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CT Abdomen with Contrast

What Is It?

Indications

• Abdominal pain (suspected appendicitis, diverticulitis, bowel obstruction, perforation)
• Trauma evaluation (hepatic, splenic, renal, mesenteric injuries)
• Detection and staging of abdominal tumors, enlarged lymph nodes, metastases
• Fluid collections (abscess, hemorrhage, ascites)
• Vascular pathology (mesenteric ischemia, aortic aneurysm, venous thrombosis)
• Air outside the GI tract (pneumoperitoneum)

Standard Technique

• Grainger & Allison's Diagnostic Radiology: "Contrast medium should be used whenever possible, with the upper abdomen imaged in both the arterial and portal venous phases."

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The Contrast Phases - What Happens After Injection

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Triple-Phase CT Abdomen

Definition

1. Late Arterial Phase (~35-45 sec post bolus trigger)
2. Portal Venous Phase (~60-75 sec)
3. Delayed Phase (~2-5 minutes)

Note on terminology: Some institutions define triple-phase as Non-contrast + Late Arterial + Portal Venous, and call the above protocol "four-phase." A true non-contrast phase added to the three above makes it a quadruple-phase or four-phase CT. Always confirm your local protocol.

Indications

• Bailey and Love's Surgery: "Routine staging [of colorectal liver metastases] involves triple-phase CT chest/abdomen/pelvis, contrast MRI scan."
• Washington Manual: "Triple-phase CT or quadruple-phase CT is indicated for liver mass evaluation. A delayed phase is useful when HCC is suspected."

How HCC is Diagnosed on Triple-Phase CT

1. Arterial phase hyperenhancement - the lesion enhances brighter than surrounding liver parenchyma
2. Portal venous or delayed phase washout - the lesion becomes hypoenhancing (darker) compared to liver

Triple-Phase CT Technique (Radiopaedia Protocol)

• Patient position: Supine, arms above head
• Scout: Diaphragm to iliac crests
• Bolus tracking: ROI placed at aorta at level of diaphragmatic hiatus; threshold = 150 HU
• Contrast: 100-120 mL non-ionic contrast at 3-5 mL/s
• Scan delays:
  • Late arterial: 15-30 sec post bolus trigger (35-45 sec after injection start)
  • Portal venous: 60-75 sec after injection (independent of arterial timing)
  • Delayed: 2-5 minutes
• Respiration: Inspiration, breath-hold for each phase

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CT Abdomen Contrast vs. Triple-Phase - Key Differences

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Radiation Considerations

• Use automatic exposure control (AEC)
• Reduce dose on non-contrast phase (lower image quality acceptable)
• In children: split-bolus technique to achieve both arterial and venous enhancement in a single acquisition (Grainger & Allison's)
• Avoid triple-phase unless specifically clinically indicated

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Summary of Clinical Use

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CT Carotid Angiography (CTA Neck) Protocol

Indications

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Patient Preparation

Right arm injection is preferred per ACR-ASNR guidelines to prevent artifact from undiluted contrast pooling in the left brachiocephalic vein overlying the arch vessels.

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Scout / Topogram

• AP and lateral scout
• Coverage from thorax through vertex of skull
• Used to plan scan range

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Scan Parameters

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Scan Coverage

• Aortic arch and origin of subclavian/brachiocephalic arteries
• Common carotid arteries
• Carotid bifurcation (most important area)
• Internal and external carotid arteries (cervical course)
• Vertebral arteries (origin to skull base)
• Circle of Willis and intracranial vessels
• Up to vertex

"CTA and CEMRA both allow a full assessment of the arterial tree from the aortic arch to the circle of Willis and beyond." - Grainger & Allison's Diagnostic Radiology

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Scan Direction

• Scan direction should be inferior to superior (craniocaudal approach going from aortic arch upward)
• This follows the direction of contrast flow from heart → carotid arteries → brain
• Scanning in the direction of blood flow ensures contrast is present in each vessel segment as the scanner reaches it
• Some institutions use craniocaudal (top to bottom) to reduce perivenous streak artifact from contrast in the subclavian/brachiocephalic vein - this is an alternative validated approach

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Contrast Protocol

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Bolus Timing - Bolus Tracking (Preferred Method)

1. Place ROI (region of interest) in the ascending aorta or aortic arch
2. Run low-dose monitoring scans every 1 second starting 5 seconds after contrast injection begins (monitoring at 100 kVp, 20-40 mAs)
3. Trigger threshold: 100-150 HU above baseline in the aorta
4. When threshold is reached, scanner automatically initiates the CTA acquisition (with a typical 5-8 second delay for table/gantry prep + patient instruction)

• Inject 10-15 mL test bolus at same flow rate
• Serial low-dose scans at the aortic arch level every 1-2 seconds
• Measure time to peak enhancement → use as fixed scan delay for full bolus

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Pre-Contrast Phase

• Suspected mural hemorrhage (dissection - intramural hematoma appears bright pre-contrast)
• Mapping calcification for stenosis assessment
• Localizing anatomy

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Reconstructions / Post-Processing

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What the Radiologist Reports

• Degree of stenosis using NASCET criteria:
  • Stenosis % = (1 - residual lumen diameter / normal distal ICA diameter) × 100
  • Symptomatic: ≥70% stenosis = high benefit from CEA; 50-69% = moderate benefit
• Plaque morphology (calcified vs. soft/lipid-rich - juxtoluminal hypodensity = unstable plaque)
• Occlusion vs. near-occlusion ("trickle flow")
• Tandem lesions (intracranial + extracranial disease)
• Carotid bulb/ICA anatomy, tortuosity, kinks, coils
• Aortic arch type and variant origin of vessels (surgical planning)
• Vertebral artery dominance/patency
• Circle of Willis completeness

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CTA vs. Other Modalities for Carotid Stenosis

• Fischer's Mastery of Surgery, Grainger & Allison's Diagnostic Radiology

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Example CTA Carotid Image

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Key Tips and Pitfalls