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Result : Searchterm 'Image Guidance' found in 0 term [] and 54 definitions []
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News  (3)  
 
Aliasing ArtifactInfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.MRI Resource Directory:
 - Artifacts -
 
Quick Overview
Please note that there are different common names for this MRI artifact.

Artifact Information
NAME Aliasing, backfolding, foldover, phase wrapping, wrap around
DESCRIPTION Image wrap around
REASON Undersampling in k-space
HELP Larger FOV, oversampling, foldover suppression

Aliasing is an artifact that occurs in MR images when the scanned body part is larger than field of view (FOV). As a consequence of the acquired k-space frequencies not being sampled densely enough, whereby portions of the object outside of the desired FOV get mapped to an incorrect location inside the FOV. The cyclical property of the Fourier transform fills the missing data of the right side with data from behind the FOV of the left side and vice versa. This is caused by a too small number of samples acquired in, e.g. the frequency encoding direction, therefore the spectrums will overlap, resulting in a replication of the object in the x direction.
Aliasing in the frequency direction can be eliminated by twice as fast sampling of the signal or by applying frequency specific filters to the received signal.
A similar problem occurs in the phase encoding direction, where the phases of signal-bearing tissues outside of the FOV in the y-direction are a replication of the phases that are encoded within the FOV. Phase encoding gradients are scaled for the field of view only, therefore tissues outside the FOV do not get properly phase encoded relative to their actual position and 'wraps' into the opposite side of the image.


Image Guidance
Use a larger FOV, RFOV or 3D Volume, apply presaturation pulses to the undesired tissue, adjust the position of the FOV, or select a small coil which will only receive signal from objects inside or near the coil. The number of phase encoding steps must be increased in phase direction, unfortunately resulting in longer scan times.
When this is not possible it can be corrected by oversampling the data. Aliasing is eliminated by Oversampling in frequency direction. No Phase Wrap (Foldover Suppression) options typically correct the phase encoding by doubling the field of view, doubling the number of phase encodes (to keep resolution constant) and halving the number of averages (to keep scan time constant) then discarding the additional data and processing the image within the desired field of view (but this is more time consuming).
Tissue outside this doubled area can be folded nevertheless into the image as phase wrap. In this case combine more than 2 number of excitations / number of signal averages with foldover suppression.
See also Aliasing, Foldover Suppression, Oversampling, and Artifact Reduction - Aliasing.

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    • Phase Wrapping Artifact
    • Readout Oversampling
    • Aliasing
    • Number of Signal Averages
    • Phase Encoding
MRI Resources 
Sequences - Directories - General - MRCP - Fluorescence - Portals
 
Artifact by Patient MovementInfoSheet: - 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 Motion, movement
DESCRIPTION Blurring, ghosting
REASON Patient movement
HELP Fast scan techniques

Patient movement during the scans are often an imaging problem. Artifacts from patient movement are widely varied due to a dependence when during k-space filling the motion occurs. When the patient moving causes only in the last few seconds of the scan at that time the outside edges of K-space were being filled, and as a result the artifact does not overly affect the image (there are only fine lines).


Image Guidance
A good cooperation between the patient and the operator is the best way to avoid these artifacts, in difficult cases a sedative may help. If a compliance of the patient is not possible (e.g. pain, stroke, or consciousness), choose fast scan methods like gradient echo or single shot technique.
See also Motion Artifact and Phase Encoded Motion Artifact.

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Further Reading:
  News & More:
Patient movement during MRI: Additional points to ponder
Tuesday, 5 January 2016   by www.healthimaging.com    
MRI Resources 
MRI Reimbursement - Claustrophobia - Mass Spectrometry - Breast Implant - Online Books - Coils
 
Audio Frequency ArtifactInfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.MRI Resource Directory:
 - Artifacts -
 
Quick Overview

Artifact Information
NAME Audio frequency
DESCRIPTION Ghosting, lines or spots
REASON Wrong modulation at audio rate, wrong audio signal
HELP AC-line synchronization

Two types of audio-frequency problems are possible:

1. Modulation of the MR signal at an audio rate 2. Audio signal component at digitizer input
Problem 1 looks like ghosts, weak copies of the real image, displaced along the phase encoding direction. The number and intensity of the ghosts depends upon the relationship between the period of the audio modulation and the repetition time.
Problem 2 shows up as lines or spots at the appropriate points along the frequency direction. If there is no correlation between the audio period and TR, lines are generated or discrete spots occur.


Image Guidance
Both problems can be lessened by use of AC-line synchronization (line trigger).

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Searchterm 'Image Guidance' was also found in the following service: 
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News  (3)  
 
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.



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 
Coils - Abdominal Imaging - MRI Accidents - Corporations - Manufacturers - IR
 
Black Boundary 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 Black boundary, dark boundary, contour, chemical shift, relief
DESCRIPTION Black contours at boundaries
REASON Chemical shift, opposed phase image, motion shear (flow effects), inversion recovery null point
HELP Smaller water fat shift (high bandwidth), in phase image, SE sequences

Black boundary artifacts are black lines following voxels where both water and fat protons are present in the same voxel. This artifact arise along the boundary of organs or tissues perpendicular to the frequency encoding direction, and occurs preferentially in gradient echo sequences with out of phase echo times.


Image Guidance
Fat suppression techniques eliminate this artifact. For artifact reducing helps a smaller water fat shift (high bandwidth), a higher matrix or/and an in phase TE.
See also Chemical Shift Artifact.

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• View the DATABASE results for 'Black Boundary Artifact' (4).Open this link in a new window

 
Further Reading:
  Basics:
Artifacts in MRI
Saturday, 1 October 2011   by www.huc.min-saude.pt    
What is chemical shift artefact? Why does it occur? How many Hz at 1.5 T?
   by www.revisemri.com    
MRI Resources 
Liver Imaging - Services and Supplies - Health - Absorption and Emission - Pregnancy - MRI Centers
 
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