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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 Parameters

Answer this...

There follows a table of the relationship between SNR and operator-selectable imaging parameters (excluding pulse timing parameters). Fill in the SNR column with fractions indicating the proportion of the SNR given on the first line. E.g. "1/2" etc.


(Nx is the number of frequency encoding steps, Ny is the number of phase encoding steps, thk is the slice thickness, Dx = Nx.Δx and Dy = Ny.Δy; these are the x and y dimensions of the field of view. Assume bandwidth is constant.)

As usual, apply the SNR equation:


Thus we see that increasing the image size by a factor of two (same matrix size but half the FOV dimensions), decreases the SNR by a factor of four. Doubling the resolution (doubling the matrix size Nx.Ny) yields a decrease of SNR by a factor of 2, as well as causing an increase in scan time.

Regarding scan time: consider reducing the FOV by a factor of two in both directions, and the slice thickness by a factor of 2. How much longer would we have to scan to maintain SNR (how many times must we multiply NEX (NSA))? Answer: sixty-four!

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