What happens to the signal-to-noise ratio (SNR) when the field of view (FOV) is decreased?

• A. SNR remains unchanged
• B. SNR increases
• C. SNR decreases
• D. SNR is unpredictable

Answer: (C)

When the field of view (FOV) is decreased, the signal-to-noise ratio (SNR) decreases. This relationship is due to the fact that a smaller FOV captures a smaller volume of tissue, which inherently results in less signal being acquired from the area of interest. The noise, however, does not decrease at the same rate because it is distributed over the entire volume being imaged.

As the FOV is reduced, the pixels in the image have less signal representing the actual anatomy relative to the amount of noise present. Since SNR is a measure of the desired signal strength compared to the background noise, a decrease in the signal resultantly leads to a lower SNR.

This principle is fundamental in MRI physics, as maximizing SNR is critical for producing high-quality images. Techniques to improve SNR, such as increasing the number of signal averages or using dedicated coils, become especially important in situations with a reduced FOV to compensate for this inherent loss.