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Hertz
 
(Hz) The standard SI unit of frequency.
Definition: The number of repetitions of a periodic process per unit time. It is equal to the old unit cycles or oscillations each second of a simple harmonic motion. The unit is named for the German physicist Heinrich Rudolf Hertz.
Larger units are
kilohertz (kHz) = 1 000 Hz
megahertz (MHz) = 1 000 kHz
gigahertz (GHz) = 1 000 MHz
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Heinrich Hertz
   by en.wikipedia.org    
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Angular Frequency
 
Frequency of oscillation or rotation (measured in radians/second) commonly designated by the Greek letter w; w = 2pf, where f is frequency (in hertz (Hz)).
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BandwidthForum -
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(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.
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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    
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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.
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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:
  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    
Searchterm 'Hertz' was also found in the following services: 
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FrequencyForum -
related threads
 
(F) The number of repetitions of a periodic process per unit time. It is related to angular frequency, w, by f = w/2p. In electromagnetic radiation, it is usually expressed in units of Hertz (Hz), where 1 Hz = 1 cycle per second.
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• View the NEWS results for 'Frequency' (3).Open this link in a new window.
 
Further Reading:
  News & More:
Safety and reliability of Radio Frequency Identification Devices in Magnetic Resonance Imaging and Computed Tomography
Thursday, 4 February 2010   by 7thspace.com    
Improved shim method based on the minimization of the maximum off-resonance frequency for balanced SSFP
Monday, 1 June 2009   by www.ncbi.nlm.nih.gov    
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