What are the phases in Computed Tomography (CT) imaging?

Phases in CT Imaging

In CT imaging, phases refer to specific time points after contrast administration when images are acquired, with the main phases being non-contrast, arterial, portal venous, and delayed phases, each providing unique diagnostic information about different pathologies.

Core CT Imaging Phases

Non-contrast Phase

• Acquired before contrast administration
• Primary utility: Identifies high-attenuation materials that might mimic bleeding or enhancement 1
• Essential for detecting calcifications, hemorrhage, and fat
• Can be replaced with virtual non-contrast series in newer multi-energy CT scanners 1

Arterial Phase

• Timing: Typically 25-35 seconds after contrast injection begins 1
• Also called "late arterial phase" in most protocols
• Provides optimal arterial opacification
• Best phase for:
  • Detection of hypervascular tumors (e.g., HCC, neuroendocrine tumors)
  • Visualization of active bleeding (extravasation)
  • Vascular anatomy assessment

Portal Venous Phase

• Timing: 60-70 seconds after contrast injection begins 1
• Features:
  • Maximum enhancement of liver parenchyma
  • Optimal visualization of most abdominal organs
  • Good for detecting hypovascular lesions
  • Allows evaluation of venous structures
  • Shows slower venous bleeding 1

Delayed Phase

• Timing: 90 seconds or more after contrast injection 1
• Also called "equilibrium phase" or "late phase"
• Useful for:
  • Detecting slow venous bleeding 1
  • Evaluating washout in lesions (important for HCC diagnosis) 1
  • Characterizing lesions based on retention patterns
  • Assessing capsule appearance in HCC 1

Specialized Phases

Enteric Phase

• Timing: Approximately 50 seconds after contrast injection 1
• Provides peak bowel wall enhancement
• Optimal for detecting inflammatory conditions and certain tumors 1

Hepatobiliary Phase

• Used with specific contrast agents (gadoxetate disodium, gadobenate dimeglumine)
• Timing: 10-20 minutes after injection
• Valuable for liver lesion characterization 1

Clinical Applications

Multiphase CT Protocols

1. Liver/HCC Imaging:

   • Non-contrast + arterial + portal venous + delayed phases
   • Essential for diagnosis of HCC using the characteristic enhancement pattern:
     • Arterial hyperenhancement
     • Portal venous/delayed phase washout
     • Delayed phase capsule appearance 1
   • Sensitivity for HCC detection: 43-63% overall, 98% for tumors >2cm 1

2. GI Bleeding Assessment:

   • Multiphase CT technique including non-contrast, arterial, and portal venous phases 1
   • Allows detection of active extravasation with high sensitivity (85-90%) 1
   • Oral contrast should be avoided as it can mask bleeding 1

3. Pancreatic Cancer Evaluation:

   • Multiphase thin-section images including pancreatic, arterial, and portal venous phases 1
   • Pancreatic phase provides best tumor detection due to greater attenuation gradient 1
   • Delayed phase increases sensitivity for detecting small primary tumors 1

Evidence-Based Considerations

• The combination of arterial and portal venous phases detects 92% of hypervascular HCCs, superior to any single phase alone 2
• Adding a delayed phase to arterial and portal venous imaging significantly improves detection of small (≤2cm) HCCs (Az = 0.940 vs. 0.917) 3
• The hepatic arterial phase has a mean duration of only 8.6 seconds, making rapid scanning technology essential 4

Common Pitfalls

1. Timing Issues:

   • Mistiming the arterial phase can lead to suboptimal enhancement and missed lesions
   • The arterial phase window is narrow (mean duration 8.6 seconds) 4
   • Patient factors (cardiac output, body habitus) affect contrast timing

2. Protocol Selection:

   • Using single-phase imaging when multiphase is indicated can miss critical findings
   • For HCC surveillance, using only portal venous phase may miss up to 33% of lesions 2, 5

3. Interpretation Challenges:

   • Small hemangiomas (<2cm) may show atypical enhancement patterns 6
   • Isoattenuating pancreatic tumors (5-17% of cases) may be missed without proper phase selection 1

4. Radiation Considerations:

   • Multiple phases increase radiation exposure
   • Consider dual-energy CT to generate virtual non-contrast images and reduce radiation dose 1

By understanding these phases and their appropriate applications, radiologists and clinicians can optimize CT protocols for specific clinical questions, maximizing diagnostic yield while minimizing radiation exposure.