Final answer:
The diameter of PMI in the supine position is just under a tenth of a millimeter, known to only two significant digits due to the value of pi.
Step-by-step explanation:
The diameter of the posterior mediastinal index (PMI) in the supine position is just under a tenth of a millimeter. This measurement is known to only two significant digits because the mathematical constant pi (π) is also known only to two digits.