radiofrequency ablation ( rfa ) is an established local therapy for managing localized liver tumors . ablation margins are usually evaluated using contrast - enhanced multi - detector computed tomography ( mdct ) after rfa therapy . however , immediate therapeutic assessment using these techniques is limited unless the equipment is available during rfa . recent advances in c - arm cone - beam technology allow operators to acquire optimal images during procedures without requiring patient transfer . a digital flat - panel detector on the upper side of the c - arm rotates around the patient to generate volumetric images similar to conventional ct images . in this preliminary study , we used this novel modality and injected a contrast medium through a peripheral vein into patients undergoing rfa of liver tumors to assess ablation margins using a recently developed multimodality fusion software . five patients ( 4 men , 1 woman ; mean age , 68.2 years ) , each of whom had a solitary liver tumor ( 4 hepatocellular carcinomas , 1 metastatic tumor ; range , 8 - 30 mm ; mean size , 15.0 mm ) , underwent rfa using a c - arm cone - beam system ( innova 3100 ; ge healthcare , waukesha , wi , usa ) . prior to rfa treatment , we obtained images of each patient 's treatment area using either triphasic dynamic contrast - enhanced mdct or magnetic resonance ( mr ) imaging . contrast - enhanced c - arm ct images were obtained immediately after rfa . a 90-ml bolus of iopamidol ( 300 mgi / ml ; flow rate , 1 ml / s ) was injected through the antecubital vein using a power injector and the c - arm ct images were obtained 2 min later . the image acquisition parameters included were the following : total scanning angle , 200 degrees ; acquisition time , 10 s ; matrix size , 1500  1500 ; isotropic voxel size , 0.2 mm ; and effective field - of - view , 18 cm . raw data sets were transferred to an external workstation ( advantage workstation 5.0 ; ge healthcare ) , upon which the images were reconstructed with a 1-mm slice thickness in multiple planes . post - treatment mdct images were obtained for each patient 3 - 7 days after rfa therapy . triphasic contrast - enhanced mdct images were obtained using a 16-mdct scanner ( somatom sensation ; siemens medical solutions , forchheim , germany ) using the following scanning parameters : 120 kv ; 182 ma ; beam collimation , 0.75 mm ; helical pitch , 1.15 mm ; and rotation table speed , 0.5 s. helical acquisition for arterial , portal , and venous phase imaging was initiated at 20 , 60 , and 160 s , respectively , after a threshold level of 100 hu was reached in the abdominal aorta . we used a multimodality image fusion software ( integrated registration ; ge healthcare ) to fuse multimodality images before and after rfa . the volume data sets of contrast - enhanced c - arm ct , portal - phase mdct , and hepatobiliary - phase mr images were transferred to an advantage workstation 5.0 ( ge healthcare ) . images were first displayed side by side on the workstation screen and then manually adjusted so that the intrahepatic vessels adjacent to the target tumor were similarly displayed . a hepatic or portal vein that was identical in both images was then selected as the landmark for image registration in another plane . if image registration was unsatisfactory , one or two additional intrahepatic vein landmarks were selected to ensure more precise image registration adjustment . once the images were registered , the fusion images were reconstructed as 3-mm - thick multi - planar images by the same radiology technician . the fused post - treatment c - arm ct and pretreatment images were sufficiently delineated to permit measurement of the ablation margins [ figure 1 ] . however , the interface of the ablated margin was not sharp and inflammatory peripheral enhancement was more eminent for c - arm ct fusion than mdct fusion in all cases . the minimum ablation margins measured using c - arm ct and mdct fused with the pretreatment images are listed in table 1 . a 66-year - old man with a 15-mm hepatocellular carcinoma . ( a ) an inverted hepatobiliary - phase mr image obtained before rfa showing the target tumor ( arrow ) at hepatic segment vii . ( b ) a transverse fusion image of intra - procedural contrast - enhanced c - arm ct and pretreatment hepatobiliary - phase mr image revealing insufficient ablation margins ( arrow ) due to the heat - sink effect of the adjacent portal vein ( arrowhead ) . ( c ) a fusion image of a portal - phase multi - detector ct image and a pretreatment hepatobiliary - phase mr image showing an insufficient ablation margin ( arrow ) and an adjacent portal vein ( arrowhead ) that are similar to those seen in the c - arm ct fusion image . minimum ablation margins of liver tumors after radiofrequency ablation as determined from fusion of pretreatment images and post - treatment c - arm ct and mdct images ablation margins are usually evaluated using side - by - side comparisons of transverse pre- and post - treatment image views . , we adopted a new image fusion software that can fuse multimodal image sets . using this technique , our results suggest that intra - procedural ablation margin assessment can be performed using a fusion image from c - arm ct images obtained during rfa and pretreatment ct / mr imaging . in this study , image fusion took approximately 1 - 2 min , allowing immediate decisions regarding further treatment . if insufficient ablation margins or residual lesions are detected on fusion c - arm ct images , additional targeted ablation can be promptly applied under c - arm ct guidance . conversely , when sufficient ablative margins are confirmed on c - arm ct images , additional therapeutic assessments with other imaging modalities may not be required . intravenous contrast - enhanced c - arm ct images fused with pretreatment images for the assessment of treatment efficiency demonstrated sufficiently delineated ablation margins after rfa of liver tumors . this novel technique may potentially be used in the intra - procedural assessment of liver tumor ablation .
the aim of this preliminary study was to evaluate the feasibility of assessing ablation margins after radiofrequency ablation ( rfa ) of liver tumors from fusion images of post - treatment c - arm computed tomography ( ct ) images fused to pretreatment images . 
 five patients with liver tumors underwent rfa . 
 intravenous contrast - enhanced c - arm ct images were obtained for all patients immediately after rfa , and multi - detector ct ( mdct ) images were obtained 3 - 7 days later . 
 the c - arm ct and mdct images were fused to pretreatment images using a multimodality image fusion software . 
 the minimum ablation margins were assessed in the c - arm ct and mdct fusion images . 
 ablation margins after rfa of liver tumors can be measured using intravenous contrast - enhanced c - arm ct images fused with pretreatment images . 
 this technique has the potential for use in the intra - procedural assessment of liver tumor ablation .