[Sarugetchu]

You can infact do a Masters in a Japanese University in English, especially in Biological areas such as Radiology. Look at the Tokyo University website and others' foreign section for requirements, but usually they are simply under the term "Biological Sciences", and then you specialize after being accepted.

I am applying for a Biological masters in Japan currently. If you are not from a native English speaking country however, you must present evidence of being fluent in English through the likes of TOEFL, I believe.

Hope this helps.