It definitely doesn't look good. Is there any way you can get that expunged from you record? That would be your best bet. As a last resort, hire a lawyer to help you, because if you could get it off your record that would be super-awesome.
When you say Pre-radiology, do you mean that you're in medical school? If you're trying to get into med school, then you would be 'pre-med'. If you're already in med school, then I don't think Radiology would care any more about a cannabis misdemeanor anymore than another specialty. If anything, they should be the most forgiving for substance abuse problems, b/c radiologists don't prescribe medications (for the most part), so there's no worry about you relapsing, unless you're reading CT scans all stoned and write crazy stuff in your report, 'I notice a 3 cm dragon in the upper left lobe of the right lung, I recommend sending a small knight to kill it'.
I would guess that Anesthesia would care the most about that stuff b/c they have access to all the drugs.