Summary of How MRI Works - Part 1 - NMR Basics

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00:00:00 - 00:40:00

This video discusses the basics of MRI, including how it uses NMR to produce images of tissues and organs. Hyperpolarization and spin echo are discussed, as well as the limitations of MRI. Finally, gradient echo and Fourier theory are discussed in depth.

  • 00:00:00 The MRI scanner uses a strong, homogeneous magnetic field to produce images of the inside of the body. The magnet is always on and the field is ramped up and maintained throughout the scan.
  • 00:05:00 MRI uses a very strong magnetic field to create a very slight magnetization in the nuclei of atoms within the body. Proton spin is an entirely quantum mechanical property of particles, and it is largely why MRI can see hydrogen nuclei more clearly than other elements.
  • 00:10:00 The magnetic moment of a proton is described by its spin quantum number and a constant called the gyromagnetic ratio. When a magnetic field is present, the spin of the proton tries to align with the field, but precession occurs, causing the signal to change frequency and amplitude. This effect is described by the Larmour equation, which states that the frequency of the signal increases as the magnetic field strength is increased, and that the angle between the spin and the magnetic field plays a role in the signal.
  • 00:15:00 MRI works by using a magnetic field to detect the precession of individual spins. The more spins involved in the NMR signal, the greater the signal intensity. The dynamics of the NMR signal decay exponentially with a time constant, t2. When all spins are in phase, the NMR signal is strong. However, over time the spins become randomly distributed and the NMR signal decays. Knowing which tissues decay quickly or slowly can be used to distinguish them in an image.
  • 00:20:00 This 1-minute video explains how MRI works by focusing on the different signals that are produced when the spins of a molecule are knocked out of alignment with the magnetic field. The signals are then differentiated by how long it takes for them to be realigned, or "echoed." This process is slowed by the process of t2, or "time to peak." If the echo time is set to zero, the signal will be maximally differentiated.
  • 00:25:00 This video explains how MRI works by describing the physics of how the magnetization of a sample changes over time as it is passed through a series of alternating magnetic fields. The magnetization follows a curve known as the Boltzmann magnetization curve, which is similar to the voltage across a charging capacitor. The video also explains how MRI works with a rotating reference frame, and how the dynamics of the magnetization vectors can be simplified when dealing with rotating frames of reference. Finally, the video shows how MRI works in a laboratory setting.
  • 00:30:00 This video discusses how MRI works, focusing on the nucleus. Magnetic fields are used to create images of the body by separating the spin of the nuclei of tissues. The repetition time (TR) and echo time (TE) determine the amount of signal each tissue will produce. T1 and T2 are the times it takes for the signals to decay. The Boltzmann magnetization (M) of a tissue is proportional to its spin density and depends only on the differences in spin between tissues. If TR and TE are chosen appropriately, then the image will be a spittin density image. Contrast depends only on the differences in Boltzmann magnetization between tissues.
  • 00:35:00 In this video, the basics of MRI are explained, including how spin states are quantized and how the magnetic field affects the polarization of the spins. It is shown that MRI is a very insensitive technique, and that only a fraction of the spins in a sample are detectable. Finally, it is shown that the polarizing equation can be simplified when expressed in terms of energy differences between the two states.
  • 00:40:00 This video discusses the basics of MRI, including the use of NMR to produce images of tissues and organs. Hyperpolarization is discussed, as well as the limitations of MRI. Finally, spin echo, gradient echo, and Fourier theory are discussed in depth.

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