Real-time Monitoring Of Regional Thermal Ablation Of In Vivo Porcine Liver By Respiratory-gated MR Temperature Imaging

Background and Objection:1.This study was to identify the thermal boundary after microwave ablation using magnetic resonance(MR)thermometry with the proton resonance frequency(PRP)shift technique,and compare the results on MR thermometry with post-operative histopathological images.The feasibility and accuracy of real time MR thermometry-guided microwave ablation was evaluated,and the application of real time monitoring microwave ablation of porcine liver in vivo was discussed.2.The T2*values of different fibrotic background of the livers were measured.The parameters of MR thermometry sequence were optimized according to the requirement of signal to noise ratio(SNR),temporal and spatial resolution.The image data were analyzed based on difference method referenceless PRF shift.The feasibility and accuracy of realtime monitoring the temperature and thermal dose during microwave ablation of hepatic lesions with fibrotic background was evaluated,which provides a realtime and accurate method for monitoring ablation in patients with hepatocellular carcinoma and hepatic fibrosis.Materials and Methods:1.Twenty healthy Bama miniature pigs(weight range,30-35kg)were include in this study.Ten pigs were assigned to the hepatic fibrosis model group,and blood samples were regularly acquired to determine the hepatic function and hepatic ultrasonography was routinely given to evaluate the fibrotic degrees of the liver.2.Testing the T2*values and MR thermometry parameters under different fibrotic backgrounds.3.Preoperative imaging guided ablation zone-targeted acupuncture and microwave ablation.4.MR thermometry,data post-processing and acquisition of routine sequences5.Acquisition of liver gross pathologic and histopathological specimens6.The feasibility and accuracy of realtime MR thermometry-guided microwave ablation monitoringIndicators of feasibility included,operation time,microwave ablation energy deposition parameters(voltage,current intensity,Ooutput power and tissue impedance.etc.),dimension of the susceptibility artifact of the electrode were measured as well as the SNR of the images.Indicators of accuracy included,the correlation and agreement of the max long axis,short axis and area of the ablative lesion on intraoperative temperature map,thermal dose map(TD240 area)and post-operative routine T1-,T2-WI and enhanced T1WI sequences.Results:1.The average T2*values were 38.55(36.14,44.84)in Stage SO,29.23(25.64,31.89)in Stage S1,25.43(19.58,27.06)in Stage S2,22.02(20.61,24.79)in Stage S3,22.19(21.29,23.22)in Stage S4O By optimizing the sequences,a TE value of 25ms in early and 15ms in late ages of hepatic fibrosis(S3-S4)were determined.2.Dimension of the susceptibility artifact of the electrode on different sequences and changes of SNR before and after ablation.The actual dimension of the electrodes 2.1mm,2.7±0.4mm on T1 trufisp images,2.3±0.3mm on T2-haste images,2.3±0.2mm on magnetic images of MR thermometry,which were acceptable and did not interfere with the observation of ablative area.The loss of SNR was estimated as 10.9%on MR thermometry sequences,as compared with preoperative images,which indicated favorable image quality.3.Comparison of the ablation zones between temperature maps,thermal dose maps(TD240)and routine sequencesThe ablation zones demonstrated hyperintense on T1-tfl images,with a hypointense peripheral rim.They are hypointense on T2-haste images,with a hyperintense peripheral rim.No central enhancement could be observed on enhanced T1WI,while there was mild peripheral enhancement around the lesion.On MR thermometry sequences,the lesions appeared as spherical hypointense.There were signif-icant differences in the max long axes,short axes and areas of the ablative lesions between T1-tfl,T2-haste,enhanced T1WI and intraoperative temperature map,thermal dose map(TD240area)(p<0.05.4.Liver gross pathologic and histopathological(HE and NADH staining)evaluationsGross evaluation of the specimens demonstrated sperical or quasi-sperical in shape,hard in texture,white in the center(white zone,Wz),with a red peripheral rim which indicated hyperedema(red zone,Rz),surrounded by normal hepatic tissue.The deviding line between the necrotic ablation zone and viable cells was difficult to identify clearly on HE stained images,but it could be observed clearly on NADH stained images.5.Comparison of the ablation zones on temperature maps and thermal dose maps(TD240 area)of MR thermometry and histopathological resultsThe ROC analysis showed that the best cutoff value that differentiate the necrotic area and hyperedemac area was 57?.The predicted area by this cutoff value proved favorable correlation and agreement with Wz and Rz observed in histopathological analysis.The necrotic area predicted by TD240 area on thermal dose map proved favorable correlation and agreement with Wz and Rz observed in histopathological analysis.Conclusion:1.In vivo MR thermometry with PRF shift technique-guided mirowave ablation of porcine liver is feasible,which can be used in identification of the thermal boundary and ablation zone after microwave ablation,as an method determining the endpoint of ablation.2.After optimizing the parameters of MR thermometry according to T2*value measured under different degrees of hepatic fibrotic backgrounds,and generating the temperature map and thermal dose map from the difference method referenceless PRF shift,it is feasible to monitor the microwave ablation realtime,and depict the dimension of ablation zone accurately.