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Spin Spin Relaxation Time
 
The spin spin relaxation time is the time the spins need to dephase in the transverse plane.
See T2 Time, and Transverse Relaxation Time.
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MULTIEXPONENTIAL PROTON SPIN-SPIN RELAXATION IN MAGNETIC RESONANCE IMAGING OF HUMAN BRAIN TUMORS
Friday, 26 March 1999   by www.dkfz-heidelberg.de    
T2* cardiac MRI allows prediction of severe reperfusion injury after STEMI
Tuesday, 9 November 2010   by www.medwire-news.md    
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T2 TimeForum -
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The T2 relaxation time (spin spin relaxation time or transverse relaxation time), is a biological parameter that is used in MRIs to distinguish between tissue types and is termed 'Time 2' or T2. It is a tissue-specific time constant for protons and is dependent on the exchanging of energy with near by nuclei. T2 weighted images rely upon local dephasing of spins following the application of the transverse energy pulse. T2 is the decay of magnetization perpendicular to the main magnetic field (in an ideal homogeneous field).
Due to interaction between the spins, they lose their phase coherence, which results in a loss of transverse magnetization and MRI signal. After time T2 transverse magnetization has lost 63% of its original value. This tissue parameter determines the contrast.
The T2 relaxation is temperature dependent. At a lower temperature molecular motion is reduced and the decay times are reduced.
Fat has a very efficient energy exchange and therefore it has a relatively short T2.
Water is less efficient than fat in the exchange of energy, and therefore it has a long T2 time.

See also T2 Weighted Image and Magnetic Resonance Imaging MRI.
 
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MYELIN-SELECTIVE MRI: PULSE SEQUENCE DESIGN AND OPTIMIZATION
   by www.imaging.robarts.ca    
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Spin Lattice Relaxation Time
 
(T1) The spin lattice relaxation time (also called longitudinal relaxation time and T1 Time) is a spin property, whereby the value changes between different tissues. By the spin lattice relaxation process, the longitudinal magnetization Mz achieve the equilibrium value Mz0. The T1 time constant is an exponential approach toward Mz0.
The equation for the magnetization at a time t will be (if at t=0 the longitudinal magnetization is Mz0):
Mz(t) = M0+(Mz (0) - Mz0) exp(t/T1)
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Further Reading:
  Basics:
Electron Spin Resonance
   by hyperphysics.phy-astr.gsu.edu    
  News & More:
MRI's inside story
Thursday, 4 December 2003   by www.economist.com    
MULTIEXPONENTIAL PROTON SPIN-SPIN RELAXATION IN MAGNETIC RESONANCE IMAGING OF HUMAN BRAIN TUMORS
Friday, 26 March 1999   by www.dkfz-heidelberg.de    
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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.
 
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Further Reading:
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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    
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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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MRI History
 
Sir Joseph Larmor (1857-1942) developed the equation that the angular frequency of precession of the nuclear spins being proportional to the strength of the magnetic field. [Larmor relationship]
In the 1930's, Isidor Isaac Rabi (Columbia University) succeeded in detecting and measuring single states of rotation of atoms and molecules, and in determining the mechanical and magnetic moments of the nuclei.
Felix Bloch (Stanford University) and Edward Purcell (Harvard University) developed instruments, which could measure the magnetic resonance in bulk material such as liquids and solids. (Both honored with the Nobel Prize for Physics in 1952.) [The birth of the NMR spectroscopy]
In the early 70's, Raymond Damadian (State University of New York) demonstrated with his NMR device, that there are different T1 relaxation times between normal and abnormal tissues of the same type, as well as between different types of normal tissues.
In 1973, Paul Lauterbur (State University of New York) described a new imaging technique that he termed Zeugmatography. By utilizing gradients in the magnetic field, this technique was able to produce a two-dimensional image (back-projection). (Through analysis of the characteristics of the emitted radio waves, their origin could be determined.) Peter Mansfield further developed the utilization of gradients in the magnetic field and the mathematically analysis of these signals for a more useful imaging technique. (Paul C Lauterbur and Peter Mansfield were awarded with the 2003 Nobel Prize in Medicine.)
In 1975, Richard Ernst introduced 2D NMR using phase and frequency encoding, and the Fourier Transform. Instead of Paul Lauterbur's back-projection, he timely switched magnetic field gradients ('NMR Fourier Zeugmatography'). [This basic reconstruction method is the basis of current MRI techniques.]
1977/78: First images could be presented. A cross section through a finger by Peter Mansfield and Andrew A. Maudsley. Peter Mansfield also could present the first image through the abdomen.
In 1977, Raymond Damadian completed (after 7 years) the first MR scanner (Indomitable). In 1978, he founded the FONAR Corporation, which manufactured the first commercial MRI scanner in 1980. Fonar went public in 1981.
1981: Schering submitted a patent application for Gd-DTPA dimeglumine.
1982: The first 'magnetization-transfer' imaging by Robert N. Muller.
In 1983, Toshiba obtained approval from the Ministry of Health and Welfare in Japan for the first commercial MRI system.
In 1984, FONAR Corporation receives FDA approval for its first MRI scanner.
1986: Jürgen Hennig, A. Nauerth, and Hartmut Friedburg (University of Freiburg) introduced RARE (rapid acquisition with relaxation enhancement) imaging. Axel Haase, Jens Frahm, Dieter Matthaei, Wolfgang Haenicke, and Dietmar K. Merboldt (Max-Planck-Institute, Göttingen) developed the FLASH (fast low angle shot) sequence.
1988: Schering's MAGNEVIST gets its first approval by the FDA.
In 1991, fMRI was developed independently by the University of Minnesota's Center for Magnetic Resonance Research (CMRR) and Massachusetts General Hospital's (MGH) MR Center.
From 1992 to 1997 Fonar was paid for the infringement of it's patents from 'nearly every one of its competitors in the MRI industry including giant multi-nationals as Toshiba, Siemens, Shimadzu, Philips and GE'.
 
Images, Movies, Sliders:
 Cardiac Infarct Short Axis Cine Overview  Open this link in a new window
    

Courtesy of  Robert R. Edelman
 
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Further Reading:
  Basics:
Magnetic Resonance Imaging, History & Introduction
2000   by www.cis.rit.edu    
A Short History of the Magnetic Resonance Imaging (MRI)
   by www.teslasociety.com    
Fonar Our History
   by www.fonar.com    
  News & More:
Scientists win Nobels for work on MRI
Tuesday, 10 June 2003   by usatoday30.usatoday.com    
2001 Lemelson-MIT Lifetime Achievement Award Winner
   by web.mit.edu    
MRI's inside story
Thursday, 4 December 2003   by www.economist.com    
MRI Resources 
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