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The study in this issue of HEPATOLOGY by Saito et al.1 is another evolutionary step in the increasing ability of the radiologist to take the liver apart in virtual reality before the surgeon does so in the operating room. Why is an accurate measurement of liver volume important to the liver surgeon and hepatologist? The 2 major reasons are (i) for resection of liver tumors, and (ii) in living donor liver transplantation; accurate measurement of both the planned donation volume and residual donor volume is important for the success and safety of such procedures. Historically, anthropometric and radiological methods have been used for measuring liver volumes, but there have been significant shortcomings because of variability in body habitus in different populations, and poor standardization of radiological methods.

Anthropometric data to estimate liver volume are based on height, weight, body surface area, age, and sex,2, 3 with the addition of other anthropometric measures such as abdominal or thoracic dimension in other formulas.4 Estimates from such formulas have a value in population studies, but have a predictive value ±20% in the context under discussion.2 Variability due to overall body habitus, particularly the effect of obesity, sex, and racial differences, has limited their value. In addition, such methods do not allow for differences in lobar volumes: the right liver (segments V through VIII) has been shown to vary between 49% and 82% of total liver volume.5

Radiological methods have shown some improvement in liver volume estimates compared with anthropometric data. Early studies of liver volume measurement with computed tomography (CT) scan6 traced serial 1-cm liver slices and summated them: day-to-day variability was ±6%, and interobserver variability ±5%. Increasing sophistication using CT or magnetic resonance imaging (MRI) has progressively refined this basic method with the ability to separately measure right and left liver volumes based on the middle hepatic vein defined by an observer. Further subsegmentation also can be defined. These methods have become the current standard in assessing residual liver volume for resection5, 7 and in living donor transplantation.8–10 These methods estimate liver volume to within 10% of actual volume at the time of resection.

In the paper in this issue of HEPATOLOGY,1 the authors present a method that predicts liver segmental volume based on the portal and hepatic venous vasculature. The key issues to be considered in evaluating this paper are: (1) the validity of the method; (2) the practicality of the method; (3) is there a role for this method in clinical practice, and (4) will it supplant current methods? This paper brings several new factors to the field. Advances in liver imaging, with more sophisticated scanners allowing more rapid image acquisition, and increasing sophistication in the software for image reconstruction, are key to this technique. This study had also emphasized the importance of a multidisciplinary approach, with radiologists, hepatologists, and surgeons capitalizing on this technology. Each has brought different perspectives to the table and applied them to a clinical question. This collaborative approach by surgeons, radiologists, and industry give us a new way of defining liver segments. Let us briefly consider the implications of this on the questions posed above.

First, is this a valid method? The concept of liver segments being based on vascular anatomy first proposed by Couinaud,11 is a valid concept, and is the basis on which much of modern liver surgery is based. Segmental vascular inflow and outflow are real entities, and this method builds its 3D reconstruction on the 2 axes of inflow and outflow. Defining these axes requires anatomical knowledge of the 8 liver segments and radiological interpretation. Simulating the parenchyma perfused by the portal venous inflow and drained by the hepatic venous outflow is defined and refined with computer software. The segments can be clearly delineated with this technology, and the authors of this manuscript show that their calculated volumes correlate well with subsequent resection volumes. This method passes the test for validity.

Second, how practical is this method for clinical use? Here, perhaps we run into some limitations because volume of the dye required to define the segments requires arterial portography. In the method described, a catheter needs to be placed by an angiographer in the superior mesenteric artery to deliver a high volume of contrast through to the portal venous phase. It is not standard practice to use such an invasive procedure for CT imaging. The question must be asked whether adequate portal venous imaging can be attained with the more conventional peripheral intravenous dye injection bolus. If this can indeed be achieved as the technique evolves, it would greatly improve the practicality of using this method. In addition, the sophisticated software program developed by the authors and required for the simulation analysis, is both expensive and time consuming. As new methods are more widely used, case volume increases, and this brings down the cost of equipment and software. However, the time and expertise required for the simulation analysis will remain a moderately high, fixed cost. The practicality of this method therefore, raises some questions that are not insurmountable, but must be considered.

Do we need this new method? In many respects, this is the most crucial question, which must be turned back to the clinician, and specifically, the surgeon taking care of such patients. It is said that “knowledge is power,” and one can make an argument that the knowledge of the hepatic vascular and segments acquired with this method is indeed powerful. More accurate planning of segmental liver resections and planning of resection for living donors for transplantation could indeed be achieved with wider application of this method. Conversely, the argument can be given that the currently available, less sophisticated imaging and reconstruction with standard spiral CT or magnetic resonance technology are adequate for these operative procedures. This question therefore remains unanswered but clearly needs to be considered as this method is rolled out to a wider audience and increasingly used in clinical practice. Comparative study, cost analysis, and even consideration for a randomized trial documenting advantage for the new method over standard practice are the steps that need taken to definitively answer this question. This new method is not yet ready to replace existing methods.

Finally, the issue of liver volume changing with blood volume and perfusion pressure has not been addressed. All of the above imaging methods are measuring liver volume in situ, which is the standard. However, the comparison of these preoperative measurements with weighed nonperfused liver segments has not considered the effect of lost blood volume or the perfused state of the isolated segment. An interesting experimental study addresses this issue in a porcine model,12 which measures liver volumes in situ and after explantation at physiological hepatic vein perfusion pressures. The correlation of these liver volumes was excellent at 4% ± 5%; however, the liver volumes before and after fluid infusion showed a 33% difference. These data thus raise the currently unanswered question of the effect of blood volumes and perfusion pressure on liver volume?

In summary, this paper brings us a new method of looking at liver segments and liver vasculature that adds a further “small step” rather than a giant leap in our ability to take the liver apart in virtual reality. As a surgeon, I can see this method allowing us to assess our patients in a more sophisticated way before surgery, improve our preoperative planning, be a useful tool for education, and perhaps at the end of the day, be of benefit to our patients.

References

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  2. References
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