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 'Flip Angle' 
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Rapid Excitation Magnetic Resonance ImagingInfoSheet: - Sequences - 
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(RE MRI) There are several approaches to speeding up the MRI data acquisition process by repeating the excitation by RF pulses in times short compared to T1, typically using small flip angles and gradient echo refocusing. When TR is also on the order of or shorter than T2, the repeated RF pulses will tend to refocus transverse magnetization remaining from prior excitations, setting up a condition of steady state free precession, and a dependence of signal strength (and image contrast) on both T1 and T2.
This can be modified in various ways, particularly:
1) to spoil the tendency to build up a steady state by reducing coherence between excitations, e.g. by variation of the phase or timing of consecutive RF pulses or of the strength of spoiler gradient pulses, thus increasing the relative dependence of signal strength on T1 or
2) acquire the signal when it is refocusing immediately prior to the next RF pulse, thus increasing the relative dependence of signal strength on T2.
See also Ultrafast Gradient Echo Sequence.
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Rotating Frame Zeugmatography
 
Technique of MR imaging that uses a gradient of the RF excitation field (to give a corresponding variation of the flip angle along the gradient as a means of encoding the spatial location of spins in the direction of the RF field gradient) in conjunction with a static magnetic field gradient (to give spatial encoding in an orthogonal direction). It can be considered to be a form of Fourier transformation imaging.
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Small Tip Angle Gradient EchoInfoSheet: - Sequences - 
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(STAGE) A gradient echo sequence with low flip angles and spoiled gradients.
See Spoiled Gradient Echo Sequence.
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Specific Absorption Rate
 
(SAR) The Specific Absorption Rate is defined as the RF power absorbed per unit of mass of an object, and is measured in watts per kilogram (W/kg).
The 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. Inhomogeneity of the RF field leads to a local exposure where most of the absorbed energy is applied to one body region rather than the entire person, leading to the concept of a local SAR. Hot spots may occur in the exposed tissue, to avoid or at least minimize effects of such theoretical complications, the frequency and the power of the radio frequency irradiation should be kept at the lowest possible level. Averaging over the whole body leads to the global SAR.
It increases with field strength, radio frequency power and duty cycle, transmitter-coil type and body size. The doubling of the field strength from 1.5 Tesla (1.5T) to 3 Tesla (3T) leads to a quadrupling of SAR. In high and ultrahigh fields, some of the multiple echo, multiple-slice pulse sequences may create a higher SAR than recommended by the agencies. SAR can be reduced by lower flip angle and longer repetition times, which could potentially affect image contrast.
Normally no threatening increase in temperature could be shown. Even in high magnetic fields, the local temperature increases not more than 1°C. 2.1°C is the highest measured increase in skin temperature. Eddy currents may heat up implants and thus may cause local heating.

FDA SAR limits:
Whole body: 4W/kg/15-minute exposure averaged;
Head: 3W/kg/10-minute exposure averaged;
Head or torso: 8W/kg/5 minute exposure per gram of tissue;
Extremities: 12W/kg/5 minute exposure per gram of tissue.
IEC (International Electrotechnical Commission) SAR limits of some European countries:
All limits are averaged over 6 minutes.
Level 0 (normal operating mode): Whole body 2W/kg; Head 3.2W/kg; Head or Torso (local) 10W/kg; Extremities (local) 20W/kg;
Level I (first level controlled operating mode): Whole body 4W/kg; Head 3.2W/kg; Head or Torso (local) 10W/kg; Extremities (local) 20W/kg;
Level II (second level controlled operating mode): All values are over Level I values.
(For more details: IEC 60601-2-33 (2002))

In most countries standard MRI systems are limited to a maximum SAR of 4 W/kg, so most scanning in level II is impossible.
For Level I, in addition to routine monitoring, particular caution must be exercised for patients who are sensitive to temperature increases or to RF energy.
For Japan different SAR limits are valid.
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Further Reading:
  Basics:
On the estimation of the worst-case implant-induced RF-heating in multi-channel MRI.
Thursday, 2 March 2017   by www.ncbi.nlm.nih.gov    
What MRI Sequences Produce the Highest Specific Absorption Rate (SAR), and Is There Something We Should Be Doing to Reduce the SAR During Standard Examinations?
Thursday, 16 April 2015   by www.ajronline.org    
Numerical investigations of MRI RF field induced heating for external fixation devices
Thursday, 7 February 2013   by 7thspace.com    
Evaluation of Specific Absorption Rate as a Dosimeter of MRI-Related Implant Heating
2004   by www.imrser.org    
  News & More:
MRI in Patients with Implanted Devices: Current Controversies
Monday, 1 August 2016   by www.acc.org    
Accounting for biological aggregation in heating and imaging of magnetic nanoparticles
Tuesday, 2 September 2014   by www.ecnmag.com    
Commission delays electromagnetic fields legislation
Monday, 29 October 2007   by cordis.europa.eu:80    
Guidance for Industry and FDA Staff, Criteria for Significant Risk Investigations of Magnetic Resonance Diagnostic Devices
Monday, 14 July 2003   by www.fda.gov    
MRI Resources 
Online Books - Coils - Spine MRI - Spectroscopy - Shielding - Functional MRI
 
Tip Angle
 
Angle between the net magnetization vector before and after a RF excitation pulse. Small tip angles allow a decrease in TR, which is used to decrease scan time in Field Echo pulse sequences. See Flip Angle.
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Further Reading:
  Basics:
Mapping of low flip angles in magnetic resonance(.pdf)
Saturday, 1 January 2011   by www.hal.inserm.fr    
MRI Resources 
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