RF ablation of focal liver lesions uses an alternating current at frequencies above 250 kHz, induced by mono- or bipolar probes. This is used to generate heat and thus destroy tissue without causing stimulation of nerves or muscles. The slow application of electrical energy with low current density leads to the heating and eventual dehydration of cells and, thereafter, to the degeneration of col-lagenous structures and proteins .
Multiple probe arrays or cooled-tip RF electrodes with 2-3 cm of exposed metal tip, both of which are connected to a frequency generator (up to 200 Watts), are usually used to deliver the RF energy to the tissue. During the ablation procedure, a thermocouple measures the local temperature, and tissue impedance is monitored continuously while the generator output is increased slowly to 950 -1100 mA. Careful monitoring is necessary in order to prevent the tissue from boiling and subsequently carbonizing. Thus, if the impedance increases by > 10 Ohm, the current needs to be reduced until stable impedance is again observed [ 5]. The standard RF application generally lasts for 12-25 minutes and results in typical coagulation necrosis diameters of 2.5-3.7 cm. The maximum diameter reached is 7 cm [18,33 ].
Histologically, ablated areas consist of necrotic tumor tissue surrounded by necrotic liver tissue. The safety margin of surrounding necrotic liver tissue should have a thickness of > 1 cm, corresponding to the surgical resection borders. This zone is encircled by a homogeneous rim of edema-tous inflammatory tissue of variable thickness, which is maximal at about two weeks after the intervention .
In cases of complete tumor destruction, T2-weighted MR images acquired post-RF ablation typically show an area of low signal intensity
Fig. 3a-c. Patient with liver metastases from colon cancer treated with radio-frequency (RF) ablation. Due to diffuse coagulative necrosis, the RF-lesion (arrow) appears slightly hypointense on the T2-weighted image (a) and slightly hyperintense on the Tl-weighted image (b). No enhancement of the RF-lesion is seen during the equilibrium phase after Gd-BOPTA administration (c), indicating complete tumor destruction
Fig. 3a-c. Patient with liver metastases from colon cancer treated with radio-frequency (RF) ablation. Due to diffuse coagulative necrosis, the RF-lesion (arrow) appears slightly hypointense on the T2-weighted image (a) and slightly hyperintense on the Tl-weighted image (b). No enhancement of the RF-lesion is seen during the equilibrium phase after Gd-BOPTA administration (c), indicating complete tumor destruction which corresponds to necrotic tumor and liver tissue. A hyperintense rim encircling this area represents edematous liver and granulation tissue. Conversely, treated lesions typically present as hyper-intense with a hypointense rim on unenhanced T1-weighted images (Fig. 3). In the equilibrium phase following the injection of an extracellular Gd-chelate, T1-weighted images of completely destroyed liver lesions reveal a hypointense, non-perfused central area that is surrounded by a thin hy-perintense rim (Fig. 4). The increased peripheral enhancement in completely destroyed liver lesions reflects the inflammatory reaction as well as sinusoidal obstruction and portal-venous perfusion at the edge of the ablated area. Typically, the peripheral wash-out that is characteristic of viable liver metastases, is not observed in completely RF-de-stroyed liver metastases.
On the other hand, if hyperintense irregular or nodular foci are detected on T2-weighted images, together with irregularity or interruption of the enhancement at the boundaries, or enhancement within the lesion on post-contrast T1-
weighted images, this could indicate residual tumor or local tumor regrowth. Similarly, signs of peripheral wash-out in areas of the RF ablation should be interpreted as possible tumor regrowth or residual tumor tissue (Fig. 5). Wedged-shaped arterial perfusion as an expression of arteriopor-tal shunting can sometimes complicate the evaluation of the surrounding normal liver tissue .
Commonly described complications of RF ablation are: hemorrhage, central bile duct injury, in-traperitoneal abscess formation and, rarely, burning injuries of adjacent organs such as bowel, stomach or kidney. Due to the high sensitivity of MR imaging, major complications are easily detected.
Hemorrhage is detected on T1-weighted fat su-pressed images with intermediate to high SI depending on when the bleeding occurred. In RF ablation special attention should be paid not only to complications such as hemorrhage and inflammation, but also to intrahepatic diffusion of the tumor and to seeding along the puncture canal within the liver and abdominal wall.
4a-d. Patient with solitary metastasis of breast cancer treated with radio-frequency (RF) ablation. Imaging was performed three days cy (
after ablation. On the unenhanced T2-weighted HASTE images in axial (a) and coronal (b) orientation, the RF-lesion (arrows) shows a low SI, surrounded by a rim of even lower SI. Note the inflammation of the surrounding liver parenchyma, which has an increased SI. These changes are most likely caused by impairment of hepatic perfusion due to RF-ablation. In the equilibrium phase (c) after contrast agent injection (0.05 mmol/kg BW Gd-BOPTA), T1-weighted images show a closed, hyperintense rim surrounding the lesion. The lesion itself demonstrates no enhancement, indicating complete tumor destruction. In the hepatobiliary phase one hour after Gd-BOPTA administration (d), heterogeneous contrast agent uptake of the liver tissue surrounding the RF-lesion is demonstrated, the lesion shows no enhancement and is sharply demarcated from the surrounding liver
Fig. 5a-d. Incomplete RF-ablation of a liver metastasis from colorectal carcinoma. The unenhanced T2-weighted HASTE image (a) shows tissue with increased SI surrounding the hypointense RF-lesion (arrow). On the corresponding T1-weighted image (b) the RF-lesion has a slightly increased SI (arrows), whereas the residual tumor tissue is depicted with a low SI. After contrast agent injection (0.05 mmol/kg BW Gd-BOPTA) the Tl-weighted equilibrium phase image (c) shows a hyperintense rim surrounding the RF-lesion (arrows). Similarly a rim separates the residual tumor tissue from the surrounding liver parenchyma (arrowheads). A coronal Tl-weighted fat-suppressed image in the hepatobiliary phase after contrast agent injection (d) clearly demonstrates residual tumor tissue (arrowheads) separated from the RF-lesion by a hyperintense rim (arrows)
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