Lobar volume increase and duct spreading
In patients with lobar or segmental liver atrophy, the non-atrophied parenchyma may undergo compensatory hyperplasia (more often referred to as hypertrophy, even though histologically the volume increase results from hyperplasia) (Figs. 1, 2, 3, 4, 6, 7, 8) [3, 5]. Compensatory hypertrophy is common [4, 8] and is important to recognise as it helps with recognition of contralateral atrophy, and has critical importance in planning surgical resection or interventional approaches to biliary decompression [3, 7].
Contrary to the crowded appearance of bile ducts in an atrophic lobe, those in a hypertrophied lobe appear relatively spread out (Figs. 1, 2). This spreading is most obvious when it affects the left lateral segments with resulting increased separation of the segment II and III ducts (Fig. 2c).
The relative occurrence of compensatory hypertrophy in patients with right versus left lobe atrophy has received little attention in the English literature. In two small series, one reported an equal rate of compensatory hypertrophy [8], whereas the other showed a higher rate in patients with right lobe atrophy [20].
In our experience compensatory hypertrophy is more common in patients with right lobe atrophy than left lobe atrophy, possibly because right lobe hypertrophy is more difficult to perceive.
In left lobe hypertrophy, the main increase in dimensions occurs in the axial plane on CT, which is the standard imaging plane (Figs. 1, 2). Normally, as mentioned above, the left liver lobe is substantially smaller than the right. Therefore, left lobe hypertrophy as a percentage change from baseline volume may be more substantial and therefore more obvious. Left lobe hypertrophy extends towards the left (Fig. 2a), and the left lateral segments tend to extend inferiorly into the central abdomen (Fig. 1c).
In cases of right lobe hypertrophy, the increase in dimensions is more obvious in the coronal plane (Fig. 4c). In the axial plane the hypertrophied right lobe (or segments) tends to show a prominent convex or “bulbous” contour of the visceral surface (Figs. 3, 4, 7, 8). To our knowledge, this finding has not previously been described. As the right lobe is normally larger than the left the degree of hypertrophy has to be more marked for it to be conspicuous. The recognition of the “bulbous” contour sign as well as the use of coronal CT or MRI should allow more ready recognition of right lobe hypertrophy.
Caudate lobe
In the presence of right or left lobe atrophy the caudate lobe may be atrophied or hypertrophied or neither (Figs. 1, 2, 3). In our experience the latter is the most common.
The development of atrophy or hypertrophy is likely to be determined by the involvement of caudate bile duct obstruction, which is variable in hilar malignant obstruction, as well as the status of the left portal vein, which provides the dominant portal supply to the caudate lobe [20, 21].