What happens to SNR as the size of the surface coil decreases?

When the size of the surface coil decreases, the signal-to-noise ratio (SNR) is affected in a specific way. A smaller surface coil will typically result in a reduction in the volume of tissue it samples, leading to a more focused acquisition of signals from a smaller area. This can actually enhance SNR for that specific region of interest because the coil is closer to the signals coming from the tissues within that area and picks up less noise from surrounding structures.

Surface coils are designed to provide improved SNR over larger coils when imaging smaller anatomical structures. Therefore, with a appropriately matched smaller coil for the region being examined, the SNR can indeed increase as it gathers stronger signals relative to noise, thus making the correct answer the increase in SNR.

Understanding the relationship between coil size and SNR is key for optimizing MRI performance and achieving the best imaging results in targeted areas, particularly when high-resolution imaging is needed.