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Spin Echo SequenceInfoSheet: - Sequences - 
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Spin Echo Timing Diagram (SE) The most common pulse sequence used in MR imaging is based of the detection of a spin or Hahn echo. It uses 90° radio frequency pulses to excite the magnetization and one or more 180° pulses to refocus the spins to generate signal echoes named spin echoes (SE).
In the pulse sequence timing diagram, the simplest form of a spin echo sequence is illustrated.
The 90° excitation pulse rotates the longitudinal magnetization (Mz) into the xy-plane and the dephasing of the transverse magnetization (Mxy) starts.
The following application of a 180° refocusing pulse (rotates the magnetization in the x-plane) generates signal echoes. The purpose of the 180° pulse is to rephase the spins, causing them to regain coherence and thereby to recover transverse magnetization, producing a spin echo.
The recovery of the z-magnetization occurs with the T1 relaxation time and typically at a much slower rate than the T2-decay, because in general T1 is greater than T2 for living tissues and is in the range of 100-2000 ms.
The SE pulse sequence was devised in the early days of NMR days by Carr and Purcell and exists now in many forms: the multi echo pulse sequence using single or multislice acquisition, the fast spin echo (FSE/TSE) pulse sequence, echo planar imaging (EPI) pulse sequence and the gradient and spin echo (GRASE) pulse sequence;; all are basically spin echo sequences.
In the simplest form of SE imaging, the pulse sequence has to be repeated as many times as the image has lines.
Contrast values:
PD weighted: Short TE (20 ms) and long TR.
T1 weighted: Short TE (10-20 ms) and short TR (300-600 ms)
T2 weighted: Long TE (greater than 60 ms) and long TR (greater than 1600 ms)
With spin echo imaging no T2* occurs, caused by the 180° refocusing pulse. For this reason, spin echo sequences are more robust against e.g., susceptibility artifacts than gradient echo sequences.

See also Pulse Sequence Timing Diagram to find a description of the components.
 
Images, Movies, Sliders:
 Shoulder Coronal T1 SE  Open this link in a new window
    

Courtesy of  Robert R. Edelman
 Shoulder Axial T1 SE  Open this link in a new window
 MRI Orbita T1  Open this link in a new window
    
 
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• Related Searches:
    • Gradient
    • T1 Time
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Further Reading:
  Basics:
Fast Spin Echo(.pdf)
Tuesday, 24 January 2006   by www.81bones.net    
Magnetic resonance imaging
   by www.scholarpedia.org    
FUNDAMENTALS OF MRI: Part I
   by www.e-radiography.net    
  News & More:
New MR sequence helps radiologists more accurately evaluate abnormalities of the uterus and ovaries
Thursday, 23 April 2009   by www.eurekalert.org    
MRI techniques improve pulmonary embolism detection
Monday, 19 March 2012   by medicalxpress.com    
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Magnetization Transfer Contrast
 
(MTC) This MRI method increases the contrast by removing a portion of the total signal in tissue. An off resonance radio frequency (RF) pulse saturates macromolecular protons to make them invisible (caused by their ultra-short T2* relaxation times). The MRI signal from semi-solid tissue like brain parenchyma is reduced, and the signal from a more fluid component like blood is retained.
E.g., saturation of broad spectral lines may produce decreases in intensity of lines not directly saturated, through exchange of magnetization between the corresponding states; more closely coupled states will show a greater resulting intensity change. Magnetization transfer techniques make demyelinated brain or spine lesions (as seen e.g. in multiple sclerosis) better visible on T2 weighted images as well as on gadolinium contrast enhanced T1 weighted images.
Off resonance makes use of a selection gradient during an off resonance MTC pulse. The gradient has a negative offset frequency on the arterial side of the imaging volume (caudally more off resonant and cranially less off resonant). The net effect of this type of pulse is that the arterial blood outside the imaging volume will retain more of its longitudinal magnetization, with more vascular signal when it enters the imaging volume. Off resonance MTC saturates the venous blood, leaving the arterial blood untouched.
On resonance has no effect on the free water pool but will saturate the bound water pool and is the difference in T2 between the pools. Special binomial pulses are transmitted causing the magnetization of the free protons to remain unchanged. The z-magnetization returns to its original value. The spins of the bound pool with a short T2 experience decay, resulting in a destroyed magnetization after the on resonance pulse.

See also Magnetization Transfer.
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• View the DATABASE results for 'Magnetization Transfer Contrast' (5).Open this link in a new window

 
Further Reading:
  News & More:
MRI of the Human Eye Using Magnetization Transfer Contrast Enhancement
   by www.iovs.org    
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Partial SaturationInfoSheet: - Sequences - 
Intro, 
Overview, 
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etc.
 
(PS) Excitation technique applying repeated RF pulses in times comparable to or shorter than T1. Incomplete T1 relaxation leads to reduction of the signal amplitude; there is the possibility of generating images with increased contrast between regions with different relaxation times.
Although partial saturation is also commonly referred to as saturation recovery, that term should properly be reserved for the particular case of partial saturation in which recovery after each excitation effectively takes place from true saturation. A GRE sequence where α = 90° is identical to the partial saturation or saturation recovery pulse sequence.
It does not directly produce images of T1. However, since the measured signal will depend on T1, the method generates contrast between regions with different relaxation times. If T2 and/or T2 effects are minimized through the use of a short echo time TE, the result is a T1 weighted image. It is not a T1 image due to the possible presence of spin density and T2 effects as well as the nonlinear dependence on T1.
The change in signal from a region resulting from a change in the interpulse time, TR, can be used to calculate T1 for the region.
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• View the DATABASE results for 'Partial Saturation' (5).Open this link in a new window

MRI Resources 
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Relaxivity
 
The ability of magnetic compounds to increase the relaxation rates of the surrounding water proton spins.
Relaxivity is used to improve the contrast of the image, and to study tissue specific areas where the contrast agent better diffuses or to perform functional magnetic resonance imaging.
The relaxivity of MRI contrast agents depends on the molecular structure and kinetic of the complex. To increase the number of water molecules that are in the inner sphere of the complex, or to slow down the molecular rotational correlation time, are possibilities to improve the water relaxivity.
Relaxivity units (r1, r2) are mM-1 * sec-1 (at varying temperatures).
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• View the NEWS results for 'Relaxivity' (2).Open this link in a new window.
 
Further Reading:
  News & More:
Measurements of the relaxivity of gadolinium chelates in tissues in vivo(.pdf)
2001   by cds.ismrm.org    
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