**Core Concept**
Metastatic gastrinoma is a rare and aggressive form of neuroendocrine tumor that secretes excessive gastrin, leading to severe peptic ulcer disease. Accurate diagnosis and localization are crucial for effective management. The best investigation for metastatic gastrinoma should be sensitive to detect small tumor deposits and provide anatomical information for surgical planning.

**Why the Correct Answer is Right**
The correct investigation for metastatic gastrinoma is **Somatostatin Receptor Scintigraphy (SRS)**, also known as Octreoscan. SRS uses a radiolabeled somatostatin analogue to bind to somatostatin receptors on neuroendocrine tumor cells. This allows for the detection of small tumor deposits, including liver metastases, lymph node involvement, and primary tumor sites. SRS is particularly useful in gastrinoma, as these tumors often express high levels of somatostatin receptors.

**Why Each Wrong Option is Incorrect**
**Option A:** Helical CT scan is useful for detecting larger tumors and assessing lymph node involvement but is less sensitive for small metastatic deposits.
**Option B:** MRI is useful for evaluating the pancreas and surrounding tissues but is not the best modality for detecting small metastatic deposits.
**Option C:** PET scan using 18F-FDG is not the best choice for gastrinoma, as these tumors often have low metabolic activity, leading to false-negative results.

**Clinical Pearl / High-Yield Fact**
Remember that SRS is particularly useful in neuroendocrine tumors that express somatostatin receptors, including gastrinoma, carcinoid, and medullary thyroid carcinoma. This makes it an essential tool in the diagnostic arsenal for these rare and challenging tumors.

**Correct Answer: A. Somatostatin Receptor Scintigraphy (SRS)**