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R-Weighted Image
 
When the contrast of an MR image is predominantly dependent on the spin density the term of this image is R-weighted. Also called (PD) proton density weighted image.
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R1
 
Longitudinal relaxation rate equal to reciprocal of T1 relaxation time (R1= 1/T1).
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R2
 
Longitudinal relaxation rate equal to reciprocal of T2 relaxation time (R2 = 1/T2).
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Radian
 
Dimensionless unit of angular measure; 360°=2p radians.
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Radio FrequencyForum -
there are related threads
 
(RF) Radio frequency refers to that portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna.
The RF pulses used in MRI are commonly in the 1-100 megahertz range, and their effect upon a body is potential heating of tissues and foreign bodies, such as metallic implants, mainly at the surface.
This is a principal area of concern for MRI safety limits caused by absorption (see specific absorption rate) of the applied pulses of RF energy.


Image Guidance
The higher the frequency, the larger will be the amount of heat developed.
The more ionic the biochemical environment in the tissue, the more energy that will be deposited as heat.
This effect is well known for homogeneous model systems, but the complex structure of various human tissues makes detailed theoretical calculations very difficult, if not impossible. By scanning problems, it is important to verify the transmission frequency. If the RF transmitted into the patient was, e.g. 5000 Hz lower than the resonance frequency of the protons, no protons was excited, and no signal returns.
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Further Reading:
  Basics:
Radio frequency
   by en.wikipedia.org    
  News & More:
Numerical investigations of MRI RF field induced heating for external fixation devices
Thursday, 7 February 2013   by 7thspace.com    
Safety and reliability of Radio Frequency Identification Devices in Magnetic Resonance Imaging and Computed Tomography
Thursday, 4 February 2010   by 7thspace.com    
Radiologists Offer Non-surgical Treatment For Early-stage Liver Cancer
Saturday, 5 March 2005   by www.sciencedaily.com    
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