Readings from the two independent radiologists are summarized in Table 1.
Of the 50 patients included in the study, 32 (64%) had normal portal vein anatomy on CT scan and 18 (36%) showed anatomic variants. Typically, the MPV divides into right and left branches in the hepatic hilum, with the right portal vein (RPV) subdividing into right anterior sectorial branch (RASB), supplying segments V and VIII, and right posterior sectorial branch (RPSB), supplying segments VI and VII. The left portal vein (LPV) supplies hepatic segments I, II, III, and IV.
This is considered the typical branching pattern of the MPV (type I) and is the most common branching pattern, occuring in 65-80% of population (Figure 1). [1,4,5].
The most common MPV variation was trifurcation, present in 12 patients (24%). The second most common variation was the RPSB originating directly from the MPV which was noted in 4 (8%) patients. The less common variants were the RASB arising from the LPV and an isolated and independent segment VIII branch originating from the MPV, both present in 1(2%) patient each (Table 2).
Comparing the reading results from CT with those depicted on portography, an agreement of 49 out of 50 patients (98%, p < 0.001) was documented (Table 1).
One patient classified as having a trifurcation variant on portography was described as having normal anatomy on CT. This discordance was probably related to the non-optimal arch rotation during DSA acquisition. In this patient direct portography was acquired with no tube angulation which hided part of the right portal trunk (Figure 2). Ideally portography should be acquired with a 25 degree right anterior oblique projection. [5, 6]
Variations of portal vein branching pattern are common and are found in about 20-35% of the population. [3,4]
The most common anatomic variants of the portal vein are:
- Trifurcation - MPV divides into the RASB, RPSB, and LPV at the same level. It is the most common variant and can be present in 7.8-10.8% of population (Figure 3). [3,7]
This branching pattern was also the most common variant in our patients.
- RPSB arising from the MPV - The first branch to split off is the right posterior branch and then the MPV continues to the right for a variable distance and bifurcates into the right anterior and left portal veins. Present in 4.7-5.8% of cases (Figure 4). [3]
Also the second most common variant observed in our patients.
- RASB arising from the LPV - MPV divides into the right posterior and LPV, and the right anterior portal vein arises from the LPV. Accounts for 2.9%– 4.3% of cases (Figure 5). [3]
On our study this variant was presented in 1 patient (2%).
Less common PV variations have been described, such as quadrifurcation, absence of portal vein bifurcation, LPV arising from the RASB and PV duplication, but their incidence has not been found to be higher than 2%. [4]
In addition to anatomic variations of the MPV, there are other important variations of the right portal vein branching pattern, although they probably have a more limited impact on most surgical and interventional procedures.[5,7]
A predominance of anatomical variations has been documented in males (13 of 18 variants found) in our study, although there is no documented evidence in the literature on their predominance between genders.
Recognition of portal vein anatomy and its most frequent variants has significant implications in liver surgery, liver transplantation and interventional radiology procedures. During portal vein embolization, anatomical variants might increase procedure complexity and risk of nontarget embolization with potential harm to the future liver remnant. Moreover, recent studies showed that portal vein variants might even influence hypertrophy results after PVE, where patients with complex anatomy exhibited significant less regeneration after this preoperative procedure. [8]