If the number of excitations (NEX) is doubled, what will happen to the signal-to-noise ratio (SNR) and scan time?

• A. SNR increases by 20%, scan time remains constant
• B. SNR remains constant, scan time increases
• C. SNR increases by 40%, scan time doubles
• D. SNR increases, scan time decreases

Answer: (C)

When the number of excitations (NEX) is doubled, the signal-to-noise ratio (SNR) improves because SNR is directly proportional to the square root of NEX. This means that when you double NEX, the SNR will increase by a factor of the square root of 2, which is approximately 1.41, or in percentage terms, about a 41% increase. Therefore, the SNR increases substantially with increased NEX.

Additionally, doubling the NEX directly influences the total scan time. Since NEX represents the number of averages taken of the same data to enhance SNR, when you double this number, you are essentially doubling the time it takes to acquire the data. This is because each excitation requires a full scan, and thus, if you have twice the amount of excitations, the time required to complete the scan also doubles.

Taking this into account, the increase in SNR by approximately 40% and the doubling of scan time supports the correctness of the choice indicating that SNR increases and scan time doubles, providing clear reasoning for the concept relating NEX to SNR and scan time.