What is the effect on SNR when decreasing the matrix size in an MRI scan from 512 x 512 to 320 x 320?

• A. SNR decreases
• B. SNR remains the same
• C. SNR increases
• D. SNR fluctuates randomly

Answer: (C)

When decreasing the matrix size from 512 x 512 to 320 x 320, the Signal-to-Noise Ratio (SNR) actually increases. This occurs because a smaller matrix size results in larger pixels for the image, which allows each pixel to capture more of the signal information from the tissue being imaged.

In MRI, SNR is influenced by several factors, including voxel size. A voxel is the three-dimensional equivalent of a pixel and its size is determined by the geometry of the imaging matrix and the field of view. When you decrease the matrix size, the number of pixels in the image is reduced, and each pixel covers a larger volume of tissue, thereby collecting more signal on average.

This increase in signal per pixel leads to improvements in SNR because the noise remains relatively constant, while the signal has increased due to the larger volume being sampled. The result is a clearer and more distinct image, making it easier to discern details.

Conversely, if the matrix size were to be increased, the pixel size would decrease, and SNR would drop as the smaller pixels would capture less signal but not necessarily less noise, resulting in poorer image quality. This principle is important for optimizing MRI protocols to achieve the best diagnostic images.