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Nucleus
 
The core or center part of an atom, which contains protons having a positive charge and neutrons having no electrical charge, except in the common isotope of hydrogen, where the nucleus is a single proton.
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Particles Proton Neutron Decay
   by hyperphysics.phy-astr.gsu.edu    
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Explanation For Synesthesia- Area Deep Within Brain Plays A Role In Sensory Perception
Monday, 24 September 2007   by www.sciencedaily.com    
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Chemical Shift
 
Chemical shift depends on the nucleus and its environment and is defined as nuclear shielding / applied magnetic field. Nuclei are shielded by a small magnetic field caused by circulating electrons, termed nuclear shielding. The strength of the shield depends on the different molecular environment in that the nucleus is embedded. Nuclear shielding is the difference between the magnetic field at the nucleus and the applied magnetic field.
Chemical shift is measured in parts per million (ppm) of the resonance frequency relative to another or a standard resonance frequency.
The major part of the MR signal comes from hydrogen protons; lipid protons contribute a minor part. The chemical shift between water and fat nuclei is about 3.5 ppm (~220 Hz; 1.5T). Through this difference in resonance frequency between water and fat protons at the same location, a misregistration (dislocation) by the Fourier Transformation take place, when converting MR signals from frequency to spatial domain. This effect is called chemical shift artifact or chemical shift misregistration artifact.

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Further Reading:
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FUNDAMENTALS OF MRI: Part III – Forming an MR Image
   by www.e-radiography.net    
Abdominal MRI at 3.0 T: The Basics Revisited
Wednesday, 20 July 2005   by www.ajronline.org    
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Larmor Frequency
 
The Larmor precession frequency is the rate of precession of a spin packet under the influence of a magnetic field. The frequency of an RF signal, which will cause a change in the nucleus spin energy level, is given by the Larmor equation. The frequency is determined by the gyro magnetic ratio of atoms and the strength of the magnetic field. The gyromagnetic ratio is different for each nucleus of different atoms.
The stronger the magnetic field, the higher the precessional frequency. If an RF pulse at the Larmor frequency is applied to the nucleus of an atom, the protons will alter their alignment from the direction of the main magnetic field to the direction opposite the main magnetic field. As the proton tries to realign with the main magnetic field, it will emit energy at the Larmor frequency. By varying the magnetic field across the body with a magnetic field gradient, the corresponding variation of the Larmor frequency can be used to encode the position. For protons (hydrogen nuclei), the Larmor frequency is 42.58 MHz/Tesla.
See also Larmor Equation.
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Further Reading:
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Magnetic resonance imaging
   by www.scholarpedia.org    
Dealing with Increased MRI Field Strength
Tuesday, 1 October 2013   by www.evaluationengineering.com    
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Magnetic resonance-guided motorized transcranial ultrasound system for blood-brain barrier permeabilization along arbitrary trajectories in rodents
Thursday, 24 December 2015   by www.ncbi.nlm.nih.gov    
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Magnetism
 
Magnetic forces are fundamental forces that arise due to the movement of electrical charge. Maxwell's equations describe the origin and behavior of the fields that govern these forces. Thus, magnetism is seen whenever electrically charged particles are in motion. This can arise either from movement of electrons in an electric current, resulting in 'electromagnetism', or from the quantum-mechanical orbital motion (there is no orbital motion of electrons around the nucleus like planets around the sun, but there is an 'effective electron velocity') and spin of electrons, resulting in what are known as 'permanent magnets'.
The physical cause of the magnetism of objects, as distinct from electrical currents, is the atomic magnetic dipole. Magnetic dipoles, or magnetic moments, result on the atomic scale from the two kinds of movement of electrons. The first is the orbital motion of the electron around the nucleus this motion can be considered as a current loop, resulting in an orbital dipole magnetic moment along the axis of the nucleus. The second, much stronger, source of electronic magnetic moment is due to a quantum mechanical property called the spin dipole magnetic moment.
Gauss (G) and tesla (T) are units to define the intensity of magnetic fields. One tesla is equivalent to 10 000 gauss.
Typically, the field strength of MRI scanners is between 0.15 T and 3 T.
See also Diamagnetism, Paramagnetism, Superparamagnetism, and Ferromagnetism.
Radiology-tip.comGamma Ray,  Electromagnetic Radiation
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Magnet basics
   by my.execpc.com    
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What affects the strength of a magnet?
   by my.execpc.com    
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Gyromagnetic Ratio
 
A constant for any given nucleus that relates the nuclear MR frequency and the strength of the external magnetic field.
Definition: The ratio of the magnetic moment (field strength = T) to the angular momentum (frequency = v) of a particle.
The gyromagnetic effect happens if a magnetic substance is subjected to a magnetic field. Upon a change in direction of the magnetic field, the magnetization of the substance must change. In order for this to happen, the atoms must change their angular momentum. Since there are no external torques acting on the system, the total angular momentum must remain constant. This mass rotation may be measured. The gyromagnetic ratio is different for each nucleus of different atoms. The value of the gyromagnetic ratio for hydrogen (1H) is 4,258 (Hz/G) (42.58 MHz/T).
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Further Reading:
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Electron and proton gyromagnetic ratios
   by www.phys.au.dk    
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