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Slice SelectionForum -
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Exclusive excitation of spins in one slice performed by the coincident combination of a gradient magnetic field and a narrow bandwidth or slice selective RF pulse at a specific Larmor frequency.
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• Related Searches:
    • Chemical Shift Spatial Offset
    • Spectral Presaturation Inversion Recovery
    • Bandwidth
    • Slice
    • Pulse Sequence Timing Diagram
 
Further Reading:
  Basics:
RARE
Monday, 3 December 2012   by www2.warwick.ac.uk    
  News & More:
Magnetic resonance imaging
   by www.scholarpedia.org    
MRI Resources 
Breast MRI - Shoulder MRI - Safety pool - Jobs - Nerve Stimulator - Mobile MRI
 
Pulse Sequence Timing DiagramInfoSheet: - Sequences - 
Intro, 
Overview, 
Types of, 
etc.
 
Spin Echo Timing Diagram The schematic figures of a pulse sequence timing diagram illustrate the steps of basic hardware activity that are incorporated into a pulse sequence. Time during sequence execution is indicated along the horizontal axes. Each line belongs to a different hardware component. One line is needed for the radio frequency transmitter and also one for each gradient (Gs = slice selection gradient x, Gf = phase encoding gradient y, Gf = frequency encoding gradient z, also called readout gradient).
In picture 1, a timing diagram for a 2D pulse sequence is shown.
Slice selection and signal detection are repeated in duration, relative timing and amplitude, each time the sequence is repeated. A single phase encoding component is present each time the sequence is executed.
Additional lines are added for ADC (Analog to Digital Converter) and sampling. A gradient pulse is shown as a deviation above or below the horizontal line. Simultaneous component activities such as the RF pulse and slice selection gradient are indicated as a non-zero deviation from both lines at the same horizontal position. Simple deviations from zero show constant amplitude gradient pulse. Gradient amplitudes that change during the measurement, e.g. phase encoding are represented as hatched regions.
Spin Echo Timing Diagram The second picture shows a timing diagram for a 3D pulse sequence.
Volume excitation and signal detection are repeated in duration, relative timing and amplitude, each time the sequence is repeated. Two phase encoding components are present, one in the phase encoding direction and the other in slice selection direction (irrespectively incremented in amplitude) in each time the sequence is executed. A description of the comparison of hardware activity between different pulse sequences.
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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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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    
Searchterm 'Slice Selection' was also found in the following service: 
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Chemical Shift Spatial OffsetInfoSheet: - Artifacts - 
Case Studies, 
Reduction Index, 
etc.
 
Image artifact of apparent spatial offset of regions with different chemical shifts along the direction of the frequency encoding gradient;; a similar effect may be found in the slice selection direction.
See Chemical Shift Artifact.
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Further Reading:
  Basics:
1H MR Spectroscopy and Chemical Shift Imaging of the In Vivo Brain at 7 Tesla
Sunday, 26 November 2006   by tobias-lib.uni-tuebingen.de    
MRI Resources 
Supplies - Calculation - Education pool - Image Quality - Implant and Prosthesis - Mass Spectrometry
 
Fat Saturation
 
(FAT SAT) A specialized technique that selectively saturates fat protons prior to acquiring data as in standard sequences, so that they produce a negligible signal. The presaturation pulse is applied prior to each slice selection. This technique requires a very homogeneous magnetic field and very precise frequency calibration.
Fat saturation does not work well on inhomogeneous volumes of tissue due to a change in the precessional frequencies as the difference in volume affects the magnetic field homogeneity. The addition of a water bag simulates a more homogeneous volume of tissue, thus improving the fat saturation. Since the protons in the water bag are in motion due to recent motion of the bag, phase ghosts can be visualized.
Fat saturation can also be difficult in a region of metallic prosthesis. This is caused by an alteration in the local magnetic field resulting in a change to the precessional frequencies, rendering the chemical saturation pulses ineffective.
See also Fat Suppression, and Dixon.

 
Images, Movies, Sliders:
 Shoulder Sagittal T2 FatSat FRFSE  Open this link in a new window
    

Courtesy of  Robert R. Edelman
 Shoulder Coronal T2 FatSat FRFSE  Open this link in a new window
 Shoulder Axial T2 FatSat FRFSE  Open this link in a new window
    

Courtesy of  Robert R. Edelman
 
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Further Reading:
  Basics:
Techniques of Fat Suppression(.pdf)
   by cds.ismrm.org    
  News & More:
New Imaging Technique Reveals Fatty Hearts In Pre-diabetics
Wednesday, 5 September 2007   by www.sciencedaily.com:80    
MRI Resources 
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