Chapter: Liver Region

Liver Anatomy How To Intercostal Approach

Liver Region Imaging

The IMBUS-Advanced exam of the liver region is more than just looking at the gallbladder and bile ducts.

A clinic IMBUS exam of the liver region is most frequently requested for patients with subacute epigastric or right upper quadrant discomfort that suggests gallbladder (GB) disease. This exam may also be needed as part of the exam of systemically ill patients and patients with lab abnormalities suggesting hepatobiliary disease. Thus, it is important to learn to do a comprehensive, but efficient, exam that includes the liver parenchyma, hepatic veins (HV), portal veins (PV), hepatic ducts (HD), common bile duct (CBD), and GB. This chapter will discuss anatomy, technique, and normal findings while the next chapter will cover abnormal findings in the liver region.

The liver region exam varies from patient to patient depending on the location of the transverse colon, the amount of adipose tissue, and when the patient last ate. An IMBUS-advanced physician needs to know several approaches to the exam to deal with these variables. Whatever the approach, knowledge of key anatomic details is important for a good and efficient exam.


LIVER ANATOMY

The following frontal anatomy view, with anterior structures removed, suggests that it should be easy to examine the liver and GB. Yet, when muscles, connective tissue, adipose tissue, and digestive tract contents are added the exam is more difficult.

The liver is divided into functional segments based on the HV and PV divisions. The concept is simple and it is not necessary to know the segments by number. There are three HVs that drain cephalad to the IVC. The middle HV (MHV) divides the liver into the main right and left lobes and then the right and left HV (RHV, LHV) further divide each main lobe in half to create longitudinal quarters. This is shown schematically in the following diagram with the HVs in blue.

The longitudinal quarters are divided in the transverse plane by the right and left PV (RPV, LPV), shown in purple in the diagram above. Ignoring the numbering system, it is enough for us to know we are viewing the “superior, medial portion of the right lobe”.

More detail of the vein anatomy is shown in the following view as seen with the body turned about half way toward the left lateral decubitus position.

Notice that the RHV may be two separate veins draining into the IVC. Thus, the HVs are often seen one or two at a time unless a particular oblique view is obtained, as in the following clip. Notice that the HVs have thin walls that are minimally hyperechoic.

In contrast, intra-hepatic portal veins have hyperechoic walls. The following clip shows a left portal vein with hyperechoic walls.

 

Porta hepatis:

The porta hepatis (PH) is a fissure on the inferior surface of the liver through which the neurovascular bundle and the bile ducts enter and exit. At the level of the PH separate right and left HD join to form the common HD.

A little further caudad the common HD is joined by the cystic duct (rarely seen with IMBUS) to form the CBD. The CBD then runs vertically through the medial margin of the head of the pancreas into the duodenum.

The following frontal view diagram emphasizes the important relationships in this extra-hepatic region. The IVC is always the deepest, largest vessel. The PV runs oblique and anterior to the IVC. The distal CBD is anterior to the PV and runs mostly parallel to the lateral edge of the IVC while the hepatic artery is just posterior to the CBD and more closely parallel to the PV. Near the PH, the common HD more closely parallels the orientation of the PV, but it is still anterior to the PV and the HA.  

 

Gallbladder and bile ducts:

The GB, cystic duct, HD, and CBD have anatomic variants that won’t be detailed here. But, the GB always lies on the inferior surface of the liver. Below is the most common anatomy with a view looking up at the undersurface of the liver. The GB fundus is peeking out below the inferior border of the right liver lobe. The GB body and neck then run obliquely posterior and medial on the inferior liver surface toward the pancreas and duodenum. The orientation of the GB, viewed from the anterior surface of the liver, is typically with the fundus at about 8:00 and the neck at 2:00.


HOW TO IMAGE

The curvilinear probe is used with an abdominal preset. The standard radiology ultrasound exam includes a parasagittal and transverse subcostal sweep and a patient who has been fasting for 6 hours. This subcostal approach is traditionally performed using the left lateral decubitus patient position and patient inhalation to help move the liver into better view.

We have noticed that some patients have improved views of the liver region when a standing patient position is used and it is reasonable to have this as a default clinic approach. The standing position may also make for a quicker exam and be easier for patients with mobility troubles, as long as they can stand for the required minutes.

CAUTION: Two new variables are introduced by the standing position. First, standing quietly for a number of minutes may cause pooling of blood in the legs. Secondly, patients are able to see the screen during parts of the exam, which may cause them to ask questions and become concerned. Anecdotally, we have seen a patient become faint during a standing exam. Patients can be positioned so they do not see the screen during the exam

Many clinic patients have cephalad transverse colons and/or thick adipose layers and these produce suboptimal subcostal windows. In addition, many are not fasting. If the standing position does not help the subcostal approach, switch to an intercostal approach in the standing or supine position. Patient exhalation will help the intercostal approach because it moves lung out of view and elevates the liver above the costal margin.

With any imaging approach a short halt in breathing is important for best viewing of key structures. Don’t try and evaluate everything in constant motion.

Here are the overall things to evaluate:

  1. Form and overall size of the liver
  2. Capsular contour (smooth, coarse, lobulated)
  3. Liver echogenicity compared to the kidney parenchyma
  4. Homogeneity of the liver parenchyma; masses, collections of things
  5. HV size and patency
  6. PV size and patency
  7. GB size and structure
  8. HD and CBD size

 

Liver Size:

Expert groups don’t agree on a particular measurement of liver size because the organ varies in shape and is dependent on patient size. As a European US course book puts it, “Liver size measurement has no impact in daily routine because there is no reliable and reproducible ultrasound method established so far.” There is agreement that a liver extending caudad of the right kidney is enlarged, as long as it isn’t just a downward elongation of an isolated lateral portion of the right lobe of the liver (Riedel variant). The caudad extension of the liver can be seen with either subcostal or intercostal views through the right liver lobe. Here is a clip of a longitudinal subcostal view showing extension of the liver below the right kidney, but this extension was isolated to the lateral right lobe so it was a Riedel variant.

 

Another marker of hepatomegaly is a rounded longitudinal inferior edge of the left liver lobe instead of the normal acute angle. The following clip shows a normal acute angle.


THE SUBCOSTAL APPROACH

This is the traditional approach. It is usually immediately obvious when the subcostal windows are limited by colon and adipose tissue; immediately switch to the intercostal approach. Repeated patient inhalation may bring the liver caudad for better viewing during the subcostal approach.

Step 1: Transverse high epigastric

Begin in the high left epigastrium with a parasagittal view to see the angle of the lower edge of the left liver lobe. Then rotate to a transverse probe position over the left lobe of the liver and fan through the parenchyma, including the left portal vein. Here is the hand and probe position.

Look posterior to the liver for the splenic vein, which leads to the PV. In the following patient the physician found the splenic vein and showed it flowing into the origin of the PV.

 

The PV and the IVC are the guides to the distal CBD, which runs anterior to the PV but mostly parallel to the right edge of the IVC. The distal CBD enters the head of the pancreas at the level of the PV origin.

Once the origin of the PV is found, slide the probe so the IVC and PV are more centered. Then, rotate slowly toward 12:00, staying to the lateral edge of the IVC. Watch for the CBD to appear anterior to the PV. Here is the result of that positioning in the patient just above. The easily seen distal CBD is entering the head of the pancreas. This tube was shown to be non-vascular with Color.

 

In the next clip the probe rotation toward 12:00 and slide to the lateral edge of the IVC was repeated a little higher in the epigastrium of the same patient, giving a slightly different view of the vessels and CBD. The IVC is mostly long axis while the PV is oblique.

 

Normal CBD diameter is about 1 mm per decade of life after about age 40, but can be a little larger in post-cholecystectomy patients. Normal CBDs are thin by eyeball and don’t need to be measured after a modest amount of training. An enlarged CBD immediately grabs attention because it is half or more the diameter of a normal PV. In the patient immediately above the PV had a diameter of about 1.0 cm and the CBD was easily less than half as wide, so clearly normal.

 

Step 2: Transverse subcostal sweep

Move the probe obliquely underneath the beginning of the right costal margin. The indicator should point to the patient’s right hip (toward the physician). Fan as anterior as possible to see the diaphragm and then begin a slow posterior fan through the liver. The HVs will usually appear anteriorly as they join the IVC if the view is medial enough. The HVs may enter at slightly different locations and were shown at the beginning of this chapter.

Fanning a little posterior from the HV, an obliquely or horizontally oriented right PV should appear. This has been described as looking like an animal jumping over the IVC toward the patient’s left. Most of the animal is the long-axis right PV with its hyperechoic walls. Because this view is higher in the liver, the bile duct to be seen is the thinner HD and it runs right on top of the PV. Here is a PV looking a bit like a dog jumping over the IVC. A thin HD runs along the back of the dog.

 

Fanning posterior from the PV, the GB will often appear, seeming to come out of the tail of the animal. The cut through the GB depends on how a patient’s GB lies on the inferior liver surface. Gallstones can be observed during the posterior fan and GB wall thickness (normal < 5 mm) may be measured at the anterior wall that is interfacing with the liver. However, a contracted GB is not always easy to see. Here is a clip of a partially contracted GB during this posterior fan. The patient had eaten a heavy quantity of fat about 1.5 hours before the exam.

 

Step 3: The long axis GB

Slide one probe width at a time to the patient’s right underneath the right costal margin, fanning anterior and posterior through the liver parenchyma. At some point, the fundus of the GB should appear at the inferior liver margin. Here is a typical subcostal probe position where the GB fundus might be seen.

When the GB is glimpsed, rotate the probe slowly clockwise (indicator moving anterior) until the best GB long axis is found. This is usually with the indicator about 12:00-2:00 during the rotation, depending on various patient factors. Below is a fasting patient with a GB in long axis. In this patient, the PV was short axis, forming the classic “exclamation point”. The aorta and IVC are to the right below the GB with the IVC partially obscured by the shadow from duodenal contents.

 

It is important for the first view of the GB to be deep enough to be able to see any shadows coming off of stones, particularly in the neck of the GB, where stones can hide. Fan through the long axis GB, which can be up to 10 cm in length and 4 cm in width in the fasting state.

Bile ducts: The variability of the GB position means that many patients won’t have an exclamation point appearance of the GB and PV  It doesn’t matter. Focus on the PV and the IVC. Rotate the probe toward 11:00 - 12:00 while sliding or fanning to the lateral edge of the IVC. The CBD will appear anterior to the PV. If necessary, check Color/Power Doppler to see signal in the PV and IVC but no flow in the bile duct.

The following clip is from the patient above with the exclamation point GB/PV. The probe was rotated clockwise until the IVC was mostly long axis. The PV was partly long axis and the GB was fading out of view. When the probe was moved to the lateral edge of the IVC a 3 mm CBD was visualized heading into the pancreas anterior to the PV.

 

Contracted GB versus duodenum: Since most clinic patients have not fasted for six hours, contracted GBs are common and the duodenum can be mistaken for the GB. The next subcostal view near the right liver lobe showed a structure that a novice could mistake for a GB filled with small stones, because of a posterior shadow. However, notice that the outer wall of this structure is HYPOechoic and any shadow seen is “dirty”. This is duodenum.

 

In contrast, here is a long axis view of a partially contracted, otherwise normal GB, showing a HYPERECHOIC outer wall, a middle hypoechoic layer, and an inner hyperechoic layer, distinguishing it from the duodenum. In a contracted GB, the hypoechoic middle layer is not edema in the wall. Do not measure the wall thickness on a contracted GB.

 

The rest of the right liver lobe: After finishing the GB and bile duct exam, continue sliding transversely under the rest of the right costal margin, fanning anterior and posterior through liver parenchyma. Past the mid-clavicular line, the kidney should appear posteriorly. If the liver is large, it would extend below the kidney. The following is a subcostal view of normal liver and kidney. This can be an excellent view for assessing the echogenicity of the liver next to the cortex of the kidney. Fanning further caudad can determine if liver extends beyond the kidney.


THE INTERCOSTAL APPROACH ANATOMY

Patients with “high” transverse colons or obesity may have suboptimal subcostal views. When this occurs, use the intercostal spaces. The following image shows the intercostal spaces that are open caudad of the lungs but still cephalad of where the colon causes trouble. All of this space is available to view the liver region. A supine position and patient exhalation may be best for this approach to maximize liver behind the ribs with lungs more cephalad.

Wherever the probe is placed in the intercostal spaces, the physician needs to be aware of how the GB, PV, HV, IVC, and bile ducts are oriented. Here is how the GB typically lies from the lateral intercostal spaces.

 

Intercostal method:

Deep inspiration is NOT an advantage because lung should be kept out of the lower intercostal spaces. Have the patient breathe quietly and periodically hold an exhalation for best viewing. All of the views of structures discussed with the subcostal approach can be reproduced in the intercostal spaces as long as the physician focuses on the anatomy relationships. Begin with the caudad intercostal spaces in about the mid-axillary line. Slide the probe between the lung border and the costal margin. Push into the spaces to keep rib shadows away and fan side to side. Then, move cephalad one interspace and slide through that interspace between lung and costal margin.

Follow the orientation of the interspaces and survey each one., The IVC, HV, PV, GB and bile ducts will be better seen closer to the epigastrium. The intercostal spaces just to the patient’s right of the epigastrium can give a good parasagittal view of the IVC, PV, and bile duct, even though a rib shadow may be present. However, this view is usually near the porta hepatis so it is typically common hepatic duct or even right hepatic duct that is visualized, rather than the distal CBD. This is still adequate to assess bile duct obstruction as long as the normal modestly smaller caliber of the more proximal bile ducts is considered.

Here is a fully intercostal view of a GB as it was first seen just to the right of the epigastrium.

The probe was then rotated toward 12:00 and the long axis IVC, short axis PV, and mostly long axis GB were imaged. A normal bile duct is present just anterior to the PV. Decreased depth could have been used if more CBD detail had been needed but the duct was much smaller than the diameter of the PV so it was normal.