Physics of magnetic resonance imaging | Wikipedia audio article
Channel: wikipedia tts
Date: 2018-12-29

This is an audio version of the Wikipedia Article:
https://en.wikipedia.org/wiki/Physics_of_magnetic_resonance_imaging


00:04:55 1 Nuclear magnetism
00:06:09 1.1 Resonance and relaxation
00:11:51 2 Imaging
00:12:00 2.1 Imaging schemes
00:13:23 2.1.1 Echo-planar imaging
00:14:03 2.2 Image contrast and contrast enhancement
00:18:17 2.3 ik/i-space
00:27:15 2.4 Example of a pulse sequence
00:32:01 2.5 Overview of main sequences
00:32:18 3 MRI scanner
00:32:28 3.1 Construction and operation
00:33:07 3.2 Magnet
00:39:14 3.3 Shims
00:41:16 3.4 Gradients
00:44:59 3.5 Radio frequency system



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SUMMARY
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The physics of magnetic resonance imaging (MRI) involves the interaction of biological tissue with electromagnetic fields. MRI is a medical imaging technique used in radiology to investigate the anatomy and physiology of the body. The human body is largely composed of water molecules, each containing two hydrogen nuclei, or protons. When inside the magnetic field (B0) of the scanner, the magnetic moments of these protons align with the direction of the field. They may align in two configurations parallel (in the direction of B0), or anti-parallel opposing B0. While each individual proton can only have one of two alignments the collection of protons appear to behave as though they can have any alignment. More protons align parallel to B0 as this is a lower energy state.
A radio frequency pulse is then applied, which can excite protons from parallel to anti-parallel alignment, only these protons are relevant to the rest of the discussion. In response to the force bringing them back to their equilibrium orientation, the protons undergo a rotating motion (precession), much like a spin wheel under the effect of gravity. The protons will return to the low energy state and do so by emitting photons corresponding to the energy difference between the two possible alignments. This appears as a magnetic flux, which yields a changing voltage in receiver coils to give the signal. The frequency at which a proton or group of protons in a voxel resonates depends on the strength of the local magnetic field around the proton or group of protons, a stronger 
field corresponds to a larger energy difference and higher frequency photons. By applying additional magnetic fields (gradients) that vary linearly over space, specific slices to be imaged can be selected, and an image is obtained by taking the 2-D Fourier transform of the spatial frequencies of the signal (a.k.a., k-space). Due to the magnetic Lorentz force from B0 on the current flowing in the gradient coils, the gradient coils will try to move. The knocking sounds heard during an MRI scan are the result of the gradient coils trying to move against the constraint of the concrete or epoxy in which they are secured. These sounds can be very unnerving to the patient, particularly given the tight space in which the patient lies. This behaviour of MRI scanners can be described in terms of a fully coupled acousto-magneto-mechanical system. Solutions to such systems can provide useful insight for design engineers.
Diseased tissue, such as tumors, can be detected because the protons in different tissues return to their equilibrium state at different rates (i.e., they have different relaxation times). By changing the parameters on the scanner this effect is used to create contrast between different types of body tissue.
Contrast agents may be injected intravenously to enhance the appearance of blood vessels, tumors or inflammation. Contrast agents may also be directly injected into a joint in the case of arthrograms, MRI images of joints. Unlike CT, MRI uses no ionizing radiation and is generally a very safe procedure. Patients with some metal implants, cochlear implants, and cardiac pacemakers are prevented from having an MRI scan due to effects of the strong m ...