MRI Basic Principles PDF

MRI basic principles 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.  Atomic Structure All matter...

MRI basic principles 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.  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. It is used because hydrogen is very abundant in the human body and because the solitary proton gives it a relatively 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 Motion + electricity = magnet an 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) (Net magnetic vector, NMV) – Low energy 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). Anatomical level  Glossary Acquisition time = Period of time required to collect the image data AP Anteroposterior = from the front to the back of the body Axial= Orthogonal to the long axis of the body CISS= Constructive interference steady state: a sequence which supposedly minimizes the interference-induced artifacts of a true FISP sequence; the CISS sequence is heavily T2-weighted and currently is used primarily as a high- resolution 3-D sequence for the inner ear Coil =Transmission or reception unit for the signals transmitted/ received by the magnet. There are transmission, reception, and combined transmission/reception coils, and they come as body and surface coils, the latter being available as rigid and flexible (wraparound) coils  Glossary CSF = Cerebrospinal fluid Dark fluid = Turbo inversion recovery sequence: a technique with a long TI (approx. 2200ms for 1.0 and 1.5 T) used for fluid suppression, e.g., for suppressing the CSF in brain and spinal studies with T2 weighting (and long TE) DESS = Dual echo steady state: gradient echo sequence where both axial and transverse magnetization adds to the signal, e.g. the FISP sequence and its temporally reversed form PSIF. In DESS the two sequences (FISP as a ratio of T1 to T2, and PSIF as mostly T2-weighted sequence) are added  Glossary 3-D measurement = Volume measurement: imaging technique where each pulse excites not just a single slice but the entire volume of interest Dual echo = Dual measurement: sequences characterized by two readout times (TE) for one (comparatively long) TR. Normally dual echo combines proton-densityweighted (short TE) and T2-weighted (long TE) measurements ECG triggering (EGG gating)= Triggered by the signals of the heart: data acquisition takes place only during particular phases of the cardiac cycle (e.g. during systole or diastole); the patient has to be attached to a set of ECG leads