MR Imaging-Histopathologic Correlation of Radiofrequency Thermal Ablation Lesion in a Rabbit Liver Model: Observation during Acute and Chronic Stages

Abstract

Objective: To determine the ability of MR imaging to detect the pathological changes occurring in radiofrequency (RF) thermal lesions and to assess its accuracy in revealing the extent of tissue necrosis. Materials and Methods:Using an RF electrode, thermal lesions were created in the livers of 18 rabbits. The procedure involved three phases. In the acute phase, six animals were killed the day after performing thermal ablation with RF energy, and two on day 3. In the subacute and chronic phases, eight rabbits underwent percutaneous hepatic RF ablation. After performing MR imaging, two animals were sacrificed at 1, 2, 4, and 7 weeks after the procedure, and MRpathologic correlation was performed. Results:In the acute phase, the thermal ablation lesions appeared at gross examination as well-circumscribed, necrotic areas, representing early change in the coagulative necrosis seen at microscopic examination. They were hypointense on T2-weighted images, and hyperintense on T1-weighted images. Gadolinium-enhanced MR imaging showed that a thin hyperemic rim surrounded the central coagulative necrosis. In the subacute phase, ablated lesions also showed extensive coagulative necrosis and marked inflammation at microscopic examination. Beyond two weeks, the lesions showed gradual resorption of the necrotic area, with a peripheral fibrovascular rim. The size of lesions measured by MR imaging correlated well with the findings at gross pathologic examination. Conclusion:MR imaging effectively demonstrates the histopathological tissue change occurring after thermal ablation, and accurately determines the extent of the target area.mage-guided, percutaneous ablative therapies using thermal energy sources such as radiofrequency (RF) (1 3), microwave (4) and laser (5) are rapidly evolving as minimally invasive techniques for the treatment of primary and metastatic hepatic tumors. The potential benefits of these techniques over conventional surgical options include tumor ablation in nonsurgical candidates, reduced morbidity compared with surgery, and use of the procedure on an outpatient basis (6). Preliminary clinical reports have demonstrated that hepatic RF ablation produces effective local disease control in a significant proportion of patients with nonresectable liver tumors (7 9). However, according to even the most optimistic longterm report of percutaneous RF thermal therapy in colorectal metastases, local tumors recurred in 35% of cases (10). In addition, recent studies have shown that a significant proportion of patients with hepatocellular carcinomas greater than 3 5 cm in diameter experienced local recurrence after the use of current thermal ablation strategies (11 12).

Added value of diffusion‐weighted imaging in the MRI assessment of perilesional tumor recurrence after chemoembolization of hepatocellular carcinomas

“…The increase in false positive findings originated from perilesional parenchymal insults that showed sustaining hyperintensity on DWI with increasing b factors. Because a lesion-to-lesion correlation between DWI and histological features was not possible in this study, the mechanism for the high false positive rate with DWI could be just speculated as perilesional inflammation, which is more frequently noted after radiofrequency ablation (25) and may result from TACE (19,26). Hypercellularity intermingled with a fibrotic component in the inflammatory granulation tissues could restrict water diffusion, resulting in sustaining hyperintensity on DWI (27).…”

Real‐time monitoring of radiofrequency ablation of rabbit liver by respiratory‐gated quantitative temperature MRI

“…Because of the impracticality of repeated injections, the extent of the ablative zone can only be assessed at the end of the procedure. It has also been suggested that magnetic resonance imaging (MRI) can be used to guide RF ablation by acquiring unenhanced images, especially T2-weighted (T2w) images during the procedure (5,6). However, the limited precision of such images necessitates the acquisition of dynamic gadolinium (Gd)-enhanced T1-weighted (T1w) images at the end of the procedure (6).…”

Prediction of cell necrosis with sequential temperature mapping after radiofrequency ablation

“…Lee et al (33) showed that the size of coagulative necrosis increases during the first week after the intervention and then begins to gradually decrease. An involution of the coagulation zone could have taken place before the follow-up examination.…”

“…Nonenhanced areas in these images were regarded as necrotic tissue (22,23). Concerning the temperature maps, we considered that tissue that reached at least 60°C could be regarded as necrotic.…”