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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.

# SNR Parameters

## 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.

SNRDx(cm)Dy(cm)NxNythk(cm)
120201281281
?10101281281
?20202561281
?20202562561
?20202562560.5

(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:

SNRDx(cm)Dy(cm)NxNythk(cm)
120201281281
1/410101281281
1/√220202561281
1/220202562561
1/420202562560.5

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!