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QUESTIONS
» Pulse Sequences

How do we change image contrast? What are the effects of parameter changes? This section explores image contrast and image acquisition methods.

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SNR and Receiver Bandwidth

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You decide to try and increase the SNR by using a smaller receiver bandwidth. Why might this work? List also any tradeoffs.

A wider bandwidth includes more noise. Remember that bandwidth (BW) = Nx / Tline where Nx is the number of frequency encoding steps and Tline is the total sampling time for a whole line of k-space. Decreasing the bandwidth by a factor of 2 results in an increase in the SNR by a factor of √2. Again, just use the SNR equation.

The confusing subject of receiver bandwidth is addressed in the receiver bandwidth tutorial.

Tradeoffs:

  • increases chemical shift artefact (see the chemical shift tutorial)
  • longer TE (sampling time is inversely proportional to BW. A longer TE means more time for T2 dephasing, reducing SNR. However, the reduction noise contamination as a result of a smaller receiver bandwidth outweighs the decrease in SNR from the extra T2 dephasing, and overall the SNR goes up.)
  • decreases number of slices (longer TE means fewer slices can be fitted into the TR period).

Further reading on this topic:
Books: MRI From Picture to Proton p75, Q&A in MRI p99, MRI The Basics p168
Online: e-MRI

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