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Magnetic Resonance Imaging (MRI) Magnetic Resonance Imaging (MRI) • Non-invasively provides high resolution anatomical images of human structures, such as brain, heart and other soft tissues How MRI Works 1. Put subject in big magnetic field 2. Transmit radio waves into subject [2~10 ms] 3. Turn off radio wave transmitter 4. Receive radio waves re-transmitted by subject • Store measured radio wave data vs. time • Now go back to 2) to get some more data • Process raw data to reconstruct images • Allow subject to leave scanner How MRI Works • Strong magnetic field, usually from • superconducting magnets. • RadioFrequency coils and sub-system. • Gradient coils and sub-system. • Shimming coils and sub-system. • Computer(s) that coordinate all subsystems. Timeline of MR Imaging 1924 - Pauli suggests that nuclear particles may have angular momentum (spin). 1937 – Rabi measures magnetic moment of nucleus. Coins “magnetic resonance”. 1944 – Rabi wins Nobel prize in Physics. 1946 – Purcell shows that matter absorbs energy at a resonant frequency. 1946 – Bloch demonstrates that nuclear precession can be measured in detector coils. 1952 – Purcell and Bloch share Nobel prize in Physics. 1959 – Singer measures blood flow using NMR (in mice). 1972 – Damadian patents idea for large NMR scanner to detect malignant tissue. 1973 – Lauterbur publishes method for generating images using NMR gradients 1973 – Mansfield independently publishes gradient approach to MR. 1975 – Ernst develops 2D-Fourier transform for MR. NMR becomes MRI MRI scanners become clinically prevalent. 1985 – Insurance reimbursements for MRI exams 1990 – Ogawa and colleagues create functional images using endogenous, blood-oxygenation contrast. Early Human MR Images (Damadian) Transmit Receive rf coil main magnet Shimming gradient Control Computer How MRI Works How MRI Works MRI whole system Shimming Coils • Used to compensate for magnetic field non homogeneities • Types of shim systems -Passive: Large number of metal rods w/ adjustable weights - Superconducting: Coils surrounded by cryogens - Resistive: Coils at room-temperature - Cheaper, adjusted for each subject Magnetic and Electromagnetic Fields • Magnetic fields generate the substance we “see” which is HYDROGEN molecules: magnetization of the H protons in H2O • Magnetic fields also let us manipulate magnetization - make a map [or image] of its distribution inside the body’s tissue • Static magnetic fields change slowly (< 0.1 ppm/ hr) - main field; static inhomogeneities Magnetic and Electromagnetic Fields • RF fields are electromagnetic fields that oscillate at Radio Frequencies (tens of millions of times per second) transmitted radio waves into subject received signals from subject • Gradient magnetic fields change quickly (switching up to thousands of times per second) Magnetic and Electromagnetic Fields How do we detect magnetization? • We need to perturb the system • RF coil transmits B1 field Magnetic induction causes RF signal generation (precessing) which is received by RF receiving coil Various coils used for transmit, receive, both, volume and surface measurements Magnetic and Electromagnetic Fields Relaxation/Decay Times when particles return to equilibrium have diagnostic value • T1 or spin-lattice decay • T2 or spin-spin decay • Long time constants • Spin echo MRI image formation • Defines the spatial location of the proton pools that contribute to the MR signal after spin excitation • A 3-D gradient field (dB/dx, dB/dy, dB/dz) would allow a unique correspondence between the spatial location and the magnetic field. Using this information, we will be able to generate maps that contain spatial information – images. Readout Localization (frequency encoding) • After RF pulse (B1) ends, acquisition of MRI RF signal begins During readout, gradient field perpendicular to slice selection gradient is turned on Signal is sampled about once every few microseconds, digitized, and stored Computer breaks measured signal into frequency components Since frequency varies across subject in a known way, we can determine where frequency component comes from • Also Phase Encoding measurements Image formation • Fourier analysis used to determine the amplitude distribution of the returned frequencies • No rotation like CT Direction of the magnetic gradient is rotated slightly • Reconstuction similar to CT Back projection • No ionizing radiation used In theory, safer than CT, but… Risks of MRI • Projectile Effects: External • Projectile Effects: Internal • Radiofrequency Energy • Gradient field changes • Claustrophobia • Acoustic Noise • Quenching July 31, 2001 — A 6-year-old boy died after undergoing an MRI exam at a New York-area hospital when the machine's powerful magnetic field jerked a metal oxygen tank across the room, crushing the child's head. …