Can a urethral stricture be viewed non-invasively using ultrasound or mri or is a flow-rate test conclusive evidence?

Yes, but ...: Everything we do in medicine and life should not only be hinged on the ultimate balance between necessity & possibility but be less cost, less radiation, and higher definite accuracy as deduced from decades-long experience. Therefore, detailed Hx + urethorscopy +/- retrograde urethrogram would have most conformed with this realistic demand. More? Ask urologist timely.