What is the role of fusion imaging in guiding microwave ablation (MWA) for liver tumors?

Fusion Imaging in Microwave Ablation of Liver Tumors

Fusion imaging should be used to guide microwave ablation (MWA) of liver tumors, particularly for lesions ≤2 cm, as it significantly improves tumor detection rates and technical success rates of ablation procedures.

Evidence-Based Recommendation

The 2022 Korean Liver Cancer Association-National Cancer Center (KLCA-NCC) guidelines provide Level B1 evidence that contrast-enhanced ultrasound and fusion imaging improve both the detection rate and technical success rate of local ablation therapy for hepatocellular carcinomas (HCCs) ≤2 cm 1. This represents the highest quality guideline recommendation specifically addressing fusion imaging for liver tumor ablation.

Clinical Application and Technical Considerations

When to Use Fusion Imaging

Fusion imaging is most beneficial for:

• Small tumors ≤2 cm in diameter where detection on conventional B-mode ultrasound may be challenging 1
• Lesions not clearly demarcated on B-mode ultrasound, where fusion with contrast-enhanced CT or MRI improves visualization 2
• Tumors in difficult anatomical locations where precise needle placement is critical 3

Technical Implementation

The fusion imaging technique typically involves:

• Registration of pre-procedural contrast-enhanced CT or MRI with real-time ultrasound, which takes approximately 5-10 minutes depending on DICOM volume data and patient habitus 2
• Either automatic registration or plane match registration can be selected based on the clinical scenario 2
• Real-time guidance during needle placement and ablation to ensure complete tumor coverage with adequate margins 2

Impact on Clinical Outcomes

Technical Success and Effectiveness

Research demonstrates that fusion imaging provides:

• Increased operator confidence and procedural accuracy in real-time during thermal ablation 2
• Improved assessment capabilities compared to examination without image fusion in pre-, intra-, and post-interventional management 2
• Enhanced ability to achieve complete ablation coverage, particularly for tumors that are difficult to visualize on conventional ultrasound 2

Margin Assessment and Local Control

Intra-procedural imaging monitoring is critical for margin control:

• CT-monitored minimal ablative margin (MAM) control during MWA allows for immediate additional ablation within the same session when margins appear insufficient 4
• This strategy achieved sufficient MAM in 77.5% of tumors with excellent long-term local tumor control rates (2-year local tumor progression rate of 6.9% for margin-sufficient ablations) 4
• Insufficient MAM is a strong independent risk factor for local tumor progression (hazard ratio = 14.4, p < 0.001) 4

Integration with Treatment Guidelines

MWA in the Treatment Algorithm

The KLCA-NCC guidelines establish that:

• MWA produces comparable survival, recurrence, and complication rates to radiofrequency ablation (RFA) for HCC treatment (Level B2 evidence) 1
• Combined therapy with TACE and MWA increases survival rates in patients with 3-5 cm HCCs not amenable to resection compared to MWA alone (Level A2 evidence) 1
• MWA is most effective for tumors <3 cm (preferably <2 cm) in appropriate locations away from major vessels and bile ducts 1

Special Considerations and Advanced Techniques

Tumors Near Critical Structures

For perihilar liver tumors proximal to bile ducts:

• US-MR fusion imaging combined with intraductal chilled saline perfusion via PTCD represents a feasible and safe strategy 3
• This approach achieved 91.7% technical success and 83.3% technique efficacy with 0% local tumor progression in a pilot study 3
• The combination allows MWA of tumors abutting hilar bile ducts that would otherwise be contraindicated 3

MR-Guided MWA

Multiparametric MR-guided and monitored MWA is an alternative when ultrasound and CT are inappropriate:

• Achieved 100% technical success and 92.8% primary technical effectiveness in a prospective study 5
• Particularly suitable for lesions poorly visualized by ultrasound or CT 5
• Provides excellent soft tissue contrast for real-time monitoring of ablation zones 5

Common Pitfalls and How to Avoid Them

Key technical challenges include:

• Heat sink effect near major vessels can decrease effectiveness and increase local recurrence; fusion imaging helps identify these high-risk locations pre-procedurally 1
• Subcapsular tumors risk track seeding with direct puncture; fusion imaging allows for optimal needle trajectory planning 1
• Incomplete margin assessment leads to higher local progression; intra-procedural contrast-enhanced imaging should be used to verify adequate margins before completing the procedure 4

Quality Assurance

Post-ablation imaging verification should include:

• Immediate post-procedural contrast-enhanced imaging (CT or MRI within 24 hours) to confirm complete tumor coverage and assess minimal ablative margins 1, 4
• Image fusion and registration to document tumor-free margins, ideally using dedicated fusion software 1
• Follow-up imaging at 3 months to assess primary technical effectiveness 1, 5

The evidence strongly supports fusion imaging as a standard adjunct to MWA for liver tumors, particularly for small lesions where precise targeting and margin assessment are critical for achieving optimal local tumor control and long-term outcomes.