Final answer:
The vessel immediately posterior to the pancreatic head is the Superior Mesenteric Vein (SMV), which is important to identify during an abdominal ultrasound for a complete assessment.
Step-by-step explanation:
When performing an abdominal ultrasound on a patient with a mass in the pancreatic head, the identification of surrounding anatomical structures is essential for a complete assessment. The vessel that lies immediately posterior to the pancreatic head is the Superior Mesenteric Vein (SMV). The SMV runs behind the pancreas and joins with the Splenic Vein to form the Portal Vein. Therefore, the correct answer for the vessel that lies immediately posterior to the pancreatic head is C) SMV (Superior Mesenteric Vein). This information is crucial as the pancreatic head is located in the curve of the duodenum and any masses in this region could potentially impact the SMV.