**Core Concept**
An acoustic neuroma, also known as a vestibular schwannoma, is a benign tumor arising from the Schwann cells of the vestibulocochlear nerve. In the early stages, these tumors are small and often asymptomatic. A 1 cm diameter acoustic neuroma is considered small and may not cause significant symptoms, but it still requires investigation to confirm the diagnosis and assess potential complications.

**Why the Correct Answer is Right**
The investigation of choice for a small acoustic neuroma is MRI with gadolinium enhancement. This imaging modality provides high-resolution images of the internal auditory canal and cerebellopontine angle, allowing for accurate detection and characterization of the tumor. Gadolinium enhancement helps to differentiate the tumor from surrounding neural structures and assess its relationship with the facial nerve and other critical structures. MRI is non-invasive, does not involve ionizing radiation, and can be repeated as needed to monitor tumor growth.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan with contrast - While CT scans can detect larger acoustic neuromas, they are less sensitive than MRI for small tumors and may not provide adequate detail of the tumor's relationship with surrounding structures.

**Option B:** Angiography - Angiography is not the primary investigation for acoustic neuromas, as these tumors are benign and do not typically require preoperative embolization or other vascular interventions.

**Option C:** CT scan without contrast - A non-contrast CT scan is unlikely to detect a small acoustic neuroma and may not provide sufficient detail for accurate diagnosis or assessment.

**Clinical Pearl / High-Yield Fact**
When investigating a suspected acoustic neuroma, it's essential to remember that MRI with gadolinium enhancement is the imaging modality of choice due to its high sensitivity and specificity for detecting small tumors.

**Correct Answer: C. MRI with gadolinium enhancement.**