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T2* Time
 
(T2 Star) The characteristic time constant that describes the decay of transverse magnetization, taking into account the inhomogeneity in static magnetic fields and the spin spin relaxation in the human body. This results in a rapid loss of phase coherence and the MRI signal. The T2* time is always less than the T2 time.
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Automatic Mapping Extraction from Multiecho T2-Star Weighted Magnetic Resonance Images for Improving Morphological Evaluations in Human Brain
Wednesday, 5 June 2013   by www.hindawi.com    
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* (also called T2 Star) is composed of molecular interactions (spin spin relaxation) and local magnetic field non-uniformities. Caused by this the protons precess at slightly different frequencies. The T2* effect cause a rapid loss in coherence and transverse magnetization. The T2* time is less than the T2 time.
See also T2* Time, T2 Star.
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Further Reading:
  Basics:
IMAGE CONTRAST IN MRI(.pdf)
   by www.assaftal.com    
T2* cardiac MRI allows prediction of severe reperfusion injury after STEMI
Tuesday, 9 November 2010   by www.medwire-news.md    
Introduction to MRI Physics, Page 9
   by www.simplyphysics.com    
  News & More:
Scientists create imaging 'toolkit' to help identify new brain tumor drug targets
Tuesday, 2 February 2016   by www.eurekalert.org    
Resonance Health Limited (RHT.AX) Receives FDA Approval for MRI-Q Cardiac Iron T2* Test
Tuesday, 16 August 2011   by www.biospace.com    
MRI effectively measures hemochromatosis iron burden
Saturday, 3 October 2015   by medicalxpress.com    
Principles, Techniques, and Applications of T2*- based MR Imaging and Its Special Applications1
September 2009   by pubs.rsna.org    
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Contrast Enhanced Magnetic Resonance AngiographyInfoSheet: - Sequences - 
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(CE MRA) Contrast enhanced MR angiography is based on the T1 values of blood, the surrounding tissue, and paramagnetic contrast agent.
T1-shortening contrast agents reduces the T1 value of the blood (approximately to 50 msec, shorter than that of the surrounding tissues) and allow the visualization of blood vessels, as the images are no longer dependent primarily on the inflow effect of the blood. Contrast enhanced MRA is performed with a short TR to have low signal (due to the longer T1) from the stationary tissue, short scan time to facilitate breath hold imaging, short TE to minimize T2* effects and a bolus injection of a sufficient dose of a gadolinium chelate.
Images of the region of interest are performed with 3D spoiled gradient echo pulse sequences. The enhancement is maximized by timing the contrast agent injection such that the period of maximum arterial concentration corresponds to the k-space acquisition. Different techniques are used to ensure optimal contrast of the arteries e.g., bolus timing, automatic bolus detection, bolus tracking, care bolus. A high resolution with near isotropic voxels and minimal pulsatility and misregistration artifacts should be striven for. The postprocessing with the maximum intensity projection (MIP) enables different views of the 3D data set.
Unlike conventional MRA techniques based on velocity dependent inflow or phase shift techniques, contrast enhanced MRA exploits the gadolinium induced T1-shortening effects. CE MRA reduces or eliminates most of the artifacts of time of flight angiography or phase contrast angiography. Advantages are the possibility of in plane imaging of the blood vessels, which allows to examine large parts in a short time and high resolution scans in one breath hold. CE MRA has found a wide acceptance in the clinical routine, caused by the advantages:
3D MRA can be acquired in any plane, which means that greater vessel coverage can be obtained at high resolution with fewer slices (aorta, peripheral vessels);
the possibility to perform a time resolved examination (similarly to conventional angiography);
no use of ionizing radiation; paramagnetic agents have a beneficial safety.


 
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 CE-MRA of the Carotid Arteries  Open this link in a new window
    
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 CE MRA of the Aorta  Open this link in a new window
    
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 CE-MRA of the Carotid Arteries Colored MIP  Open this link in a new window
    
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• View the NEWS results for 'Contrast Enhanced Magnetic Resonance Angiography' (2).Open this link in a new window.
 
Further Reading:
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Contrast-Enhanced MR Angiography(.pdf)
   by ric.uthscsa.edu    
CONTRAST ENHANCED MR ANGIOGRAPHY – PRINCIPLES, APPLICATIONS, TIPS AND PITFALLS(.pdf)
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CONTRAST-ENHANCED MRA OF THE CAROTIDS(.pdf)
PERIPHERAL VASCULAR MAGNETIC RESONANCE ANGIOGRAPHY(.pdf)
CONTRAST ENHANCED MRI OF THE LIVER STATE-OF-THE-ART(.pdf)
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Free Induction Decay
 
(FID) A free induction decay curve is generated as excited nuclei relax. The amplitude of the FID signal becomes smaller over time as net magnetization returns to equilibrium. If transverse magnetization of the spins is produced, e.g. by a 90° pulse, a transient MR signal will result that will decay toward zero with a characteristic time constant T2 (or T2*); this decaying signal is the free induction decay.
The signal peaks of the echoes fall onto this T2 decay curve, while at each echo the signals arise and decay with T2*. The typical T2 relaxation times being of the order of 5-200 ms in the human body. The first part of the FID is not observable (named the 'receiver dead time') caused by residual effects of the powerful exciting radio frequency pulse on the electronics of the receiver.
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Further Reading:
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Free induction decay
   by en.wikipedia.org    
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Magnetic resonance imaging
   by www.scholarpedia.org    
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Fast Imaging with Steady State PrecessionInfoSheet: - Sequences - 
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Overview, 
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(FISP) A fast imaging sequence, which attempts to combine the signals observed separately in the FADE sequence, generally sensitive about magnetic susceptibility artifacts and imperfections in the gradient waveforms. Confusingly now often used to refer to a refocused FLASH type sequence.
This sequence is very similar to FLASH, except that the spoiler pulse is eliminated. As a result, any transverse magnetization still present at the time of the next RF pulse is incorporated into the steady state. FISP uses a RF pulse that alternates in sign. Because there is still some remaining transverse magnetization at the time of the RF pulse, a RF pulse of a degree flips the spins less than a degree from the longitudinal axis. With small flip angles, very little longitudinal magnetization is lost and the image contrast becomes almost independent of T1. Using a very short TE (with TR 20-50 ms, flip angle 30-45°) eliminates T2* effects, so that the images become proton density weighted. As the flip angle is increased, the contrast becomes increasingly dependent on T1 and T2*. It is in the domain of large flip angles and short TR that FISP exhibits vastly different contrast to FLASH type sequences. Used for T1 orthopedic imaging, 3D MPR, cardiography and angiography.
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Further Reading:
  Basics:
MRI techniques improve pulmonary embolism detection
Monday, 19 March 2012   by medicalxpress.com    
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