MRI 3.pdf

MRI basic principles Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation ✓Precessional phase. ✓Resonance. ✓MR signal. ✓The free induction decay (FID) signal. ✓Pulse timing parameters.  Precessional phase...

MRI basic principles Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation ✓Precessional phase. ✓Resonance. ✓MR signal. ✓The free induction decay (FID) signal. ✓Pulse timing parameters.  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.  Pulse timing parameters The TR is the time from the application of one RF excitation pulse to the application of the next RF excitation pulse for each slice and is measured in millisecond. The TR determines the amount of longitudinal relaxation that occurs between the end of one RF excitation pulse and application of the next. The TR thus determines the amount of T1 relaxation that has occurred when signal is read. The TE is the time from the application of the RF excitation pulse to the peak of signal induced in the receiver coil and is also measured in millisecond. The TE determines how much decay of transverse magnetization occurs. TE thus controls the amount of T2 relaxation that has occurred when signal is read.  The image contrast is controlled by two groups of parameters: A. Extrinsic contrast parameters B. Intrinsic contrast mechanism  Extrinsic contrast parameters A. Extrinsic contrast parameters : which are controlled by the system operator ;These include the following. 1. Repetition time (TR). This is the time from the application of one RF pulse to the application of the next. It is measured in milliseconds (ms). The TR affects the length of a relaxation period after the application of one RF excitation pulse to the beginning of the next. 2. Echo time (TE). This is the time between an RF excitation pulse and the collection of the signal. The TE affects the length of the relaxation period after the removal of an RF excitation pulse and the peak of the signal received in the receiver coil. It is also measured in ms. 3. Flip angle. This is the angle through which the NMV is moved as a result of a RF excitation pulse. 4. Turbo-factor or echo train length (ETL/TF). 5. Time from inversion (TI). 6. ‘b’ value: is a factor that reflects the strength and timing of the gradients used to generate diffusion-weighted images. Extrinsic contrast parameters  Extrinsic contrast parameters B. Intrinsic contrast mechanism: Which do not come under the operator's control; These include: 1. T1 recovery 2. T2 decay 3. Proton density 4. Flow 5. Apparent diffusion coefficient (ADC): is a measure of the magnitude of diffusion (of water molecules) within tissue.  MRI Terms Dephasing : the fanning out or loss of phase coherence of signals within the transverse plane. Diffusion : a term used to describe moving molecules due to random thermal motion. Dipole : a magnetic field characterized by its own north and south magnetic poles separated by a finite distance. Display matrix : the total number of pixels in the selected matrix, which is described by the product of its phase and frequency axis.  MRI Terms Electromagnet : a type of magnet that utilizes coils of wire, typically wound on an iron core, so that as current flows through the coil it becomes magnetized. See also Resistive Magnet, Superconducting Magnet. Equilibrium : a state of balance that exists between two opposing forces or divergent forms of influence. Excitation : delivering (inducing, transferring) energy into the "spinning" nuclei via radiofrequency pulse(s), which puts the nuclei into a higher energy state. By producing a net transverse magnetization an MRI system can observe a response from the excited system. Echo spacing : spacing between each echo in FSE.  MRI Terms Echo train : series of 180° rephasing pulse and echoes in a fast spin echo pulse sequence. Echo train length (ETL) : the number of 180° RF pulses and or turbo factor resultant echoes in FSE. Effective TE : the time between the echo and the RF pulse that initiated it in SSFP and FSE sequences. Electrons orbit : the nucleus in distinct shells and are negatively charged. External magnetic field (EMF) : drives a current in a circuit and is the result of a changing magnetic field inducing an electric field.