This study showed that IVUS liver navigation using a catheter proved to be a powerful tool to explore the liver without exposure to radiation (or the need for other medical imaging systems). The image quality of the liver parenchyma, vascular structures and biliary elements, as well as intravascular navigation, exceeded expectations, inspiring researchers to envision several potential indications in the liver and also beyond it.
This approach has several strengths and advantages. The first advantage is related to the 4 degrees of freedom (pitch, roll, yaw, and translation), its steering properties and real-time images, facilitating intrahepatic navigation. As a result, high-quality images beyond third-order vascular structures (PV and HV) can be acquired from the IVC, but also from the interior of any cannulated HV. Peripheral and central branches, usually inconspicuous due to their deep location inside the liver, can be observed as well. In addition, the intravascular space avoids gas-related interferences and liver deformations, contributing to a secondary point of view, which can work synergically with other medical imaging techniques. Doppler/color modes were essential tools during the entire experiment, and mandatory to deal with the HA and its branches. Similarly, other US imaging modalities such as shear wave elastography, contrast-enhanced ultrasound and image fusion which are currently unavailable for the abovementioned IVUSc, could well enhance this approach and extend indications even further.
A second and considerable advantage is the lack of exposure to ionizing radiation of the patient and the team, the harmful effect of which has been extensively demonstrated among physicians performing interventional procedures. It is reported to be associated with an increased incidence of lens opacities and left-sided brain tumors [17,18,19]. Gottardi et al. [13] reported the need for extra doses of ionizing radiation to guide and verify IVUSc positions, and major limitations to access the left hepatic vein resulting in incomplete evaluations. Although we used porcine models, we managed to navigate the entire liver without radiation, and it is also worth mentioning that the combination of IVUS and other US systems (percutaneous, laparoscopic, etc.) should ideally be considered as an alternative to X-ray, or at least be implemented as a first step before the use of X-ray-based techniques, in order to potentially decrease the radiation dose (Fig. 4).
Among a wide list of potential translations of this approach to other organs, we can envisage multiple indications in the liver and beyond it, using the jugular approach and also femoral vessels (maybe also avoiding general anesthesia). US is commonly used for the intraoperative detection and location of liver lesions, and as a guidance to demarcate and assess liver transections (open, laparoscopic, and robotic). However, this extrahepatic US guidance cannot be performed simultaneously, requires multiple instrument iterations, and is more demanding, extending surgical time. With the IVUSc, both problems can be addressed simultaneously, planning the transection line, its demarcation, and real-time instrumental visualization. As Abdelaziz et al. [20] described in the perioperative liver transplantation phase, ultrasound represents a powerful tool which can be used on demand and in real time to assess the patency and/or potential complications of vascular anastomoses. Considering that these patients usually have a central line during this period, we might envision a role for IVUS, avoiding the use of transesophageal echocardiography as reported by Khurmi et al. [21].
In the field of image-guided minimally invasive procedures, this IVUS approach can be conceived as a guidance system, allowing and enhancing the guidance, navigation, and control modalities of needle-based procedures, relying upon two real-time US systems (external and intravascular), with two different and synergic points of view. Even more, in the dynamic and growing field of liver ablation, some weaknesses of this technique in terms of precision and accuracy, as well as the real-time control of the results (“shadowing effect”), can be potentially mitigated. With this approach, we can expect to considerably decrease the X-ray dose (and maybe avoid it completely), as well as decrease contrast medium volumes, providing better images than the reported use of IVUS during TIPS/DIPS [6,7,8,9,10,11,12], always relying on X-rays and placing the catheter in the interior of the IVC. In addition, this hepatoportal communication can be used to access the PV with IVUS navigation, seeking diagnostic or interventional intentions. Extending previous works such as the one of Kaneko et al. [22] describing the role of the intraportal endovascular ultrasound assessment to determine portal venous invasion, and the report of Chick et al. [23] using IVUS guidance to perform a transbiliary biopsy of pancreatobiliary carcinomas, further potential indications can be conceived beyond the liver and related to the exploration of abdominal regions which are traditionally difficult to approach with US (e.g. pancreatic necrosis, borderline resectable pancreatic cancer, or retroperitoneal hematoma, etc.), or even outside blood vessels (e.g. biliary tree exploration).
This study has the limitations of a proof-of-concept translational study, and it should be tested and evaluated exhaustively before being used in humans. The abovementioned technique is demanding (HV cannulation/US planes recognition), and will potentially have steep learning curves, even in professionals with US skills. At this point, it is worth mentioning that ex vivo models such as the one developed for this study, are good enough to acquire these skills and abilities before moving on to living tissues. The lack of contrast enhancement, shear-wave elastography, and image fusion capabilities put this approach at a disadvantage as compared to other US assessments, so their incorporation might be important. The damage to blood vessels (thermal/mechanical) when in direct contact with them should be meticulously studied. Concerning the used catheter, when it is manipulated as a vascular catheter without control of the steering knobs, the tip can be bent and the steering knobs remain in a neutral position, thereby producing an inversion of the handle system (e.g. right–left mirror images).