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Diffusion Time
 
Images obtained by a diffusion weighted sequence (e.g. the Stejskal-Tanner sequence) reflect the diffusion as an attenuation of signal intensity. The diffusion time is the time elapsed between the leading edges of the diffusion gradient lobes of this sequence.
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Further Reading:
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Diffusion-Weighted MRI in the Body: Applications and Challenges in Oncology
Friday, 1 June 2007   by www.ajronline.org    
MRI Resources 
Raman Spectroscopy - Service and Support - Movies - MRCP - Pediatric and Fetal MRI - Software
 
Diffusion Weighted SequenceInfoSheet: - Sequences - 
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etc.MRI Resource Directory:
 - Diffusion Weighted Imaging -
 
Diffusion weighted imaging can be performed similar to the phase contrast angiography sequence. The gradients must be increased in amplitude to depict the much slower motions of molecular diffusion in the body.
While a T1 weighted MRI pulse sequence is diffusion sensitive, a quantitative diffusion pulse sequence was introduced by Steijskal and Tanner. Its characteristic features are two strong symmetrical gradient lobes placed on either side of the 180° refocusing pulse in a spin echo sequence. These symmetrical gradient lobes have the sole purpose of enhancing dephasing of spins, thereby accelerating intravoxel incoherent motion (IVIM) signal loss.
Dephasing is proportional to the square of the time (diffusion time) during which the gradients are switched on and the strength of the applied gradient field. Therefore, the use of high field gradient systems with faster and more sensitive sequences, make diffusion weighting more feasible.
Areas in which the protons diffuse rapidly (swollen cells in early stroke, less restriction to diffusion) will show an increased signal when the echo is measured relative to areas in which diffusion is restricted. For increased accuracy of diffusion measurement and image enhancement, useful motion correction techniques such as navigator echo and other methods should be used. In addition to this, applying the b-value calculated by the strength and duration of motion probing gradients with a high rate of accuracy is very important.
See also Apparent Diffusion Coefficient, ADC Map, Lattice Index Map.

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Further Reading:
  Basics:
Diffusion-Weighted Imaging
   by spinwarp.ucsd.edu    
Diffusion Imaging: From Basic Physics to Practical Imaging
1999   by ej.rsna.org    
EVALUATION OF HUMAN STROKE BY MR IMAGING
2000
  News & More:
Functional imaging with diffusion-weighted MRI for lung biopsy planning: initial experience
Thursday, 10 July 2014   by 7thspace.com    
Diffusion-weighted MRI sensitive for metastasis in pelvic lymph nodes
Sunday, 15 June 2014   by www.2minutemedicine.com    
Hopkins researchers use diffusion MRI technique to monitor ultrasound uterine fibroid treatment
Monday, 8 August 2005   by www.eurekalert.org    
DWI Best in Subacute Stroke Imaging
Tuesday, 1 June 2004   by www.hospimedica.com    
MRI Resources 
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Echo Planar ImagingInfoSheet: - Sequences - 
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 - Sequences -
 
Echo Planar Imaging Timing Diagram

(EPI) Echo planar imaging is one of the early magnetic resonance imaging sequences (also known as Intascan), used in applications like diffusion, perfusion, and functional magnetic resonance imaging. Other sequences acquire one k-space line at each phase encoding step. When the echo planar imaging acquisition strategy is used, the complete image is formed from a single data sample (all k-space lines are measured in one repetition time) of a gradient echo or spin echo sequence (see single shot technique) with an acquisition time of about 20 to 100 ms. The pulse sequence timing diagram illustrates an echo planar imaging sequence from spin echo type with eight echo train pulses. (See also Pulse Sequence Timing Diagram, for a description of the components.)
In case of a gradient echo based EPI sequence the initial part is very similar to a standard gradient echo sequence. By periodically fast reversing the readout or frequency encoding gradient, a train of echoes is generated.
EPI requires higher performance from the MRI scanner like much larger gradient amplitudes. The scan time is dependent on the spatial resolution required, the strength of the applied gradient fields and the time the machine needs to ramp the gradients.
In EPI, there is water fat shift in the phase encoding direction due to phase accumulations. To minimize water fat shift (WFS) in the phase direction fat suppression and a wide bandwidth (BW) are selected. On a typical EPI sequence, there is virtually no time at all for the flat top of the gradient waveform. The problem is solved by "ramp sampling" through most of the rise and fall time to improve image resolution.
The benefits of the fast imaging time are not without cost. EPI is relatively demanding on the scanner hardware, in particular on gradient strengths, gradient switching times, and receiver bandwidth. In addition, EPI is extremely sensitive to image artifacts and distortions.

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Further Reading:
  Basics:
New Imaging Method Makes Brain Scans 7 Times Faster
Sunday, 9 January 2011   by www.dailytech.com    
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Intravoxel Incoherent Motion
 
(IVIM) Spins moving in fluids with different velocities and possibly in different directions. This is being found to a small degree in all tissues as a result of capillary perfusion or diffusion. Important velocity changes occur as one moves from the vessel wall towards the center of the vessel. Hence, spins (to a variable degree) have different velocities within a single imaging voxel.
This effect can be measured using special pulse sequences such as in diffusion imaging or diffusion weighed imaging. When the velocity differences are marked, as occurs in larger blood vessels, effects due to IVIM are visible in standard MR images and give rise to flow related dephasing. The effects are more visible when longer echo times are used.
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Further Reading:
  Basics:
Diffusion Imaging: From Basic Physics to Practical Imaging
1999   by ej.rsna.org    
  News & More:
EVALUATION OF HUMAN STROKE BY MR IMAGING
2000
Technical Assessment of Artifact Production from Neuro Endovascular Coil At 3 Tesla MRI: An In Vitro Study
2012   by www.tmps.or.th    
MRI Resources 
Pacemaker - Libraries - Universities - Cardiovascular Imaging - Collections - Blood Flow Imaging
 
Point Resolved SpectroscopyInfoSheet: - Sequences - 
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(PRESS) Point resolved spectroscopy is a multi echo single shot technique to obtain spectral data. PRESS is a 90°-180°-180° (slice selective pulses) sequence. The 90° radio frequency pulse rotates the spins in the yx-plane, followed by the first 180° pulse (spin rotation in the xz-plane) and the second 180° pulse (spin rotation in the xy-plane), which gives the signal.
With the long echo times used in PRESS, there is a better visualization of metabolites with longer relaxation times. Many of the metabolites depicted by stimulated echo technique are not seen on point resolved spectroscopy, but PRESS is less susceptible to motion, diffusion, and quantum effects and has a better SNR than stimulated echo acquisition mode (STEAM).
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Further Reading:
  Basics:
The Basics of MRI
   by www.cis.rit.edu    
  News & More:
MRI evaluation of fatty liver in day to day practice: Quantitative and qualitative methods
Wednesday, 3 September 2014   by www.sciencedirect.com    
MRI Resources 
Examinations - Distributors - Collections - Stent - Education pool - Absorption and Emission
 
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