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Units & Measurements
  • Absorbed Dose -
    Coupling Constant
 
 
 
 
 
 
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Absorbed Dose 
This dose means the RF power absorbed per unit of mass of an object, and is measured in watts per kilogram (W/kg).
The absorbed dose is dependent on the duty cycle and transmitter-coil type and increases with field strength, radio frequency power and and body size.
The specific absorption rate (SAR) describes the potential for heating of the patient's tissue due to the application of the RF energy necessary to produce the MR signal.
See also Specific Absorption Rate, MRI Safety, and MRI Risks.
 
Further Reading:
  Basics:
Physics of MRI Safety
   by www.aapm.org    
  News & More:
Commission delays electromagnetic fields legislation
Monday, 29 October 2007   by cordis.europa.eu:80    
Ampere 
(A or amp) The SI base unit of electric current.
Definition: Two parallel conductors, infinitely long and having negligible cross section, should be placed 1 meter apart in a perfect vacuum. One ampere is the current that creates between them a force of 0.2 micronewton per meter of length.
One ampere represents a current flow of 1 coulomb of charge per second.
One ampere of current results from a potential distribution of 1 volt per ohm of resistance, or from a power production rate of 1 watt per volt of potential.
The unit is known informally as the amp, but A is its official symbol and is named for the French physicist André-Marie Ampère.

 
Further Reading:
  Basics:
How Many? A Dictionary of Units of Measurement © Russ Rowlett and the University of North Carolina at Chapel Hill
Wednesday, 21 March 2001   by www.unc.edu    
Angstrom 
(Å * or A*) A unit used to measure the wavelength of X-rays.
Definition: 0.1 nanometer or 10-10 meter.
Angular Frequency 
Frequency of oscillation or rotation (measured in radians/second) commonly designated by the Greek letter w: w = 2pf, where f is frequency (in hertz (Hz)).
Bit 
The basic unit of information.
Definition: The smallest unit of information in the storage on a computer. Eight bits are grouped together to form one byte, additional start and stop bit.
Larger units are
kilobyte (kB) = 1 000 bytes (computer storage 1024 bytes)
megabyte (MB) = 1 000 kB (computer storage 1024 kB)
Boltzmann Distribution 
When a group of spins is placed in a magnetic field, each spin aligns in one of the two possible orientations. The relative numbers of spins with different alignments will be given by the Boltzmann distribution.
Definition: if a system of particles, which are able to exchange energy in collisions is in thermal equilibrium, then the relative number (population) of particles, N1 and N2, in two particular energy levels with corresponding energies, E1 and E2, is given by N1/N2 = exp [-(E1-E2)/kT ] where k is the Boltzmann constant and T is the absolute temperature.
For example, in NMR of protons at room temperature in a magnetic field of 0.25 tesla, the difference in relative numbers of spins aligned with the magnetic field and against the field is about one part in a million; the small excess of nuclei in the lower energy state is the basis of the net magnetization and the resonance phenomenon.
 
Further Reading:
  Basics:
The Boltzmann distribution
   by urey.uoregon.edu    
Celsius 
A metric unit of temperature.
Definition: One degree is 1/273.16 of the difference between the triple point of water (at exactly 0.01 °C) and absolute zero.
The triple point of water is the temperature at which water can exist simultaneously in the gaseous, liquid, and solid states. Absolute zero is the temperature at which all molecular motion discontinues.
The Celsius temperature scale is named for the Swedish astronomer and physicist Anders Celsius (1701-1744), who used a similar scale.

 
Further Reading:
  Basics:
Essentials of the SI
   by physics.nist.gov    
  News & More:
How Many? A Dictionary of Units of Measurement © Russ Rowlett and the University of North Carolina at Chapel Hill
Wednesday, 21 March 2001   by www.unc.edu    
Coupling Constant 
Spectral lines are split by spin spin coupling into multiplets whose frequencies are separated by an amount depending on the coupling constant, J. The magnitude of J is independent of the strength of the applied magnetic field and is given in units of frequency, Hz.

 
Further Reading:
  Basics:
Tech Spotlight: How to calibrate an MRI RF amplifier
Saturday, 1 December 2012   by medicaldesign.com    
  News & More:
Spin Spin Coupling:The splitting of NMR Signals
   by iis.dmhcsm.edu.hk    
  dB/dt - Gauss top
 
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