Ferumoxytol - Iron Oxide Nanoparticle Magnetic Resonance Dynamic Contrast Enhanced MRI

Inclusion Criteria: 1. Patients 18 years of age or older with histologically or cytologically confirmed head and neck squamous cell carcinoma or melanoma 2. Measurable clinical and/or radiographic poly-nodal disease defined as stage N2b, N2c or N3 disease with multiple involved lymph nodes as defined by the American Joint Committee on Cancer (AJCC) cancer staging criteria 3. Patients who have received or are dispositioned to receive a Positron emission tomography (PET) and computerized tomography (CT) \[PET-CT\] scan within two weeks of starting definitive therapy for their head and neck malignancy and their participation in this study. This implies patients must receive a PET-CT to be eligible for the study. Exclusion Criteria: 1. Patients who have undergone definitive resection of their primary or nodal disease as well as any chemotherapy or radiation therapy for their head and neck primary tumor. 2. Patients unable or unwilling to give written, informed consent or to undergo MRI imaging. 3. Women of childbearing potential (A woman of child-bearing potential is a sexually mature woman who has not undergone a hysterectomy or who has not been naturally postmenopausal for at least 24 consecutive months \[i.e., who has had menses at any time in the preceding 24 consecutive months\]). Male partners must practice effective contraception (oral, injectable, or implantable hormonal contraceptive; tubal ligation; intra-uterine device; barrier contraceptive with spermicide; or vasectomized partner) throughout the study. 4. Patients unable to tolerate DCE-MRI or having an estimated GFR \< 60 ml/min/1.73m\^2. 5. Contraindications to iron supplementation include hemochromatosis, colitis, history of GI bleeds, alcoholism, or liver disease. Ferumoxytol is contraindicated in patients with evidence of iron overload and/or known hypersensitivity to Feraheme or any of its components. Consequently, we will plan to exclude patients who have symptoms or signs that might be caused by iron overload. These include patients with (unexplained): arthritis (including premature osteoarthritis), congestive heart failure or cardiomyopathy, adult-onset diabetes, secondary hypogonadism, increased skin pigmentation, or patients with persistently elevated serum ferritin not explained by an underlying inflammatory/systemic disease, unless these patients demonstrate a fasting transferrin saturation \</= 0.45. 6. Patients with any evidence of iron overload on pre-imaging laboratory studies. 7. Patients with any contraindications to gadolinium-based contrast agents. 8. Patients with claustrophobia.