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Result : Searchterm 'Chemical Shift Imaging' found in 1 term [] and 11 definitions [], (+ 8 Boolean[] results
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Magnetic Field Mapping
 
The mapping of the magnetic field by measuring or imaging the spatial distribution of magnetic field strength, can be performed by scanning with a probe and handles a large range of field strengths, but is slow and tedious. Accurate field maps can be made by measuring the Larmor frequency as a function of position.
The field must be homogeneous enough to allow MR imaging to be performed, than the magnetic field can be mapped by different methods.
1. The adaptation of chemical shift imaging.
2. The faster one measures the change in signal phase in an image obtained with a gradient echo pulse sequence resulting from a change in echo time TE, which is proportional to the local field strength.
Also useful is a spin echo pulse sequence with data collection from two time locations of the readout gradient and the data acquisition interval, where each having a known shift of the acquisition center away from the spin echo.
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Further Reading:
  News & More:
sPHENIX assembly update: magnet mapped, detectors prepared
Friday, 23 December 2022   by www.eurekalert.org    
MRI Resources 
Process Analysis - Hospitals - MRI Training Courses - Health - Societies - Blood Flow Imaging
 
Spatially Localized Spectroscopy
 
Process by which regions of tissue are selectively sampled to produce spectra from defined volumes in space. These methods may be employed to sample a single region in space (single voxel method) or multiple regions simultaneously (multivoxel methods). The spatial selectivity can be achieved by a variety of methods including surface coils, surface coils in conjunction with RF gradient methods, or RF pulses in combination with switched magnetic field gradients, for example, volume-selective excitation. An indirect method of achieving spatial selectivity is the destruction of coherence of the magnetization in regions that lie outside the region of interest. A variety of spatial encoding schemes have been employed for multivoxel localization. See Chemical shift imaging.
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ArtifactForum -
related threadsInfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.MRI Resource Directory:
 - Artifacts -
 
An image artifact is a structure not normally present but visible as a result of a limitation or malfunction in the hardware or software of the MRI device, or in other cases a consequence of environmental influences as heat or humidity or it can be caused by the human body (blood flow, implants etc.). The knowledge of MRI artifacts (brit. artefacts) and noise producing factors is important for continuing maintenance of high image quality. Artifacts may be very noticeable or just a few pixels out of balance but can give confusing artifactual appearances with pathology that may be misdiagnosed.
Changes in patient position, different pulse sequences, metallic artifacts, or other imaging variables can cause image distortions, which can be reduced by the operator; artifacts due to the MR system may require a service engineer.
Many types of artifacts may occur in magnetic resonance imaging. Artifacts in magnetic resonance imaging are typically classified as to their basic principles, e.g.:
•
Physiologic (motion, flow)
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Hardware (electromagnetic spikes, ringing)
•
Inherent physics (chemical shift, susceptibility, metal)

Several techniques are developed to reduce these artifacts (e.g. respiratory compensation, cardiac gating, eddy current compensation) but sometimes these effects can also be exploited, e.g. for flow measurements.

See also the related poll result: 'Most outages of your scanning system are caused by failure of'
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• View the DATABASE results for 'Artifact' (166).Open this link in a new window

 
Further Reading:
  Basics:
ARTEFACT VERSUS ARTIFACT
Saturday, 26 January 2002   by www.worldwidewords.org    
  News & More:
MRI results affected by movement? MIT researchers have an AI-powered solution
Friday, 25 August 2023   by healthimaging.com    
Magnetic eyelashes: A new source of MRI artifacts
Wednesday, 24 July 2019   by medicalxpress.com    
On the Horizon - Next Generation MRI
Wednesday, 23 October 2013   by thefutureofthings.com    
Searchterm 'Chemical Shift Imaging' was also found in the following services: 
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Chemical Shift ArtifactInfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.MRI Resource Directory:
 - Artifacts -
 
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
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.
mri safety guidance
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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• View the DATABASE results for 'Chemical Shift Artifact' (7).Open this link in a new window

 
Further Reading:
  Basics:
MRI Artifact Gallery
   by chickscope.beckman.uiuc.edu    
  News & More:
What is chemical shift artefact? Why does it occur? How many Hz at 1.5 T?
   by www.revisemri.com    
Abdominal MRI at 3.0 T: The Basics Revisited
Wednesday, 20 July 2005   by www.ajronline.org    
MRI Resources 
Implant and Prosthesis - Developers - Contrast Enhanced MRI - Stimulator pool - MR Guided Interventions - MRI Physics
 
BandwidthForum -
related threads
 
(BW) Bandwidth is a measure of frequency range, the range between the highest and lowest frequency allowed in the signal. For analog signals, which can be mathematically viewed as a function of time, bandwidth is the width, measured in Hertz of a frequency range in which the signal's Fourier transform is nonzero.
•
The receiver (or acquisition) bandwidth (rBW) is the range of frequencies accepted by the receiver to sample the MR signal. The receiver bandwidth is changeable (see also acronyms for 'bandwidth' from different manufacturers) and has a direct relationship to the signal to noise ratio (SNR) (SNR = 1/squareroot (rBW). The bandwidth depends on the readout (or frequency encoding) gradient strength and the data sampling rate (or dwell time).
Bandwidth is defined by BW = Sampling Rate/Number of Samples.
A smaller bandwidth improves SNR, but can cause spatial distortions, also increases the chemical shift. A larger bandwidth reduces SNR (more noise from the outskirts of the spectrum), but allows faster imaging.
•
The transmit bandwidth refers to the RF excitation pulse required for slice selection in a pulse sequence. The slice thickness is proportional to the bandwidth of the RF pulse (and inversely proportional to the applied gradient strength). Lowering the pulse bandwidth can reduce the slice thickness.
mri safety guidance
Image Guidance
A higher bandwidth is used for the reduction of chemical shift artifacts (lower bandwidth - more chemical shift - longer dwell time - but better signal to noise ratio). Narrow receive bandwidths accentuate this water fat shift by assigning a smaller number of frequencies across the MRI image. This effect is much more significant on higher field strengths. At 1.5 T, fat and water precess 220 Hz apart, which results in a higher shift than in Low Field MRI.
Lower bandwidth (measured in Hz) = higher water fat shift (measured in pixel shift).

See also Aliasing, Aliasing Artifact, Frequency Encoding, and Chemical Shift Artifact.
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• View the DATABASE results for 'Bandwidth' (19).Open this link in a new window

 
Further Reading:
  Basics:
Bandwidth
   by en.wikipedia.org    
  News & More:
Automated Quality Assurance for Magnetic Resonance Image with Extensions to Diffusion Tensor Imaging(.pdf)
   by scholar.lib.vt.edu    
A Real-Time Navigator Approach to Compensating for Motion Artifacts in Coronary Magnetic Resonance Angiography
   by www.cs.nyu.edu    
MRI Resources 
MRI Physics - Breast Implant - Anatomy - Open Directory Project - Absorption and Emission - Functional MRI
 
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