Can a high-powered magnetic resonance imaging scanner cause cancer to patients or staff?

No known association. Mri relies on changing nuclear atomic spin, and does not change the chemical bonds of molecules, which can alter dna and cause cancer. However, the use of a contrast agent in patients called gadolinium during the procedure may induce the onset of a debilitating malady commonly known as nsf/nfd. This can cause fibrosing throughout the body. http://www.medscape.com/viewarticle/550783.
No. While x-ray based scans do expose the patient to radiation that potentially can cause dna mutations and cancer. Mri does not use radiation, but relies on the nuclear state of atoms. This type of scanning is not known to cause cancer.
No. There are no increased rates of cancers, and no experimental evidence that a magnetic field and the pulse sequences used alter dna, as x-rays and ultra-violet light do. The problem with mri's is discomfort in the magnets cause noise, and the cost of the equipment - not cancer causing.