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» 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 Number of Signal Averages

Answer this...

You decide to try and increase the SNR by increasing the number of signal averages (NSA or NEX: number of excitations). Why might this work? List also any tradeoffs.

As usual, get out the SNR equation. Change NEX to 2NEX. What is the result on the SNR?

We'll get a √2 increase in SNR if NEX is doubled.

Tradeoff: Longer scan time, disproportionate to increase in SNR.

Further reading on this topic:
Books: Q&A in MRI p99, MRI The Basics p168
Online: MRItutor 1, 2

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