A spherical tumor 3 cm in diameter consisting of prostate cells may result in a PSA blood level of about 20 nanograms/milliliter of blood. The test can only detect the presence of PSA in the blood if it is greater than about 0.1 ng/ml. An MRI can detect a cancerous tumor down to about a diameter of 1 mm.

Which is a more sensitive test for the return of prostate cancer, the blood test or an MRI? To decide this, estimate how big a tumor could be and still produce a PSA level less than 0.1 ng/ml.
diameter < mm
Would a tumor that produced this (undetectable) amount of PSA be visible on an MRI?
No or Yes
Which test would be able to see the smaller tumor?