing radiation The human body is susceptible to the magnetic field produced by MRI scanners. Human body consists of about 70% water Water molecule: 1 atom of oxygen and 2 atoms of hydrogen Hydrogen: 1 proton and 1 electron Hydrogen nuclei show the most sensitive magnetic effect in the body and give off the strongest magnetic resonance signal MRI utilizes the magnetic properties of hydrogen protons.         Hydrogen Atoms Learn about properties of atoms. Tab TitleTextAtoms An atom consists of protons and neutrons in a core, or nucleus, and electrons in outer shells. The negatively charged electrons move on specific paths around the nucleus. Since protons are positively charged and neutrons have no electrical charge, the atomic nucleus is electropositive.   Atomic Number Atomic number Number of protons in the nucleus Primary index for describing atoms All atoms of an element have the same atomic number   Examples: Hydrogen 1 proton Carbon 6 protons Oxygen 8 protons Atomic Weight Atomic weight: Numbers of protons plus the number of neutrons in the nucleus. Isotopes have the same atomic number but different atomic weights.   Examples: Protium 1 proton, 0 neutrons 99.9% Deuterium 1 proton, 1 neutron 0.1% Tritium 1 proton, 2 neutrons even less Nuclear Spin Nuclear spin: a requirement for MR that depends upon the isotope.  Nuclei with an even number of protons and even number of neutrons are magnetically neutral and give no MR signal. But nuclei with an odd number of neutrons have a nuclear spin.   3 Classes of Spin Zero even atomic weight even atomic number 12C, 16O Integer even atomic weight odd atomic number 2H Half Integer odd atomic weight 1H A single hydrogen proton, called the nucleus, has a defined volume, mass, and a positive charge. Nuclear spin is the measure of the quantum state of an atomic particle. All fundamental particles have an individual spin. This spin is the quantum number of the particle. The spin of the nucleus: Remains the same magnitude, never stops Only varies by changing the direction of the axis The collective effect of the particles' spins in the nucleus is described by a spatial vector called angular momentum.         When exposed to a strong magnetic field, the hydrogen proton behaves more like a spinning top than a compass needle. The spin of the hydrogen proton moves in the shape of a cone about the direction of gravity Precession describes the wobbling movement of a spinning top The spin axis moves in the shape of a cone about the direction of gravity Precession is fundamental to magnetic resonance.        A proton precesses like a spinning top with an angular frequency (ω). Larmor Frequency tells us how frequently the protons precess around the external magnetic field: Depends on the type of nucleus and strength of the external magnetic field applied Measured in megahertz (MHz) The Larmor frequency increases proportionally with application of an external magnetic field B0 and higher precession frequency of spins. The MR system must be tuned to the Larmor Frequency of the spins.     Larmor Frequency Learn about the Larmor equation. Element HTMLLarmor Frequency: the precession frequency of spins  ω = γ • B0ω = frequency of the spinsB0 = external magnetic field γ = gyromagnetic ratio Sound File Audio ScriptThe precession frequency of spins is known as the Larmor (pronounced "Lahr - More") Frequency. The Larmor Frequency depends on the type of nucleus and the strength of the magnetic field applied. The Larmor Equation is represented by omega (ω) equals gamma (γ) times Bo (pronounced "B-naught")   Omega represents the frequency of precession for the spins around a magnetic field. B0 is the external magnetic field. The proportionality constant gamma is called the gyromagnetic ratio. The spins respond to the magnetic field with a precessional motion around the magnetic field. The frequency of the nuclear precession or rate of rotation is proportional to the strength of the magnetic field. So the higher the magnetic field strength, the higher the frequency with which the precession of the magnetic moment takes place. The gyromagnetic ratio depends on the type of nucleus and defines the relation between the spin and the magnetic moment of the nucleus. This ratio is constant for a nucleus and is expressed as the precessional frequency of the nucleus at 1 Tesla. The unit of measurement for gamma is MHz/T (pronounced "mega hertz per tesla"). The spin precession defined by the Larmor equation is the quintessential movement in MRI and one of the key components for MRI imaging. The magnetic moment is a proton quantity that determines the magnitude and direction of the force this elementary magnet can exert. The magnetic moments are responsible for the weak magnetism created in the human body through MRI.     Vectors Learn about vectors. Vectors are excellent for defining quantities that depend on spatial orientation because they exhibit magnitude and direction. The direction of the arrow corresponds to the direction of the vector quantity The length of the arrow corresponds to the magnitude of the vector Vector quantities allow for spatial addition, but the direction must be taken into account. If the arrow point in the same direction, the magnitude of vector is simply the sum of the magnitudes (a + a = 2a) Vectors of the same magnitude but opposite direction cancel each other out (a - a = 0) Vectors can also be divided into separate components, which are illustrated as the projection of the arrow along predefined spatial axes, typically coordinate system. The example to the right shows that vector c is the sum of vectors a+b Vertical component a Horizontal component b Spin & Magnetic Moment Learn about magnetic moment formula. Base ImageHotspotsText BlocksImage FileMagnetic MomentAngular MomentumPlanck's constantSpin quantum number describing the angular momentumGyromagnetic ratio In MRI, we measure the net effect of a collection of spins within a voxel, rather than the effect of individual spins. Two identical particles cannot be in the same state, so they align their spin orientations anti-parallel to each other - creating a net spin of zero. Only atomic nuclei with an uneven number of protons or neutrons have a net spin - called nuclear spin.     Voxel Learn about the voxel. Instead of measuring the effect of individual spins in the body, we consider the net sum of all the proton spins within an observed volume element or voxel. A voxel can be seen as a cube with a specific edge length (i.e. 10 mm) which contains a specific volume (i.e. 1 ml).  In order to generate the MR signal, we have to influence the magnetization so that a non-zero component will precess in the xy-plane. Apply energy to create transverse magnetization in the xy-direction of the magnetic field The magnetization can be tilted by applying a short electromagnetic pulse, the RF pulse. These RF pulses disturb the equilibrium of spins and excite protons by applying alternating magnetic fields in the radiofrequency range. The frequency of the RF pulse and Larmor frequency of spins have to resonate with each another to generate an MR signal.       The RF pulse consists of the following RF coils: RF transmitting - antenna for sending RF pulses RF receiving - antenna for receiving the MR signal There are 2 types of standard RF pulses. The angle of the pulse is called the flip angle. 90o pulse - flips magnetization in the transverse direction (xy-plane) 180o pulse - flips the magnetization in the longitudinal direction (z-plane)       The MR measurement consists of two parts: Excitation - energy "in" Relaxation - energy "out" How do we capture the MR signal?  Only transverse magnetization (Mxy) is received by coil antennas 90° RF pulse turns the longitudinal magnetization (Mz) into transverse magnetization (Mxy) Stronger transverse magnetization creates a stronger MR signal Free Induction Decay (FID) describes how transverse magnetization decays quickly over time     There are 4 main components in an MRI scanner that are essential for creating MR images. The main components include: a magnet (or magnet coils) a radio-frequency (RF) system a gradient system a high-performance computer system The high performance computer system includes the following: Image Processor Host Computer Control and Evaluation Software        Magnetism is a fundamental property of nature. A magnet creates a surrounding magnetic field. A magnetic field can also be created by electric currents and electromagnets. A magnetic field can be represented by field lines and the vector quantity B0. B0 indicates a strong, externally applied magnetic field by the MR system. Strength of B0 is measured in Tesla Direction of B0 runs parallel to the z-axis xy-plane runs transverse to the magnetic field line        Magnetic Strength in MRI Learn about magnetic fields in MRI. The magnetic field in MRI is measured in Tesla. The majority of clinical MRI systems operate at 1.5-3 Tesla.  These produce an extremely strong magnetic field, up to 50,000 times higher than the earth’s magnetic field (0.00003T-0.00007T). A magnetic field of uniform field strength is called a homogeneous field. The field lines of a homogeneous field are drawn as equidistant, straight lines running in parallel. A magnetic field that does not change over time is known as a static field. Without any external magnetic field, the nuclear spins of protons are randomly oriented in space. Protons exist naturally in a random, energetic balance Precessions are out-of-phase and incoherent Directions of the magnetic moments are randomly distributed When the value of aligned spins is equal to zero, there is no magnetization.     All spins precess at the same frequency around the direction of the magnetic field, but they exhibit a random phase orientation. Constant magnetization (M) results from the excess spins parallel to the +z direction Perpendicular to the z direction Spins cancel out because of the random orientation The resulting magnetization in the xy-direction, the transverse magnetization, is zero. How do we generate an observable signal?     Exposing the human body to strong, external magnetic field (B0) causes protons to align in the direction of B0. The preferred energetic status of nuclei is to be parallel with the magnetic field. Protons precess with their characteristic (Larmor) frequency based on B0. The protons that align in the +z direction cancel out with protons that align in the -z direction. Lower energy spins are called spin-ups Higher energy spins are called spin-downs Slightly more protons align in the +z direction, which creates a surplus of spin-ups. The result is a net magnetization in the z direction (Mz).     Net Magnetization & Boltzmann Distribution Learn about net magnetization & Boltzmann distribution. Slide NumberText BlocksCalloutsAudio ScriptImage File1The number of spin magnets that align parallel or anti-parallel will depend on the magnitude of the energy difference ΔE.  The number of spin magnets that align parallel or anti-parallel in an applied magnetic field will depend on the magnitude of the energy difference delta E.2ΔE = 0 when the protons have a random orientation in a magnet-free environment. The energy level E increases proportionally with the strength of the magnetic field B0.When delta E equals zero, the protons have a random orientation in a magnet-free environment. This random orientation of protons occurs because the energy level E increases proportionally with the strength of the magnetic field.3In this first example, B0 is 1 T and the difference in the energy levels causes one more proton to align parallel compared to those in the anti-parallel direction.In this first example, the magnetic field (b-knot) is 1 Tesla. The difference in the energy levels causes one more proton to align in the parallel direction compared to those in the anti-parallel directon.4If B0 is increased to 1.5 T, more protons align in the parallel direction.If b-naught is increased to 1.5 (pronounced "one point five") Tesla, additional protons will align in the parallel direction.5The statistical distribution between these two energy levels is described by the Boltzmann distribution. The distribution depends on the temperature of the sample where the protons are located. The higher the temperature of the sample, the lower the difference in the number of protons on the two energy levels. The temperature in all patients is constant. Only the excess spins provide the MR signal. At 1.0 T, the net magnetization equals approximately 3.3 spins per one million protons or 3.3 ppm. At 1.5 T, the ratio increase to 5 ppm. Net magnetization is larger and the signal-to-noise ratio increases. The statistical distribution between these two energy levels is described by the Boltzmann distribution. The distribution depends on the temperature of the sample where the spins are located. The higher the temperature of the sample, the lower the difference in the number of spins on the two energy levels. In patients, the temperature is relatively constant. As a result, only the excess spins provide the MR signal. At 1.0 T, the net magnetization equals approximately 3.3 spins per one million protons or 3.3 ppm. At 1.5 T, the ratio increase to 5 ppm. Thus, the net magnetization is larger at higher field strengths and the signal-to-noise ratio increases. Net Magnetization (M) is split into two, perpendicular components: Longitudinal Magnetization (Mz) along the z-axis of the magnetic field Transverse Magnetization (Mxy) along the xy-axis of the magnetic field The net magnetization is the source of the MR signal.