MRI Basic Principles PDF

MRI basic principles Dr.Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation ✓Atomic structure. ✓Motion of atom. ✓MR active nuclei. ✓Faraday low. ✓Proton alignment. ✓Net magnetization vector. ✓Precession. ✓The Larmor equation. ✓Precessional phase. ✓Resonance. ✓MRI signal. ✓FID ...

MRI basic principles Dr.Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation ✓Atomic structure. ✓Motion of atom. ✓MR active nuclei. ✓Faraday low. ✓Proton alignment. ✓Net magnetization vector. ✓Precession. ✓The Larmor equation. ✓Precessional phase. ✓Resonance. ✓MRI signal. ✓FID  Atomic Structure All matter is composed of atoms. Atoms combine to form molecules. Key elements in the human body: hydrogen, oxygen, carbon, nitrogen.  Composition of Atoms Atoms have a central nucleus and orbiting electrons. Nucleus contains protons (positive charge) and neutrons (no charge). Electrons (negative charge) orbit the nucleus.  Mass and Size of Nucleus Nucleus is tiny but contains most of the atom's mass. Composed of protons and neutrons (nucleons). Nucleus occupies a minute fraction of an atom's volume.  Motion in the atom Three types of motion are present within the atom. Electrons spinning on their own axis Electrons orbiting the nucleus The nucleus itself spinning about its own axis.  Motion in the atom Protons and Neutrons P=N P≠N MRI active nuclei Has no spine =0 Has net spine≠0  MR-active nuclei 1H (hydrogen) 13C (carbon) 15N (nitrogen) 17O (oxygen) 19F (fluorine) 23Na (sodium).  The hydrogen nucleus The isotope of hydrogen called protium is the most commonly used MR-active nucleus in MRI. It has a mass and atomic number of 1, so the nucleus consists of a single proton and has noneutrons. Itrelatively is used because hydrogen is very abundant in the human body and because the solitary proton gives it a large magnetic moment. These characteristics mean that the maximum amount of available magnetization in the body is utilized.  Faraday’s law of electromagnetic Faraday’s law of electromagnetic induction states that a magnetic field is created by a charged moving particle (that creates an electric field). The protium nucleus contains one positively charged proton that spins, it moves. Therefore, the nucleus has a magnetic field induced around it and acts as a small magnet. The magnet of each hydrogen nucleus has a north and a south pole of equal strength. The north/south axis of each nucleus is represented by a magnetic moment and is used in classical theory. Which can align with an Motion + electricity = magnet external magnetic field (B0)  Alignment In the absence of an applied magnetic field, the magnetic moments of hydrogen nuclei are randomly orientated and produce no overall magnetic effect. However, when placed in a strong static external magnetic field, the magnetic moments of hydrogen nuclei orientate with this magnetic field. This is called alignment.  Alignment Align parallel to the main magnetic field (B0) Low energy (Net magnetic vector, NMV) Or Spine up nuclei (Net magnetization M0) Align Anti-parallel to the main magnetic field (B0) High energy Spine down nuclei Parallel Low energy B0 Anti-parallel High energy  Net magnetic vector (NMV) Field strength (B0) NMV Precession and precessional (Larmor) frequency Precession frequency? The speed at which magnetic moments preces around B0 is called Precession frequency (w0) Larmor equation : W0 = y B0 Where: W0: is the Precession frequency B0: is the magnetic field strength of the magnet Y: is the gyromagnetic ratio  Precession and precessional (Larmor) frequency γ is a constant, for a given MR-active nucleus ω0 is proportional to B0 Magnetic characteristics of common elements. In addition, magnetic moments of MR-active nuclei have different precessional frequencies at different field strengths. For hydrogen, for example: At 1.5 T, the precessional frequency is 63.87 MHz (42.58 MHz. 1.5 T). At 1.0 T, the precessional frequency is 42.57 MHz (42.58 MHz. 1.0 T). At 0.5 T, the precessional frequency is 21.29 MHz (42.58 MHz. 0.5 T).  Precessional phase Means that magnetic moments of hydrogen are at the same place on the precessional path at a moment in time. Precessional phase means that magnetic moments of hydrogen are at different places on the precessional path at a moment in time.  Resonance Resonance is a phenomenon that occurs when an object is exposed to an oscillating perturbation that has a frequency close to its own natural frequency of oscillation. When a nucleus is exposed to an external force that has an oscillation similar to the natural frequency of its magnetic moment (its Larmor frequency), the nucleus gains energy from the external source. RF pulse  Resonance The result of Resonance 1. The NMV moves out of 2. The magnetic moments of aligment away from B0 H nuclei move into phase with each other  MRI signal Faraday law: Motion + electricity = magnet Recovery Dephasing  The free induction decay (FID) signal When the RF excitation pulse is switched off, the NMV is influenced only by B0, and it tries to realign with it. The hydrogen nuclei lose energy given to them by the RF excitation pulse. The process by which hydrogen loses this energy is called relaxation. As relaxation occurs, the NMV returns to realign with B0 because some of the high-energy nuclei return to the low-energy population and therefore align their magnetic moments in the spin-up direction. At the same time, but independently, the magnetic moments of hydrogen lose coherency due to dephasing. This occurs because of inhomogeneities in the B0 field and due to interactions between spins in the patient’s tissue. As the magnitude of transverse coherent magnetization decreases, so does the magnitude of the voltage induced in the receiver coil. The induction of decaying voltage is called the free induction decay (FID) signal. This is because spins freely precess influenced only by B0, signal decays with time, and magnetic moments of the spins induce a current in the receiver coil.