Nanoscale

Hello experts, please help me to solve following questions for my review (master thesis). Feel also free to state the literature correlated to the answers, if u now it. The minimum range limits of common MRI is approximately not lower as 3 micro meter. Why (setup, IT or mathematical background)? Correlated to a Review it´s dependent on the pickup coil. Can the resolution be magnified by changing parts, how would the improved future Version look like? Have a good day, Chris