This study evaluates whether images taken using a photon counting detector CT scanner (PCD-CT) can determine the growth of rectal cancer as well as, or better than, MRI for the management of patients with rectal cancer.

Inclusion Criteria:

• Adult patients 18 to 99 years of age
• Patients with proven rectal cancer clinically referred for staging pelvic MRI and contrast-enhanced CT of the abdomen and pelvis (for evaluation of liver and peritoneal disease)
• Endoscopic or imaging report that tumor is 2 cm or larger
• Patients who are able and willing to sign the informed consent
• Negative pregnancy test if subject is of child-bearing age (females of child-bearing potential will be screened for pregnancy using a urine pregnancy test, which will be administered by the unit study coordinator at no cost to the patient)

Exclusion Criteria:

• Minors less than 18 years old
• Patient unable to provide written informed consent
• Pregnancy
• Estimated Glomerular Filtration Rate (eGFR) ≤ 60
• History of prior moderate or severe contrast reaction including unresponsiveness, severe respiratory distress, convulsions, arrhythmia, cardiopulmonary distress, progressive angioedema, laryngeal edema, dyspnea, bronchospasm, symptomatic tachycardia, symptomatic bradycardia, hypotension, hypertensive crisis
• Any history of premedication prior to iodinated contrast
• Bilateral hip replacement/prosthesis
• Patients that consent to participation but do not undergo their clinically indicated MRI scanning for any reason (e.g., bad IV, infiltration, reaction, change in indication)

Patients with proven rectal cancer clinically referred for staging pelvic MR and contrast-enhanced CT of the abdomen and pelvis.