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Result : Searchterm 'Pulse Sequence' found in 5 terms [] and 166 definitions []
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Bipolar Gradient Pulse
Bipolar gradients are two gradients with the same magnitude but opposite gradient direction. A bipolar gradient pulse is produced if one of the bipolar gradients is switched e.g., in negative direction and then switched in the opposite direction for an equivalent amount of time.
Bipolar gradients are used e.g. in phase contrast and diffusion weighted sequences. A bipolar gradient pulse pair produces a phase shift, which depends on the velocity component along this gradient. Motion along a bipolar gradient pulse pair results in a flow-induced phase shift of the transverse magnetization. The bipolar gradient pulse pair will not affect stationary spins. The amount of phase shifts depends on the area of each gradient pulse, and distance between the pulses. An echo occurring after such a gradient is flow compensated for velocity. A slight shift in the balance of this gradient will introduce a defined flow sensitivity of the pulse sequence.

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    • Gradient Pulse
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Blipped Phase Encoding
A strategy for incrementing the position of the k-space trajectory of an echo planar imaging (EPI) pulse sequence.
echo planar imaging (EPI) uses a constant gradient amplitude in one direction. This, combined with an oscillating gradient system in the frequency encoding direction, produces a zigzag trajectory in k-space. In the blipped phase encoding variant of EPI, the k-space position in the phase encoded direction is incremented by gradient 'blips' of the appropriate area. These, when timed to occur during the reversals of the read-out gradient, produce a rectilinear path in k-space.
The artifacts in an EPI image can arise from both hardware and sample imperfections. These are most easily understandable from examination of the k-space trajectory involved, which is either a zigzag form (when using a constant phase encoding gradient) or a rastered zigzag (when the phase encoding is performed with small gradients at the end of each scan line, so-called 'blipped' EPI).

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Chapter 2 - Principles of Magnetic Resonance Imaging
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C-SCAN™InfoSheet: - Devices -
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Manufactured by Esaote S.p.A.; compact in-office MRI system, fits in a 90 ft² (8.4 m²) space and requires no shielding or special power. This low field MRI magnet is optimized for orthopedic use and imaging of the extremities.
The C-SCAN™ is developed from the ARTOSCAN™ - M with a new computer platform, and is also known as Artoscan C.
Esaote North America and Hologic Inc. are the U.S. distributors of this MRI device.

Device Information and Specification
CLINICAL APPLICATION Dedicated extremity
SURFACE COILS Linear and dual phased array, knee, ankle, wrist (6 total) coils
IMAGING MODES Single, multislice, volume study, fast scan, multi slab
TR 12 - 5,000 msec
TE 5 - 220 msec
SINGLE SLICE 0.8 sec/image
MULTISLICE 0.8 sec/image
FOV 10 - 20 cm
SLICE THICKNESS 2D: 2 mm - 10 mm;
3D: 0.6 mm - 10 mm
MEASURING MATRIX 256 x 256 maximum
PIXEL INTENSITY 4,096 gray lvls, 256 lvls in 3D
or W x H
33.6 x 16 cm
H*W*D 124 x 76 x 60 cm
POWER REQUIREMENTS 100/110/200/220/230/240
5-GAUSS FRINGE FIELD, radial/axial 28 cm/60 cm


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Chemical Shift ArtifactInfoSheet: - Artifacts - 
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Quick Overview
Please note that there are different common names for this artifact.

Artifact Information
NAME Chemical shift, black boundary, spatial misregistration, relief
DESCRIPTION Black or bright band
REASON Chemical shift, opposed phase image
HELP Fat suppression, smaller water fat shift (high bandwidth), in phase image, SE sequences

During frequency encoding, fat protons precess slower than water protons in the same slice because of their magnetic shielding. Through the difference in resonance frequency between water and fat, protons at the same location are misregistrated (dislocated) by the Fourier transformation, when converting MRI signals from frequency to spatial domain. This chemical shift misregistration cause accentuation of any fat-water interfaces along the frequency axis and may be mistaken for pathology. Where fat and water are in the same location, this artifact can be seen as a bright or dark band at the edge of the anatomy.
Protons in fat and water molecules are separated by a chemical shift of about 3.5 ppm. The actual shift in Hertz (Hz) depends on the magnetic field strength of the magnet being used. Higher field strength increases the misregistration, while in contrast a higher gradient strength has a positive effect. For a 0.3 T system operating at 12.8 MHz the shift will be 44.8 Hz compared with a 223.6 Hz shift for a 1.5 T system operating at 63.9 MHz.

Image Guidance
For artifact reduction helps a smaller water fat shift (higher bandwidth), a higher matrix, an in phase TE or a spin echo technique. Since the misregistration offset is present in the read out axis the patient may be rescanned with this axis parallel to the fat-water interface. Steeper gradient may be employed to reduce the chemical shift offset in mm. Another strategy is to employ specialized pulse sequences such as fat saturation or inversion recovery imaging. Fat suppression techniques eliminate chemical shift artifacts caused by the lack of fat signal.
See also Black Boundary Artifact and Magnetic Resonance Spectroscopy.


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Further Reading:
MRI Artifact Gallery
  News & More:
What is chemical shift artefact? Why does it occur? How many Hz at 1.5 T?
Abdominal MRI at 3.0 T: The Basics Revisited
Wednesday, 20 July 2005   by    
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Combination Oral Contrast AgentsInfoSheet: - Contrast Agents - 
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Biphasic oral contrast agents may produce either high or low signal intensities depending on the pulse sequence used, for example low signal intensity on T1 weighted MR images and high signal intensity on T2 weighted images. The combination of different oral contrast agents can generate a macroscopic cancellation of negative and positive magnetic susceptibility, thereby eliminating susceptibility artifacts.
Possible combinations are e.g., ferric ammonium citrate and corn oil, or ferrous sulfate emulsified with baby formula. Paramagnetic agents combined with oil emulsion may be used in MRI as positive abdominal contrast agents. The combination of diamagnetic barium sulfate and superparamagnetic iron oxide (SPIO) in one suspension may be a useful negative contrast agent.
See also Gastrointestinal Paramagnetic Contrast Agents, Gastrointestinal Superparamagnetic Contrast Agents, Gastrointestinal Diamagnetic Contrast Agents, Gastrointestinal Imaging.


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Further Reading:
Usefulness of MR Imaging for Diseases of the Small Intestine: Comparison with CT
2000   by    
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