CPT code 36012 represents selective catheter placement into a second-order venous branch (for example, the left adrenal vein or a petrosal sinus) to obtain diagnostic sampling or to deliver targeted therapy. This code is used across hospitals and interventional radiology settings and is important for accurate reporting of specialized vascular access procedures that support endocrine, neurologic, and vascular diagnostic workflows. Nationally, correct use of this code affects clinical documentation, procedure-level quality measurement, and payer reimbursement for complex catheter-based interventions. Key payers covered in this analysis include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will learn the clinical context for use of CPT code 36012, typical sites of service, and how the code maps to procedure workflows. The publication reviews common billing considerations, comparator procedure codes, and the operational implications for radiology and vascular service lines. It also summarizes benchmarking information where available and notes areas where policy guidance or coding clarification may affect claims processing. Data not available in the input is identified explicitly where relevant.