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Slice Gap
 
The not measured distance between the slices. A lower gap can introduce more noise into adjoining slices.
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Crosstalk (Artifact)InfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.MRI Resource Directory:
 - Artifacts -
 
Quick Overview
Artifact Information
NAME
Crosstalk, cross excitation
DESCRIPTION
Anomalous intensities in an image
REASON
HELP
Slice gap minimum 10%
Crosstalk is an artifact introduced into images by interference between adjacent slices of a scan, caused by a slice profile that is not ideal due to the constraints of the measurement technology. If the slice distances are too small, there is cross talk between the slices, which can affect T1 contrast.
mri safety guidance
Image Guidance
This artifact can be eliminated by limiting the minimum spacing (for the most sequences a minimum gap 10% and for IR sequences 20%) between the slices. Crosstalk can also be reduced by selection of interleaved slices (so a slice gap will not be necessary), but interleaved data acquisition can produce large mean intensity differences between adjacent slices.
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Further Reading:
  Basics:
MRI GLOSSARY
   by fonar.com    
Slice-overlap Artifacts
   by www.mritutor.org    
MRI Resources 
MRI Accidents - Non-English - Claustrophobia - Databases - Pacemaker -
 
Gradient Echo SequenceForum -
related threadsInfoSheet: - Sequences - 
Intro, 
Overview, 
Types of, 
etc.
 
Gradient Echo Sequence Timing Diagram (GRE - sequence) A gradient echo is generated by using a pair of bipolar gradient pulses. In the pulse sequence timing diagram, the basic gradient echo sequence is illustrated. There is no refocusing 180° pulse and the data are sampled during a gradient echo, which is achieved by dephasing the spins with a negatively pulsed gradient before they are rephased by an opposite gradient with opposite polarity to generate the echo.
See also the Pulse Sequence Timing Diagram. There you will find a description of the components.
The excitation pulse is termed the alpha pulse α. It tilts the magnetization by a flip angle α, which is typically between 0° and 90°. With a small flip angle there is a reduction in the value of transverse magnetization that will affect subsequent RF pulses. The flip angle can also be slowly increased during data acquisition (variable flip angle: tilt optimized nonsaturation excitation). The data are not acquired in a steady state, where z-magnetization recovery and destruction by ad-pulses are balanced. However, the z-magnetization is used up by tilting a little more of the remaining z-magnetization into the xy-plane for each acquired imaging line.
Gradient echo imaging is typically accomplished by examining the FID, whereas the read gradient is turned on for localization of the signal in the readout direction. T2* is the characteristic decay time constant associated with the FID. The contrast and signal generated by a gradient echo depend on the size of the longitudinal magnetization and the flip angle. When α = 90° the sequence is identical to the so-called partial saturation or saturation recovery pulse sequence. In standard GRE imaging, this basic pulse sequence is repeated as many times as image lines have to be acquired. Additional gradients or radio frequency pulses are introduced with the aim to spoil to refocus the xy-magnetization at the moment when the spin system is subject to the next α pulse.
As a result of the short repetition time, the z-magnetization cannot fully recover and after a few initial α pulses there is an equilibrium established between z-magnetization recovery and z-magnetization reduction due to the α pulses.
Gradient echoes have a lower SAR, are more sensitive to field inhomogeneities and have a reduced crosstalk, so that a small or no slice gap can be used. In or out of phase imaging depending on the selected TE (and field strength of the magnet) is possible. As the flip angle is decreased, T1 weighting can be maintained by reducing the TR. T2* weighting can be minimized by keeping the TE as short as possible, but pure T2 weighting is not possible. By using a reduced flip angle, some of the magnetization value remains longitudinal (less time needed to achieve full recovery) and for a certain T1 and TR, there exist one flip angle that will give the most signal, known as the "Ernst angle".
Contrast values:
PD weighted: Small flip angle (no T1), long TR (no T1) and short TE (no T2*)
T1 weighted: Large flip angle (70°), short TR (less than 50ms) and short TE
T2* weighted: Small flip angle, some longer TR (100 ms) and long TE (20 ms)

Classification of GRE sequences can be made into four categories:
See also Gradient Recalled Echo Sequence, Spoiled Gradient Echo Sequence, Refocused Gradient Echo Sequence, Ultrafast Gradient Echo Sequence.
 
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SlidersSliders Overview

 
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Further Reading:
  Basics:
Enhanced Fast GRadient Echo 3-Dimensional (efgre3D) or THRIVE
   by www.mri.tju.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    
T1rho-prepared balanced gradient echo for rapid 3D T1rho MRI
Monday, 1 September 2008   by www.ncbi.nlm.nih.gov    
Searchterm 'Slice Gap' was also found in the following service: 
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Staircase ArtifactInfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.MRI Resource Directory:
 - Artifacts -
 
Quick Overview
Artifact Information
NAME
Staircase
DESCRIPTION
Staircase like nuance
REASON
Slices too thick, no overlapping
HELP
Overlapping slices
Staircase artifacts might be seen in projection of the slice thickness in case of MIP or MPR reconstructions.
mri safety guidance
Image Guidance
Use overlapping slices to reduce this artifact by changing the slice gap to a negative value. In most cases, a negative value of 20-30% is sufficient.
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• View the DATABASE results for 'Staircase Artifact' (2).Open this link in a new window

MRI Resources 
MRCP - Health - Blood Flow Imaging - Contrast Enhanced MRI - Sequences - Brain MRI
 
Signa SP 0.5T™ Open ConfigurationInfoSheet: - Devices -
Intro, 
Types of Magnets, 
Overview, 
etc.MRI Resource Directory:
 - Devices -
 
www.gehealthcare.com/usen/mr/index.html From GE Healthcare;
The Signa SP 0.5T™ is an open MRI magnet that is designed for use in interventional radiology and intra-operative imaging. The vertical gap configuration increases patient positioning options, improves patient observation, and allows continuous access to the patient during imaging. The magnet enclosure also incorporates an intercom, patient observation video camera, laser patient alignment lights, and task lighting in the imaging volume.
Device Information and Specification
CLINICAL APPLICATION
Whole body
CONFIGURATION
Open
Integrated transmit and receive body coil; optional rotational body coil, head; other coils optional; open architecture makes system compatible with a wide selection of coilsarray
SYNCHRONIZATION
Standard cardiac gating, ECG/peripheral, respiratory gating
PULSE SEQUENCES
Standard: SE, IR, 2D/3D GRE and SPGR, 2D/3D TOF, 2D/3D FSE, 2D/3D FGRE and FSPGR, SSFP, FLAIR, EPI, optional: 2D/3D Fiesta, true chem sat, fat/water separation, single shot diffusion EPI
IMAGING MODES
Localizer, single slice, multislice, volume, fast, POMP, multi slab, cine, slice and frequency zip, extended dynamic range, tailored RF
TR
1.3 to 12000 msec in increments of 1 msec
TE
0.4 to 2000 msec in increments of 1 msec
3cm to 40 cm continuous
2D: 1.4mm - 20mm 3D: 0.2mm - 20mm
1280 x 1024
MEASURING MATRIX
128x512 steps 32 phase//freq.
PIXEL INTENSITY
256 gray levels
60 x 58 cm
POWER REQUIREMENTS
200 - 480, 3-phase
COOLING SYSTEM TYPE
Liquid helium
0.03 L/hr, holds 300 L
STRENGTH
12 mT/m
Active
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Further Reading:
  News & More:
MR Surgical Suite, Improving surgical procedure quality (.pdf)
   by www3.gehealthcare.com    
MRI Resources 
Portals - Supplies - MR Myelography - Intraoperative MRI - Fluorescence - Universities
 
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