Three hour MRI?

My mom had an ultrasound done and they found a mass on her liver. She also had cysts found on her bladder a few years ago that were determined to be precancerous. A great grandmother died from liver cancer. She's been scheduled for an MRI and was told it will last three hours and she told them she'll need Valium because she's claustrophobic. Has anyone heard of/had a three hour MRI?