How can radiologists be objective in their findings if they have a "pre-diagnosis" by a doctor?

Better. Radiologists will do a better job at interpreting the imaging if they know what the doctor is looking for. The more information the radiologist has, the better the imaging interpretation will be.
Radiologists are. Trained to be objective. Imaging studies cover a lot of territory, and radiologists usually have a pattern of searching for abnormalities in all of the imaged anatomy. That said, it may be useful, for example, to know that the patient has pain in a specific location so important abnormalities are not overlooked and differential diagnoses can be tailored to the patient's clinical situation.
History is important. Part of what a radiologist needs to make an accurate interpretation is patient history. It's like looking at a pictures in a story and not the words. You could miss interpret the pictures without the story. Ex: a new spot on a chest x-ray can be many things but if you knew they had colon cancer 2 yrs ago then metastatic cancer is pretty likely. But if patient had recent pneumonia then residual dz.