2 The normal hepato-biliary system

Introduction

Just in case you skipped the first chapter, I shall repeat an important message. The success, in terms of diagnostic information, of an abdominal ultrasound scan is dependent on numerous factors, the most important of which is the skill of the operator.

Familiarity with normal anatomy, and technical expertise in comprehensively examining an organ are essential. The latter can be obtained only by practice in a clinical environment under proper supervision. Having comprehensive medical and anatomical knowledge is necessary, but it does not enable you to scan, any more than being an air traffic controller enables you to fly an airliner (Box 2.1).

It is good practice, particularly on the patient’s first attendance, to scan the whole of the upper abdomen, focusing particularly on the relevant areas, but also excluding or identifying any other significant pathology. A full abdominal survey would usually include the liver, gallbladder, biliary tree, pancreas, spleen, kidneys and retroperitoneal structures.¹ Apart from the fact that many pathological processes can affect multiple organs, a number of significant (but clinically occult) pathological processes are discovered incidentally, for example renal carcinoma or aortic aneurysm. A thorough knowledge of anatomy is assumed at this stage, but diagrams of upper abdominal sectional anatomy are included in the appendices to this chapter for quick reference (see Chapter 2 Appendices, pages 47–50.)

General pointers on upper abdominal technique

• It is neither desirable nor possible to be prescriptive in ultrasound scanning, but there are some general approaches that help to get the best from the scanning procedure:

• Scan in a systematic way to ensure the whole of the upper abdomen has been thoroughly interrogated. The use of a worksheet, indicating structures to be examined, is helpful when learning.¹

• Always scan any organ in at least two planes – preferably at right angles to each other. This reduces the risk of missing pathology and helps to differentiate artefact from true pathology.

• Where possible, scan in at least two patient positions. It is surprising how the available ultrasound information can be enhanced by turning your patient decubitus or erect. Inaccessible organs flop into better view and bowel moves away from the area of interest.

• Use a combination of subcostal and intercostal scanning for all upper abdominal scanning. The different angles of insonation can reveal pathology and eliminate artefact. Good technique is about finding a good acoustic window.

• Do not limit yourself to longitudinal and transverse sections. Use a variety of planes and angulations. Trace ducts and vessels along their courses. Use the transducer like a pair of eyes.

• Deep inspiration is useful in a proportion of patients, but not all. Sometimes it can make matters worse by filling the stomach with air and obscuring structures. An intercostal approach with the patient breathing gently often has far more success.

• Positioning patients supine, particularly if elderly or very ill, can make them breathless and uncomfortable. Raise the patient’s head as much as necessary – a comfortable patient is much easier to scan.

• Images are a useful record of the scan and how it has been performed, but do not make these your primary task. Scan first – sweeping smoothly from one aspect of the organ to the other in two planes, then take the relevant images to support your findings.

• Make the most of your equipment (see Chapter 1). Increase the confidence level of your scan by fully utilizing all the available facilities; using Doppler, tissue harmonics, changing transducers and frequencies and manipulating the machine settings and processing options.

The liver

Normal appearances

The liver is a homogeneous, mid-grey organ on ultrasound. It has the same, or slightly increased echogenicity when compared to the cortex of the right kidney. Its outline is smooth, the inferior margin coming to a point anteriorly (Fig. 2.1). The liver is surrounded by a thin, hyperechoic capsule, which is difficult to see on ultrasound unless outlined by fluid (Fig. 2.2).

Fig. 2.1 • LS through the right lobe of liver. The renal cortex is slightly less echogenic than the liver parenchyma.

Fig. 2.2 • (A) The capsule of the liver is a thin, hyperechoic layer seen anteriorly.

(B) It is much better appreciated at higher frequencies (arrows) (7.5 MHz).

The smooth parenchyma is interrupted by vessels (see below) and ligaments (Figs 2.3–2.15), and the liver itself provides an excellent acoustic window onto the various organs and great vessels situated in the upper abdomen.

Fig. 2.3 • LS through the right lobe of liver and right kidney.

Fig. 2.4 • LS right lobe, just medial to the right kidney.

Fig. 2.5 • LS through the right lobe, angled medially towards the IVC.

Fig. 2.6 • LS midline, through the left lobe, angled right towards the IVC.

Fig. 2.7 • LS through the midline.

Fig. 2.8 • LS just to the left of midline.

Fig. 2.9 • LS, left lobe of liver.

Fig. 2.10 • TS through the liver, above the confluence of the hepatic veins.

Fig. 2.11 • TS at the confluence.

Fig. 2.12 • TS at the porta hepatis.

Fig. 2.13 • TS through the right kidney.

Fig. 2.14 • TS at the epigastrium.

Fig. 2.15 • TS at the inferior edge of the left lobe.

The ligaments are hyperechoic, linear structures; the falciform ligament, which separates the anatomical left and right lobes, is situated at the superior margin of the liver and is best demonstrated when surrounded by ascitic fluid. It surrounds the left main portal vein (PV) and is known as the ligamentum teres as it descends towards the infero-anterior aspect of the liver (Figs 2.9, 2.15). The ligamentum venosum separates the caudate lobe from the rest of the liver (Fig. 2.6).

The size of the liver is difficult to quantify, as there is such a large variation in shape between normal subjects and direct measurements are notoriously inaccurate. Size is therefore usually assessed subjectively. Look particularly at the inferior margin of the right lobe which should come to a point anterior to the lower pole of the right kidney (Fig. 2.1). A relatively common variant of this is the Reidel’s lobe, an inferior elongation of segment V1 on the right. This is an extension of the right lobe over the lower pole of the kidney, with a rounded margin (Fig. 2.16), and is worth remembering as possible cause of a palpable right upper quadrant ‘mass’.

Fig. 2.16 • LS through the right lobe, demonstrating a Reidel’s lobe extending below the right kidney. (Compare with the normal liver in Figure 2.1).

To distinguish mild enlargement from a Reidel’s lobe, look at the left lobe – if this also looks bulky, with a rounded inferior edge, the liver is enlarged. A Reidel’s lobe is often accompanied by a smaller, less accessible left lobe, with the overall liver volume remaining normal.

The segments of the liver

It is often sufficient to talk about the ‘right’ or ‘left’ lobes of the liver for the purposes of many diagnoses. However, when a focal lesion is identified, especially if it may be malignant, it is useful to locate it precisely in terms of the surgical segments. This allows subsequent correlation with other imaging, such as computed tomography (CT) or magnetic resonance imaging (MRI), and is invaluable in planning surgical procedures.

The segmental anatomy system, proposed by Couinaud in 1900,² divides the liver into eight segments, numbered in a clockwise direction. They are divided by the portal and hepatic veins and the system is used by surgeons today when planning surgical procedures (Fig. 2.17). This system is also used when localizing lesions with CT and MRI.

Fig. 2.17 • (A) The surgical segments of the liver (after Couinaud ¹).

(B & C) Segmental anatomy in TS of the liver.

Identifying the different segments on ultrasound requires the operator to form a mental three-dimensional (3D) image of the liver. The dynamic nature of ultrasound, together with the variation in planes of scan, make this more difficult to do than for CT or MRI. However, segmental localization of hepatic lesions by an experienced operator can be as accurate with ultrasound as with MRI.³ Systematic scanning through the liver, in transverse section, identifies the main landmarks of the hepatic veins (Fig. 2.11) separating segments VII, VIII, IV and II in the superior part of the liver. As the transducer is moved inferiorly, the portal vein appears, below which segments V and VI are located.

Hepatic vasculature

The portal veins radiate from the porta hepatis, where the main portal vein (MPV) enters the liver (Fig. 2.18). The veins are encased by the hyperechoic, fibrous walls of the portal tracts, which make them stand out from the rest of the parenchyma. Also contained in the portal tracts are a branch of the hepatic artery (HA) and a biliary duct radicle. These latter vessels are too small to detect by ultrasound in the peripheral parts of the liver, but can readily be demonstrated in the larger, proximal branches (Fig. 2.19).