MRI Physics - PRELIM - PDF

PRELIM – MRI OVERVIEW OF MRI PHYSICS  Magnetic Resonance Imaging (MRI) utilizes the POLARIZATION behavior of hydrogen nuclei, or protons, in the  Though only a fraction of proton dipoles a...

PRELIM – MRI OVERVIEW OF MRI PHYSICS  Magnetic Resonance Imaging (MRI) utilizes the POLARIZATION behavior of hydrogen nuclei, or protons, in the  Though only a fraction of proton dipoles align human body. with the magnetic field, the patient becomes  Hydrogen, comprising 60% of body atoms, is polarized. ideal for MRI due to its magnetic properties  Polarized patients exhibit north and south PROTON BEHAVIOR magnetic poles.  Hydrogen nuclei behave like tiny bar magnets. PRECESSION o Hydrogen - Single charged spinning  Precession is a phenomenon observed when a nucleon patient is placed in a static magnetic field  Each hydrogen nucleus exhibits magnetism during MRI. due to its angular momentum and magnetic  Similar to a gyroscope on Earth, precession moment. involves a spinning motion accompanied by a MAGNETIC ALIGNMENT wobbling motion.  Before entering the magnetic field (B0), proton PRECESSION EXPLANATION: magnetic moments are randomly oriented.  Precession results from the interaction  Upon exposure to the B0 magnetic field, some between the spinning mass of the gyroscope proton magnetic moments align with it. and the Earth's gravitational field. NET MAGNETIZATION  The spinning gyroscope creates angular momentum, interacting with Earth's angular  The alignment of proton magnetic moments momentum, causing precessional motion. creates net magnetization in the patient.  The Z-axis coincides with the axis of the static PRECESSION MRI magnetic field (B0) in MRI systems.  In MRI, the spinning magnetic moment of  Net magnetization (MZ) represents the protons experiences precession in the alignment of proton magnetic dipoles along presence of a static magnetic field (B0). the Z-axis after polarization in the static o The gyroscope precesses in the magnetic field (B0). presence of gravity; the proton precesses in the presence of B0.  This precession phenomenon is crucial for generating MRI signals used in imaging. RELATIONSHIP BETWEEN MAGNETIC FIELD STRENGTH AND PRECESSIONAL FREQUENCY  The Larmor equation is fundamental in MRI, MAGNETIC FIELD STRENGTH relating the strength of the static magnetic  The strength of the B0 magnetic field is field (B0) to the precessional frequency (f). measured in tesla (T).  There is no direct relationship between magnetic field strength and ionizing radiation.  PRELIM – MRI IMPORTANCE OF GYROMAGNETIC RATIO of the protons, is said to “flip” toward the negative Z direction, while still precessing  Gyromagnetic ratio is akin to the about the Z-axis. disintegration constant in radioactive decay,  This precession is always perpendicular to Z, in unique to each nuclear species. the XY plane, and if initially all of the spins are  It determines the precessional frequency and aligned along the same direction in the XY thus the resonance frequency in MRI. plane UNITS OF GYROMAGNETIC RATIO  This is called the phase coherence of spins o Enhancing MR signal generation  The gyromagnetic ratio is expressed in during imaging. megahertz per tesla (MHz/T). o For example, hydrogen has a gyromagnetic ratio of approximately 42.6 MHz/T. PRACTICAL APPLICATION  The Larmor frequency, also known as the precessional frequency, is determined by the gyromagnetic ratio and magnetic field strength. FREE INDUCTION DECAY  For hydrogen at 1 Tesla, the precessional frequency is 42 MHz; at 1.5 Tesla, it is 63 MHz.  When RF is pulsed into the patient, protons flip, giving up energy while precessing. Understanding Free Induction Decay in MRI: Free  Then, as a group, the net magnetization grows Induction Decay (FID) is a crucial aspect of MRI, to its normal state in the positive Z direction. representing the emission of radio signals by the net o Normal state = equilibrium magnetization in the patient. magnetization state RF PULSE  With any moving magnetic field, an electric current can be induced in a properly designed  An RF pulse, emitted at the Larmor frequency, coil. is transmitted into the patient via an RF coil to  The induced current in the RF coil generates induce energy transfer and initiate imaging. the FID signal, which decreases over time due RESONANCE IN MRI to dephasing of proton spins.  The RF pulse must match the resonant RELAXATION TIMES frequency of the precessing hydrogen nuclei  Two relaxation times exist in MRI: T1 and T2. for efficient energy transfer and imaging to  T2, or transverse relaxation time, describes the occur. decay of phase coherence and the FID signal  Resonance is fundamental to MRI, analogous over time. to the resonance of musical instruments or o Similar to the decay constant that structural elements. describes radioactive decay PHASE COHERENCE  The other is the T1 relaxation time that describes the rate of the magnetization  When the RF signal is pulsed at resonance into increasing back to equilibrium. the patient, the energy state of many protons is changed. The net magnetization, due to all