My Father-in-law Was Diagnosed With Stage 4 Small Cell Carcinoma

My father-in-law was diagnosed with Stage 4 small cell carcinoma of the lung and it has spread to his liver. We just met the the oncologist yesterday. He is considering chemo to extend his life. They want to do a brain scan first to see if it is in his brain. I think they said an MRI...what will that show and why not a PT scan? Also they took another blood sample yesterday (they had just taken one 2 weeks ago) what is the purpose of this 2nd blood test?