## 4 Discussion
The typical number of attempts that is required to access the PV has been reported to be between 3-5 attempts in the literature [16; 29], although many more attempts may be needed in clinical practice, compared to the single pass success rate of 93% (25/27) for TIPS and 88% (7/8) for DIPS procedures in this study, with the remaining procedures requiring only two passes. Additionally, the mean fluoroscopy time in all procedures was shorter than 5 min, compared to the reference level of 60 min that has been proposed for TIPS procedures, which was derived from a RAD-IR study [30]. These benefits increase the safety profile and accuracy of the procedure and greatly reduce potential radiation exposure. Compared to other existing and proposed methods, the 3 DFIG method provides users with the option to track the needle in real time, thereby facilitating the anatomical orientation by projecting fluoroscopic images onto 3D images to visualize the puncture target [11; 13; 20; 21; 22; 31; 32]. The planned puncture path can be adapted to different real-time projections, which allows the operator to make real-time reassessments of the path in 3D by simply adjusting the C-arm. The 3 DFIG method can enhance technical precision and accuracy during technically difficult TIPS procedures by providing: (1) improved 3D visual spatial orientation so the interventional radiologist can successfully puncture the portal vein; (2) 3D visualization of the patient's unique, and possibly distorted, anatomy; (3) the real-time tracking of the needle in 3D, in both the anteroposterior and lateral planes. Some potential disadvantages of the 3 DFIG method and limitations of our study were also identified. First, with the added steps of CBCT and fusion imaging processing, the procedure time may increase as CBCT and fusion imaging can be time-consuming. This may be especially true for centers that have minimal experience in CBCT and fusion imaging. However, using the current settings in our research angiography unit and an independent fusion imaging station, the process was significantly streamlined. With proper training and an effective protocol set-up, 3 DFIG procedures can be easily adapted. Furthermore, the time that could be saved by limiting the number of attempts that is needed using our method could potentially balance out, or even shorten, the overall procedure time. Another limitation of this study was that the 3 DFIG TIPS procedures were performed in normal porcine livers, which did not have cirrhosis or anatomical anomalies. Patients with difficult anatomies may produce lower success rates and may require more needle passes to achieve successful PV puncture [9; 12; 15; 18]. Consequently, the risk of peri-procedural complications, morbidity rates, procedure and fluoroscopy times and exposure to radiation also increase. However, this was a feasibility study, which tested the reproducibility of 3 DFIG portosystemic shunt placement. Even though the procedure was not performed in cirrhotic livers, it was performed in small porcine livers to mimic the size of a human cirrhotic liver. Potential future studies could employ the 3 DFIG TIPS procedure in cirrhotic animal models or in patients with cirrhosis as a clinical safety study. Other potential anatomic and physiologic considerations also remain, however. For example, polycystic liver disease (PCLD) is often listed as a contraindication for TIPS procedures due to risk of cyst rupture and hemorrhage. However, a case of PCLD was reported to have undergone successful TIPS insertion with the aid of a hybrid 2D and 3D imaging system [33]. PV and HV thrombosis have historically been considered to be relative contraindications due to the technical difficulties in placing a shunt in abnormal anatomies of portal and hepatic veins, although recent evidence has suggested that a TIPS, when placed successfully, is in fact crucial for the management of these patients [34].