CT scan isn't conclusive and patient cannot have MRI?

According to the CT (without contrast) report, "It (the mediastinal mass) appears to be displacing and possibly invading the trachea and esophagus." The said mass was diagnosed as stage 3, nonresectable thymic carcinoma and is approximately 11cm x 9cm.

Due to a medical implant, the patient is unable to have an MRI. What type of test or scan would give accurate results as to whether the mass is just pushing against the other organs or has metastasized?