Use of CT Hounsfield unit density to identify ablated tumor after laparoscopic radiofrequency ablation of hepatic tumors

Backguound: When attempting to interpret CT scans after radiofrequency ther mal ablation (RFA) of liver tumors, it is sometimes difficult to distinguis h ablated from viable tumor tissue. Identification of the two types of tiss ue is specially problematic for lesions that are hypodense before ablation. The aim of this study was to determine whether quantitative Hounsfield uni t (HU) density measurements can be used to document the lack of tumor perfu sion and thereby identify ablated tissue. Methods: Liver spiral CT scans of 13 patients with 51 lesions undergoing la paroscopic RFA for metastatic liver tumors within a 2-year time period were reviewed. HU density of the lesions as well as normal liver were measured pre- and postoperatively in each CT phase (noncontrast, arterial, portoveno us). Statistical analyses were performed using Student's paired t-test and ANOVA. Results: Normal liver parenchyma, which was used as a control, showed a sim ilar increase with contrast injection in both pre- and postprocedure CT sca ns (56.4 +/- 2.4 vs 57.1 +/- 2.4 HU, respectively; p = 0.3). In contrast, a blated liver lesions showed a preablation increase of 45.7 +/- 3.4 HU but o nly a minimal postablation increase of 6.6 +/- 0.7 HU (p < 0.0001). This wa s true for highly vascular tumors (neuroendocrine) as well as hypovascular ones (adenocarcinoma). Conclusions: This is the first study to define quantitative radiological cr iteria using HU density for the evaluation of ablated tissues. A lack of in crease in HU density with contrast injection indicates necrotic tissue, whe reas perfused tissue shows an increase in HU density. This technique can be used in the evaluation of patients undergoing RFA.