Transient hyperechoic zones momentarily appear within and around a tumor on the baseline US both during and immediately after RFA due to gas bubble formation from water vaporization and necrosis of the tumor and liver tissue. The transient hyperechoic zone can persist for 30–90 min and can be used to roughly estimate the ablation zone during RFA. When the gas dissipates, the echogenicity of the ablation zone is heterogeneous, containing iso-echoic and hyperechoic regions. ceUS can be used after the disappearance of the transient hyperechoic zones to detect residual tumor if an incomplete ablation is suspected. One month after ablation, completely ablated, necrotic tumors appear as well-defined regions with heterogeneous echogenicity and no posterior acoustic enhancement. The ablation zone gradually becomes iso-echoic with time. The ablation zone must be larger than the original tumor, and the former will gradually shrink rather than expand. Sometimes, a hyperechoic needle tract is visible in the middle of or in front of the ablation zone. No internal flow can be visualized in the ablation zone upon Doppler US imaging. The ablation zone will show normal uptake and wash out of contrast agent signal throughout the three phases of CEUS, which are the arterial enhancement phase, portal venous enhancement phase, and interstitial enhancement or delayed phase. Contrast agents increase the accuracy of US, and the ablation zone should be measured using ceUS rather than the baseline ultrasound.

CT without contrast agents, performed immediately after RFA, reveals a primarily low-density ablation zone. High-density regions representing intratumor bleeding or tissue dehydration can be seen in a few ablation zones. Additionally, small amounts of gas generated by tissue necrosis or boiling during RFA can be observed by CT. These air bubbles are usually round, tubular, or oval shaped and should not be mistaken for infection or a hepatic abscess caused by RFA. With time, the ablation zone evolves into a more homogeneous region of low density upon CT without contrast agents. Occasionally, the differences in tissue properties between the tumor and adjacent liver parenchyma create a target appearance of an ablation zone. A target appearance of an ablation zone is low density upon CT within the ablated tumor (presumably with prominent fatty metamorphosis) and is surrounded by a relatively high-density region of ablated liver parenchyma. The low density of ablated tumors may be due to fatty metamorphosis of HCC, and the high density of the ablated liver parenchyma helps to visualize the ablation margin. The electrode needle tract is often visible along the path of the RF electrode shaft in the ablation zone. It appears as a high-density charring shadow, which disappears over time. The ablation zone does not enhance during any phase of ceCT due to perfusion deficiencies. No expansion of unenhanced area is observed during the portal and delay phase [9].

One month after RFA, ablation zones display heterogeneous high-intensity signal upon T1-weighted MRI and either a homogeneous or heterogeneous low-intensity signal upon T2-weighted MRI without contrast enhancement. As time passes, the signal intensity of the ablation zone will gradually become higher on T1-weighted images and more homogeneously low upon T2-weighted imaging. The target appearance can also occur with MR imaging [10].

Transient periablational hyperemia appears immediately after RFA in most cases. Liver tissue surrounding the necrotic ablation zone develops hyperemia and edema because of the thermal damage. Pathology has further revealed inflammation in blood vessels and the formation of granulation tissue. Transient periablational hyperemia usually disappears by 1 month post-ablation but can exist for up to 6 months. Transient periablational hyperemia is hypo-echoic upon US, is low-density upon CT, and has a rim of slightly increased signal intensity upon T2-weighted MRI (Fig. 9.2). However, the hyperemia is usually subtle or nonexistent upon unenhanced imaging for most cases. After administration of contrast agents, transient hyperemia is clearly visualized as a thin (usually <5 mm) and uniform rim surrounding the ablation zone. It is hyperenhanced during the arterial phase and iso-enhanced during the portal and delay phases. This hyperemic rim should not be considered a residual tumor or tumor recurrence (Fig. 9.3).

Arterioportal shunting is a common finding upon contrast-enhanced radiological imaging. It is due to mechanical or thermal injury of small hepatic vessels and results in abnormal perfusion to liver tissue that is adjacent to the ablation zone. Arterioportal shunting is visualized as a hyperenhanced wedge shape during the arterial phase that is iso-enhanced during the portal and delayed phases. It is usually not observed upon imaging (US, CT, or MRI) without contrast enhancement. Arterioportal shunting will gradually resolve spontaneously [11].