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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 Phase Encoding Steps

Do this...

You decide to try and increase the SNR by increasing the number of phase encoding steps (Ny). Why might this work? List also any tradeoffs.

It won't work!

We saw in earlier that decreasing Ny to increase voxel size would increase SNR. We can understand this by looking at the SNR equation. (Get out a pen and paper and use it to carefully work through the following discussion.) Increasing Ny does mean you have an increase in signal, with the associated smaller noise increase as we saw earlier. But you can't increase SNR by just increasing Ny on its own, because this also results in a smaller pixel, which has a larger effect on the overall equation. (Try making Ny into 2Ny in the equation and see what the overall effect is.) Increasing Ny only increases SNR if you also maintain the voxel size by changing the FOV.

Further reading on this topic:
Books: MRI The Basics p168
Online: MRItutor 1, 2

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