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Result : Searchterm 'Sampling' found in 8 terms [] and 37 definitions []
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News  (2)  Resources  (5)  Forum  (1)  
 
Backfolding 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
Backfolding, foldover, phase wrapping, wrap around
DESCRIPTION
Image wrap around
Backfolding always occurs due to wrong phase encoding caused by objects outside the planned FOV. Phase encoding gradients are scaled for the field of view only. Tissues outside the FOV do not get properly phase encoded relative to their actual position and 'wraps' into the opposite side of the image. The Backfolding artifact projects image contents which fall outside the imaging FOV back into the image; the back folded information thus reappearing on the other side of the image. In fact, information along the phase encoding direction can be viewed as projected onto a cylindrical screen with a circumference corresponding to the linear field of view dimension in the phase encoding direction.

See also Aliasing Artifact.
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Further Reading:
  Basics:
Aliasing or wrap around artifacts
Thursday, 31 March 2011   by de.slideshare.net    
Searchterm 'Sampling' was also found in the following services: 
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Radiology  (4) Open this link in a new windowUltrasound  (12) Open this link in a new window
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.
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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.
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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 
Jobs - Mobile MRI - Spine MRI - Lung Imaging - Brain MRI - Software
 
Foldover SuppressionInfoSheet: - Sequences - 
Intro, 
Overview, 
Types of, 
etc.
 
A 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. This signal will be mapped (wrapped, backfolded) back into the image at incorrect locations.
Foldover suppression (phase oversampling, no phase wrap) is a user-selectable parameter that maps this signal to its correct location outside the FOV, then discards any signal from outside the FOV before displaying the image. In order to be able to choose this parameter, in most cases more than an average is necessary.

See also Phase Wrapping Artifact and Oversampling.
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• View the DATABASE results for 'Foldover Suppression' (4).Open this link in a new window

Searchterm 'Sampling' was also found in the following services: 
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Time Resolved Imaging of Contrast KineticsInfoSheet: - Sequences - 
Intro, 
Overview, 
Types of, 
etc.
 
(TRICKS) Time resolved imaging of contrast kinetics is a MRI technique, which increases the temporal resolution of dynamic contrast enhanced magnetic resonance angiography (CE-MRA) sequences. The K-space is divided into regions by increasing the sampling rate at the lower spatial frequencies and by reducing the sampling rate at the higher spatial frequencies. Since the time duration between two frames is shortened, it can be observed how frequently and how quickly the images are repeated at the exact same location.
TRICKS is particularly useful for dynamic vascular studies with high temporal resolution. TRICKS improves the calculation of the contrast bolus arrival and improves the characterization of arterio-venous malformations (AVMs).

See also Automatic Bolus Detection, MRA, Cardiac MRI.
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• View the DATABASE results for 'Time Resolved Imaging of Contrast Kinetics' (2).Open this link in a new window

 
Further Reading:
  Basics:
Optimal k-Space Sampling for Dynamic Contrast-Enhanced MRI with an Application to MR Renography
Thursday, 5 November 2009   by www.ncbi.nlm.nih.gov    
Searchterm 'Sampling' was also found in the following services: 
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Radiology  (4) Open this link in a new windowUltrasound  (12) Open this link in a new window
BlurringInfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.
 
Blurring comes most obviously from patient motion, but other mechanisms like low resolution sampling also lead to image blurring.
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• View the DATABASE results for 'Blurring' (15).Open this link in a new window

 
Further Reading:
  Basics:
Image Characteristics and Quality
   by www.sprawls.org    
MRI Resources 
Implant and Prosthesis pool - Movies - Cardiovascular Imaging - Colonography - Resources - MRI Reimbursement
 
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