For a patient with gangrene of the fourth toe, which lower‑extremity computed tomography angiography (CTA) protocol should be ordered?

CTA Lower Extremity with Bilateral Runoff (Abdomen/Pelvis to Feet)

For gangrene of the 4th toe, you should order a CTA of the abdomen and pelvis with bilateral lower extremity runoff with IV contrast 1, 2. This comprehensive imaging protocol is essential because gangrene represents critical limb ischemia (CLI), and you need complete anatomic assessment from the aorta down to the pedal vessels to plan revascularization.

Why This Specific Protocol

The "runoff" component is critical—this means imaging extends from the infrarenal aorta through the pelvis and down to the feet, capturing:

• Aortoiliac inflow vessels
• Femoral-popliteal segments
• Below-knee tibial arteries
• Pedal vessels (dorsalis pedis and plantar arteries)

Gangrene indicates tissue necrosis from inadequate perfusion, which by definition is CLI requiring urgent revascularization assessment 2. The entire arterial tree must be visualized because:

1. Multi-level disease is common - atherosclerotic PAD typically affects multiple segments, and you cannot plan intervention without knowing the complete anatomy 1

2. Inflow and outflow must both be assessed - successful revascularization requires restoring "direct pulsatile flow to at least one of the foot arteries, preferably the artery that supplies the anatomical region of the wound" 3

3. Surgical vs endovascular planning - the complete anatomic roadmap determines feasibility and approach for intervention 2

Technical Specifications

Order as: "CTA abdomen and pelvis with bilateral lower extremity runoff with IV contrast"

The protocol should include 4, 5:

• Thin-section acquisition timed for peak arterial enhancement
• Multiplanar reformations and 3D volume renderings (required for CTA definition)
• Coverage from diaphragm to toes
• Attention to below-knee and pedal vessel visualization

Clinical Context: This is Urgent

Gangrene with infection should be treated as a medical urgency, ideally within 24 hours 3. The phrase "time is tissue" applies directly to infected ischemic diabetic foot ulcers and gangrene. Patients with CLI and foot infection have particularly high risk for major limb amputation.

Alternative Imaging Considerations

While CTA is usually appropriate (Class IIb recommendation for anatomic assessment) 1, the guidelines actually give Class I recommendations to three modalities for anatomic assessment when revascularization is considered 2:

• Duplex ultrasound - useful but operator-dependent and limited by calcification
• MRA with gadolinium - contraindicated if severe renal dysfunction (risk of nephrogenic systemic sclerosis) 2
• CTA - superior spatial resolution, faster acquisition, better visualization of calcified vessels 4

However, for CLI (which gangrene represents), invasive angiography with immediate endovascular intervention is often most expeditious 2. The 2017 AHA/ACC guidelines note that for CLI patients, it's often most effective to proceed directly to invasive angiography with revascularization "without delay and potential risk of additional noninvasive imaging."

Practical Algorithm

If the patient is stable and surgical planning is needed:

• Order CTA abdomen/pelvis with bilateral LE runoff first
• This provides the surgical team with complete anatomic roadmap
• Allows multidisciplinary discussion of open vs endovascular approach

If the patient has infected gangrene or is clinically unstable:

• Consider proceeding directly to invasive angiography with intervention capability
• Noninvasive imaging may cause dangerous delays in CLI 2

Common Pitfalls

1. Ordering "CTA lower extremity" alone - this may not include aortoiliac inflow assessment, which is essential 1

2. Timing issues - below-knee vessels are challenging to image; too-early acquisition misses contrast bolus, too-late acquisition has venous contamination 5, 6

3. Assuming MRA is always preferred - while MRA has Class I recommendation, CTA has superior spatial resolution for distal small vessels and is faster 7, 4

4. Forgetting renal function - check creatinine before ordering; hydrate patients with baseline renal insufficiency 1

5. Not communicating urgency - gangrene requires expedited imaging and intervention, not routine scheduling