**Core Concept**
Gastrinomas are neuroendocrine tumors that produce excessive gastrin, leading to hypergastrinemia and peptic ulcers. Accurate localization of gastrinomas is crucial for surgical resection and treatment planning. Preoperative imaging studies play a vital role in identifying the location and extent of the tumor.

**Why the Correct Answer is Right**
Somatostatin receptor scintigraphy (SRS) using Indium-111 pentetreotide (Octreoscan) is the preferred preoperative imaging study for gastrinomas. This imaging modality exploits the high affinity of somatostatin receptors for the radiolabeled pentetreotide, allowing for visualization of neuroendocrine tumors, including gastrinomas. The sensitivity of SRS is significantly higher than other imaging modalities, such as CT or MRI, especially for small or occult tumors.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scans may detect larger gastrinomas but are less sensitive than SRS for small tumors and often require contrast enhancement, which may not be ideal for patients with renal impairment or contrast allergy.
**Option B:** MRI is not the preferred choice for gastrinoma localization due to its lower sensitivity compared to SRS, especially for small tumors.
**Option C:** PET scans may be useful in selected cases but are not the first-line imaging modality for gastrinoma localization.

**Clinical Pearl / High-Yield Fact**
Remember that SRS is also useful for localizing other neuroendocrine tumors, such as carcinoids and medullary thyroid cancer, which express somatostatin receptors.

**Correct Answer:** C. Somatostatin receptor scintigraphy (SRS) using Indium-111 pentetreotide (Octreoscan).