Radiofrequency Ablation of Hepatic Lesions: A Review

Venkataramu N. Krishnamurthy, MD; V. Javier Casillas, MD; Lina Latorre, MD

Disclosures

Appl Radiol. 2003;32(10) 

In This Article

Monitoring of Coagulation During RF Ablation

Currently, imaging techniques (US, CT, and MR) are standard for monitoring coagulation.[41] Of these, US is the most commonly used modality and the ablation is seen as an echogenic area due to liberation of nitrogen gas during the coagulation (Figure 4). But US is unsuitable for larger lesions or when multiple ablations are planned because the echogenicity from the initial ablation can obscure visibility during subsequent needle placement. We use a combination of US and CT in these circumstances for intraprocedural monitoring.

Monitoring of coagulation by ultrasound. Diffuse echogenic bubble pattern is seen during ablation due to release of nitrogen gas.

Both noncontrast and contrast-enhanced US, CT, and MR scans are used for assessing the adequacy of ablation. In general, noncontrast imaging findings are unreliable parameters for predicting the adequacy of ablation.[29,41] The ablated area appears hyperechoic on US or hypodense on CT scan. In experimental porcine liver models, some authors have reported the appearance of decreased hypoattenuation on noncontrast CT as a sensitive indicator of complete ablation.[42] But more important morphologic features to be determined are the size and margin (the interface between normal liver tissue and the ablation defect) of the defect. After complete ablation, the defect should be centered on the tumor location, it should beinitially larger than the tumor (which ensures desired tumor-free margins), and the interface should be sharp, smooth, and devoid of nodularity.[13] When blood-pool contrast agents are used (iodinated contrast in CT and gadolinium contrast in MR), there should be complete lack of tumor enhancement with thin, smooth rim enhancement at the interface.[41] The rim represents ablation-induced hyperemia (Figure 5). Irregularity and nodularity suggests incomplete ablation with residual tumor. Contrast-enhanced power and color Doppler US has also been used for monitoring of coagulation. Ultrasound contrast agents allow for better differentiation between perfused and nonperfused tissue, which indirectly represent viable and nonviable (coagulated) tissue.[43,44] Therefore, adequacy of ablation can potentially be predicted in real-time during the procedure.[45,46,47] The lack of general availability and operator experience are limiting factors for universal use of US contrast agents.

Monitoring of coagulation by CT. (A) CT-guided radio-frequency ablation of a single metastatic lesion from colorectal carcinoma. (B) Immediate postablation CT scan shows uniform low-attenuation in the lesion with small gas bubbles. Gas bubbles in the immediate postprocedure are common and should not be confused with abscess. (C) A 24-hour follow-up CT scan shows a thin enhancing rim due to ablation-induced hyperemia. Note the thin smooth appearance and the lack of nodularity.

Monitoring of coagulation by CT. (A) CT-guided radio-frequency ablation of a single metastatic lesion from colorectal carcinoma. (B) Immediate postablation CT scan shows uniform low-attenuation in the lesion with small gas bubbles. Gas bubbles in the immediate postprocedure are common and should not be confused with abscess. (C) A 24-hour follow-up CT scan shows a thin enhancing rim due to ablation-induced hyperemia. Note the thin smooth appearance and the lack of nodularity.

Monitoring of coagulation by CT. (A) CT-guided radio-frequency ablation of a single metastatic lesion from colorectal carcinoma. (B) Immediate postablation CT scan shows uniform low-attenuation in the lesion with small gas bubbles. Gas bubbles in the immediate postprocedure are common and should not be confused with abscess. (C) A 24-hour follow-up CT scan shows a thin enhancing rim due to ablation-induced hyperemia. Note the thin smooth appearance and the lack of nodularity.

Direct documentation of tissue temperature (using additional remote temperature sensors during and immediately after ablation) has also been used to predict adequacy of ablation.[33,48] Temperature-sensing electrodes are placed under US/CT guidance in the periphery of the tumor or cuff of normal tissue. Temperatures in these electrodes >55°C indicates adequacy of tumor ablation. However, this technique is not fully optimized and reliable systems have yet to become available for general use.

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