The use of contrast media (CM) poses a risk of post-contrast acute kidney injury (PC-AKI), especially among patients chronic kidney disease (CKD). International guidelines recommend intravenous (IV) hydration with isotonic 0.9% NaCl for three-four hours pre-contrast and four-six hours post-contrast. Recent studies have proven that oral hydration or no hydration is non-inferior to IV hydration in patients with mild to moderate CKD (eGFR 30-60 mL/min/1.73 m2). However, no randomized controlled trials have evaluated alternative hydration methods against the guideline-recommended hydration protocol for the prevention of PC-AKI in high-risk patients with severe CKD (eGFR \< 30 mL/min/1.73 m2). Thus, the main focus of this trial is to evaluate IV hydration vs. oral hydration for their efficacy to prevent of PC-AKI in patients with severe CKD, who are scheduled for an elective contrast-enhanced CT-scan (CECT) with IV contrast-administration. Our research hypotheses consist of the following: 1. Oral hydration with bottled tap water is non-inferior to IV-hydration with isotonic 0.9% NaCl as renal prophylaxis to prevent PC-AKI in patients with severe CKD referred for an elective IV CECT. 2. NGAL and cfDNA are early and precise plasma and urinary biomarkers of PC-AKI with excellent diagnostic and prognostic accuracy for PC-AKI, dialysis, renal adverse events, hospitalization, progression in CKD-symptoms, and all-cause mortality.