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Radio Frequency Pulse
 
A pulse is a rapid change in the amplitude of a RF signal or in some characteristic a RF signal, e.g., phase or frequency, from a baseline value to a higher or lower value, followed by a rapid return to the baseline value. For radio frequencies near the Larmor frequency, it will result in rotation of the macroscopic magnetization vector. The amount of rotation will depend on the strength and duration of the RF pulse; commonly used examples are 90° (p/2) and 180° (p) pulses.
RF pulses are used in the spin preparation phase of a pulse sequence, which prepare the spin system for the ensuing measurements. In many sequences, RF pulses are also applied to the volumes outside the one to be measured. This is the case when spatial presaturation techniques are used to suppress artifacts. Many preparation pulses are required in MR spectroscopy to suppress signal from unwanted spins. The simplest preparation pulse making use of spectroscopic properties is a fat saturation pulse, which specifically irradiates the patient at the fat resonant frequency, so that the magnetization coming from fat protons is tilted into the xy-plane where it is subsequently destroyed by a strong dephasing gradient.
The frequency spectrum of RF pulses is critical as it determines the spatial extension and homogeneity over which the spin magnetization is influenced while a gradient field is applied.
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Further Reading:
  News & More:
Numerical investigations of MRI RF field induced heating for external fixation devices
Thursday, 7 February 2013   by 7thspace.com    
MRI Safety: Monitoring Body Temperature During MRI
Thursday, 4 August 2011   by www.diagnosticimaging.com    
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Adiabatic Radio Frequency Pulses
 
Adiabatic RF pulses are amplitude and frequency modulated pulses that are insensitive to the effects of B1-inhomogeneity and frequency offset (conventional RF pulses used in MRI are only amplitude modulated). Due to an extended application time adiabatic RF pulses are mostly used in NMR imaging applications.
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Cardiac StentForum -
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The MRI safety of cardiac stents is dependent of the material, the examined part of the body and the used field strength. A susceptibility artifact is expected also in low magnetic fields, but less.


MRI Safety Guidance
Most of the used materials are non-magnetic, for this case there is no risk for movement caused through the magnetic field. If the cardiac stent is outside the region of the radio frequency pulse, also the risk of e.g. heating is low.

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Further Reading:
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Medtronic Receives FDA Approval for Endeavor® Zotarolimus-Eluting Coronary Stent System
Friday, 1 February 2008   by wwwp.medtronic.com    
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Carr Purcell Meiboom Gill SequenceInfoSheet: - Sequences - 
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(CPMG) This type of spin echo pulse sequence consisting of a 90° radio frequency pulse followed by an echo train induced by successive 180° pulses and is useful for measuring T2 weighted images. It is a modification of the Carr-Purcell RF pulse sequence, with 90° phase shift in the rotating frame of reference between the 90° pulse and the subsequent 180° pulses in order to reduce accumulating effects of imperfections in the 180° pulses. Suppression of effects of pulse error accumulation can alternatively be achieved by switching phases of the 180° pulses by 180°.

 
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Further Reading:
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Fast Spin Echo(.pdf)
Tuesday, 24 January 2006   by www.81bones.net    
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Spin echoes, CPMG and T2 relaxation - Introductory NMR & MRI from Magritek
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Excitation
 
Sent (inducing, transferring) energy into the 'spinning' nuclei via radio frequency pulse, which puts the nuclei into a higher energy state. By producing a net transverse magnetization a MRI system can observe a response from the excited system.
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Further Reading:
  Basics:
Tech Spotlight: How to calibrate an MRI RF amplifier
Saturday, 1 December 2012   by medicaldesign.com    
Musculoskeletal MRI at 3.0 T: Relaxation Times and Image Contrast
Sunday, 1 August 2004   by www.ajronline.org    
IMAGE CONTRAST IN MRI(.pdf)
   by www.assaftal.com    
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