The primary goal of the study is to see if the PET/CT will be able to determine the precise location of the Y-90 particles within the liver and within the tumors. We hope to use this information in order to see if these particles distribute evenly throughout large tumors or if the particles go into the tumor which is located in the portal vein.

The secondary goals are threefold. First correlate the distribution of Y90 particles (as demonstrated by PET/CT) with the degree of dead-tumor tissue. Second, see if there is a difference in the Y90 particle's distribution when a low particle load (1-3 million particles) is used compared to when a high particle load (6-8 million particles) is used. Lastly, compare the sensitivity of standard imaging ( bremsstrahlung imaging) currently being used for the Yttrium-90 angioembolization procedure to the sensitivity of the pair production PET research imaging.

Inclusion Criteria:

1. UWMC patients over 18 years of age
2. Presence of hepatocellular carcinoma greater than 5 cm OR presence of tumor-associated portal vein thrombosis.
3. Currently undergoing treatment with Yttrium-90 Radioembolization
4. Able to sign informed consent

Exclusion Criteria:

1. Patients who are not undergoing treatment with Yttrium-90 Radioembolization
2. Patients unable to provide informed consent

Patients (18-85 years old), who have hepatocellular carcinoma with tumors that are greater than 5 cm or have tumor-associated portal vein thrombosis. The patients are currently undergoing treatment with Yttrium-90 Radioembolization, as part of their clinical care.