What are the timing phases of a contrast‑enhanced CT (CECT) scan?

Phases of Contrast-Enhanced CT (CECT) Scan

A standard CECT whole abdomen protocol includes four distinct phases: noncontrast, late arterial (35 seconds post-injection), portal venous (60-70 seconds post-injection), and delayed (90+ seconds post-injection), with specific phase combinations tailored to the clinical indication. 1

Standard Phase Timing and Clinical Utility

Noncontrast Phase

• Identifies high-attenuation material (calcifications, hemorrhage, ingested material) that could be obscured by contrast enhancement 1
• Can be replaced with virtual noncontrast series on newer multienergy CT scanners 1

Late Arterial Phase (35 seconds post-injection)

• Provides optimal arterial opacification for detecting hypervascular lesions and arterial bleeding 1
• Critical for detecting contrast extravasation in suspected hemorrhage 1
• The arterial phase begins 10-20 seconds after injection and lasts 25-35 seconds in liver imaging protocols 2
• Hypervascular liver metastases show early strong enhancement during this phase 2
• Optimal timing is 10-15 seconds after aortic contrast arrival for early arterial phase hepatic imaging 3

Portal Venous Phase (60-70 seconds post-injection)

• Most sensitive phase for detecting hypovascular metastases with 85-91.5% sensitivity 1
• Provides optimal bowel wall enhancement and solid organ evaluation 1
• The portal venous phase extends from 30-45 seconds to 120 seconds post-injection 2
• Primary phase for solid organ injury detection in trauma evaluation 1
• Allows better detection of slow venous bleeding 1

Delayed Phase (90+ seconds post-injection)

• Enhances lesion characterization and improves detection of slow venous bleeding 1
• The late phase begins after 120 seconds and ends with bubble disappearance (approximately 5 minutes post-injection) in contrast-enhanced ultrasound 2
• Most characteristic feature of metastases is hypoechogenicity in the late phase relative to surrounding liver 2

Protocol Selection Based on Clinical Context

Suspected GI Bleeding

• Use multiphase protocol with noncontrast, late arterial, and portal venous phases 1
• Achieves 85-90% sensitivity and 92% specificity for detecting active hemorrhage 1
• Avoid oral contrast as it delays scanning and obscures bleeding 1

Liver Lesion Evaluation

• Include arterial, portal venous, and delayed phases 1
• Hypervascular lesions require arterial phase imaging while hypovascular metastases are best detected on portal venous phase 1
• Rim enhancement during arterial and portal venous phases is valuable for characterizing metastases 2

CT Enterography

• Contrast-enhanced imaging initiated 50-70 seconds after IV injection, between enteric and portal phases 2
• Single enteric phase adequate for detecting inflammatory conditions and masses 1
• Multiphasic examinations improve detection and characterization of vascular lesions 1
• Requires 900 mL neutral or biphasic enteric contrast administered over 45-60 minutes before scanning 2

Trauma Evaluation

• Portal venous phase is primary phase for solid organ injury detection 1
• Dual-phase imaging including arterial phase significantly improves sensitivity and accuracy for splenic vascular injury 1

Critical Technical Considerations

Contrast Administration

• High injection flow rate (4-5 mL/sec) desirable for arterial phase imaging 4, 5
• Faster injection rates increase maximum aortic enhancement without decreasing arterial phase duration 6
• Injection duration is key determinant of equilibrium phase onset 4, 5
• Use 20-gauge intravenous catheter to avoid microbubble destruction 2

Oral Contrast Selection

• Neutral oral contrast preferred for small bowel evaluation as it allows better visualization of enhancing lesions 1
• Positive oral contrast must be avoided in CT enterography as it obscures mucosal hyperenhancement and active hemorrhage 7
• Avoid oral contrast in suspected GI bleeding 1

Image Acquisition

• Slice thickness of 2.5-5 mm recommended, with thinner slices improving lesion detection 1
• ECG gating useful for ascending aorta evaluation where cardiac motion creates artifacts 1

Common Pitfalls to Avoid

• Never perform noncontrast CT alone for abdominal evaluation as it has significantly lower sensitivity for detecting visceral organ and vascular injuries 1
• Do not use standard positive oral contrast for CT enterography as it obscures mucosal enhancement patterns 7
• Avoid phosphate-containing oral preparations in patients with GFR <60 mL/min/1.73 m² due to phosphate nephropathy risk 7
• Evaluate all vascular phases when assessing focal liver lesions, as artifacts can lead to misinterpretation if only late phase is evaluated 2