Hepatic Veins (1)

Hepatic Veins (1) Images #1 and #2 are transverse views obtained subcostally of the liver.

Annotated Image 1
Annotated Image 2

Observations

In Image #1 we see the middle and left hepatic veins (MHV and LHV) and, in image #2 the right hepatic vein (RHV). The hepatic veins are anechoic (meaning absence of echoes, depicted as black) because they contain blood, whereas the remainder of liver texture is isoechoic (meaning medium level echoes, depicted as gray) and relatively uniform.

The uniform echotexture is broken up occasionally by branches of the hepatic and portal venous systems These appear as linear echogenic (meaning many echoes, depicted as white) structures surrounding an anechoic center.

The hepatic veins (which drain systemic blood from the liver to the IVC) subdivide the liver into different lobes. The middle hepatic vein subdivides the liver into the right and left lobes. The right hepatic vein subdivides the right lobe into anterior and posterior portions and, the left hepatic vein subdivides the left lobe into the medial and lateral portions. Note on image #2, the curving bright echogenic line along the posterior aspect of the image; this represents the diaphragm. Just posterior to that is the lung; and since it contains air, it appears intensely white (or echogenic) on an ultrasound image. If there had been a pleural effusion, we would see an anechoic (or black) crescentic area posterior to the diaphragm, representing fluid in the posterior costophrenic sulcus.

Both images #1 and #2 are basically transverse images and the convention of orientation is the same as that for CT scans, namely that the left side of the image corresponds to the right side of the patient and the right side of the image corresponds to the left side of the patient (i.e.,the image is viewed as though the patient were lying supine with the observer positioned at the patient's feet.)

The major hepatic veins run between the lobes and segments of the liver. They are useful in identifying segments of the liver and are seen when scanning the superior liver.

Right Portal Vein

Image #3 is a transverse image through the right lobe of the liver (RLL), with the center of the image visualizing the right portal vein (RPV) and its anterior and posterior subdivisions.

Annotated Image

Observations

The portal vein walls are usually echogenic, primarily due to the presence of fat within these tissues. As a general rule, the portal vein walls are more echogenic than the hepatic vein walls, but the primary ways of distinguishing the two venous systems are by (a) anatomic orientation (hepatic veins converge on the IVC at the dome of the liver while portal veins branch off from the porta hepatis on the inferior surface) and (b) different flow direction and pulsatility patterns (see Doppler).

Additionally seen in this image are smaller segments of hepatic and portal veins scattered throughout the liver texture. The patient's spine (the structure with an intensely echogenic anterior surface representing the vertebral body) is seen towards the right side of the image. Posterior to the vertebral body, there is an absence of echoes, indicating a complete lack of sound transmission through the bone.

Portal Triad

Image #4 is obtained transversely through the region of the porta hepatis of the liver.

Annotated Image

Observations

The porta hepatis is where the portal vein (PV) and hepatic artery (HA) enter the liver and the common bile duct leaves the liver. These three structures constitute the portal triad. The most anterior element or the triad in this image is the common bile duct (CBD) which is being delineated by the electronic calipers measuring 2.7 mm. The walls of the common bile duct appear intensely echogenic as do the portal vein walls. Immediately posterior and parallel to the duct is the main portal vein. The third and often smallest member of the portal triad, the hepatic artery (which generally runs between the duct and the vein), is not seen in this image.

Posterior to the portal vein is a tubular anechoic structure representing a portion of the inferior vena cava (IVC), and the round structure immediately posterior to that is a portion of the right renal artery (RRA). Two other anechoic areas are seen in the liver: the one towards the left side of the image (slightly oblong) is a portion of an hepatic vein. Note that its walls are less echogenic than the walls of the portal vein.

The normal common bile duct can measure up to 6 mm. With advancing age, the duct may become more ectatic, with generally 1 mm duct diameter being added for each decade over the age of fifty-five. Dilatation beyond the accepted norm either represents biliary obstruction (as from a distal duct stone, biliary stricture, or pancreatic neoplasm) or post-obstructive dilatation (from prior obstruction, due to loss of elastic recoil of the biliary tree). Generally the patient's clinical condition and correlation with serum chemistries can help distinguish these two possibilities.

Right Lobe of the Liver

Image #5 is a sagittal view of mid portion of the right lobe of the liver.

Annotated Image

Observations

The understood convention in sagittal images is that the cephalic or superior portion of the patient is to the left of the image whereas the caudal or inferior portion is to the right side of the image. In the center of the image is a portion of the right portal vein. Near the upper portion of the image is a curving echogenic line representing the diaphragm. Within the substance of the liver are linear anechoic structures which represent portions of either the portal or hepatic veins.

Note that these structures are all separated by hepatic parenchyma and appear as solitary tubular structures. If paired, tubular structures are seen immediately adjacent to each other in any peripheral portion of the liver (i.e., away from the porta hepatis), the possibility of abnormally dilated bile ducts or hepatic arteries should be raised.

Color Doppler imaging (hepatic arteries will give an arterial signal, bile ducts will not demonstrate flow) and real-time scanning to follow these structures toward the porta hepatis should allow for ready differentiation of these two possibilities.

Right Lobe of Liver & Right Kidney (sagittal)

Image #6 is a longitudinal (sagittal) view through the lateral aspect of the right lobe of the liver and the right kidney.

Annotated Image

Observations

Again note within the liver the presence of branching anechoic structures representing portions of the portal and hepatic veins. Near the top edge of the liver is the diaphragm and superior to that is the lung, which again (because of air containing alveoli) appears intensely echogenic on this image. Immediately posterior to the right kidney in this image are linear isoechoic structures representing the upper portions of the psoas muscle.

On sagittal images the normal kidney measures between 9 and 11 cm. The sonographic texture of the normal kidney usually demonstrates three different zones: a central echogenic area (representing the collapsed renal pelvis and calyces), a peripheral isoechoic reniform area (the cortex), and regularly spaced hypoechoic ovoid areas in between (representing the medullary pyramids).

In this image, two medullary pyramids are seen: one near the upper pole of the kidney, the other near the mid pole. The normal renal cortex should be as echogenic as or less echogenic than the adjacent liver. If the renal cortex appears more echogenic than the liver, this represents a diffuse abnormality of the kidney which unfortunately is non-specific: it can represent any of a number of inflammatory (i.e., glomerulonephritis), infiltrative (i.e., amyloidosis), or metabolic (i.e., diabetes mellitus) processes.

In the end stage of renal failure, the kidneys appear small (usually less than 9 cm.) and diffusely echogenic, with loss of differentiation of the cortex from the medulla.

Left Lobe of Liver (sagittal)

Image #7 is a longitudinal (sagittal) view through the left lobe of the liver.

Annotated Image

Observations

Along the extreme upper edge of the image just superior to the diaphragm is an anechoic structure representing the right ventricle of the heart. Again, because this is a fluid containing structure, it appears as anechoic on the ultrasound image. Note the normal uniform sonographic texture of the left lobe of the liver, which is basically isoechoic but interrupted periodically by linear echogenic structures representing portions of the portal and hepatic venous systems.

Note that the entire left lobe does not fit on this single image, due to the limited field of view of this transducer. This illustrates a frequent problem with measurement of large structures with real time ultrasound, namely, that they are often difficult or impossible to perform reproducibly, while imaging modalites with a more global field of view (i.e., CT or MRI) can make such measurements more easily. Nonetheless, the recognition of organomegaly is relatively easy for an experienced observer during real-time sonography.

Gall Bladder (sagittal)

Image #8 is a longitudinal (sagittal) view through the gall bladder.

Annotated Image

Observations

Sonographically, the gall bladder is an ovoid structure with a thin echogenic wall (normally measuring less than two mm) and internal uniformly anechoic contents (representing bile). The gall bladder neck is located in the main interlobar fissure (in the plane of the middle hepatic vein) between the right and left lobes of the liver. However, the fundus of the gall bladder is more variable in position, but generally underlies the right lobe of the liver. Note in this image the curving echogenic structures immediately posterior to the gall bladder represent the anterior borders of the vertebral bodies.

For optimal sonographic visualization, the gall bladder is visualized following an overnight or eight (8) hour fast to allow for maximal dilatation. Gallstones can be readily identified in a physiologically distended gall bladder as intensely echogenic structures demonstrating clean acoustic shadowing. They are also gravity dependent and mobile (unlike gallbladder polyps, which are fixed in position to the gallbladder wall).

Gall Bladder, IVC (transverse)

Image #9 is a transverse view through the mid portion of the liver at the junction of right and left lobes.

Annotated Image

Observations

There are three rounded anechoic structures noted on this image. Anterior and to the patient's right (left side of the image) is a transverse view of the gall bladder; just posterior and medial (i.e., closer to the spine) is the inferior vena cava seen in cross-section. The third rounded anechoic structure seen toward the patient's left side (right side of the image) just anterior to the spine is the aorta.

Note that in this single view all these three structures appear similar to each other: hence it is important to examine every anatomic structure in multiple planes of section (typically at right angles to each other).

In this image we would need to examine each fluid filled structure in the sagittal plane in order to determine whether they are longitudinal and tubular structures (such as the aorta and IVC) or whether they are contained oblong structures such as the gall bladder.

Gall Bladder Folds

Image #10 shows the neck of the gall bladder.

Annotated Image

Observations

Within the neck of the gall bladder are two linear echogenic structures. These are normal folds in the neck of the gall bladder and to the unwary can mimic gall bladder polyps. An additional normal gall bladder variant that can be seen at the opposite end of the gall bladder is a Phrygian cap, which is an extra fold in the gall bladder fundus.

Also nicely demonstrated in this image is acoustic enhancement which is the apparent intensification of echoes posterior to a fluid-filled structure. In this case, the area posterior to the gall bladder appears intensely echogenic due to the fact that more sound waves are passing through the fluid medium of the gall bladder than through the adjacent portion of the liver.

Differential diagnoses:

Up to now, it is apparent that there can be many explanations to apparently similar sonographic imaging findings.

For increased echogenicity, these include:

  1. the presence of fat, such as in the walls of the portal veins or in the renal hilum
  2. air, such as in the lung
  3. enhanced through transmission such as in this case or,
  4. the interface between two structures of differing acoustic impedance, such as we see in this image at the interface between the liver and the wall of the gall bladder or in prior images at the interface between liver and the lung.

Causes for absence of echoes can include:

  1. the presence of fluid (such as in blood vessels, the right atrium, or the gallbladder ) or
  2. the absence of sound transmission (such as behind bony structures or, as we will see, behind areas of acoustic shadowing, i.e., posterior to gallstones).

Pancreas, CBD (transverse)

Image #11 is a transverse image through the pancreas and normal retroperitoneal vascular structures.

Annotated Image

Observations

The location of the body of the pancreas (a curvilinear echogenic structure) is defined by the presence of the splenic vein posterior to it. As the splenic vein courses toward the left side of the image, it joins with the superior mesenteric vein to form the portal vein, with the area of confuence seen as a slight widening. Just anterolateral to this confluence is the pancreatic head (continuous with the pancreatic body), usually the thickest portion of the pancreas, measuring 3 cm in anteroposterior dimension.

The normal pancreas echogenicity is slightly greater than the left lobe of the liver, which is the tissue immediately anterior to it . Within the posterolateral portion of the pancreatic head is the distal common bile duct, and within the anterolateral portion of the head is the gastroduodenal artery. These structures are delineated by arrows.

Immediately posterior to the midpoint of the splenic vein are two round anechoic structures: the smaller anterior one is the superior mesenteric artery (SMA) and the larger posterior one is the aorta. In between these two structures is a linear anechoic tubular structure which is the tranversely oriented portion of the left renal vein (LRV) crossing the midline (ultimately to join with the IVC).

The IVC is the large round anechoic structure just posterior to the pancreatic head. Ideal visualization of the pancreas depends upon the absence of gas within the stomach (which lies just anterior to the pancreas and behind the left lobe of the liver), generally requiring overnight fasting.

Aorta (1) (2)

Images #12 and #13 are sagittal images of the aorta. Image #12 depicts the proximal aorta (where a portion of diaphragm can be seen along the superior aspect of the image) and image #13 depicts the distal aorta just proximal to its bifurcation.

Annotated Image 1
Annotated Image 2

Observations

The normal diameter of the aorta is generally less than 2.2 cm proximally at the diaphragm and tapering gradually thereafter. The walls of the aorta are usually smooth and echogenic and it lies immediately anterior to the spine.

Differential diagnosis

A focal area of aortic widening indicates an aneurysm, irregularity of the aortic walls suggests atherosclerosis, and displacement of the aorta off the spine suggests adenopathy.

Inferior Vena Cava (IVC)

Image #14 is a sagittal image of the upper portion of the inferior vena cava where it enters the right atrium.

Annotated Image

Observations

The inferior vena cava is an anechoic tubular structure whose caliber is more variable than the aorta, being influenced by such factors as hydration, cardiac function, and the patient's phase of inspiration at the time of the image.

In this particular patient, the aorta is smaller in caliber than the IVC (as can be seen on the transverse image in Image #11, and through comparison of this sagittal image to that of the aorta in images #13 and #12).

Just immediately anterior to the IVC is the caudate lobe of the liver, here seen as an isoechoic oblong structure. Immediately anterior to the caudate is the left lobe of the liver containing the ascending portion of the left portal vein.

Note the increased echogenicity posterior to the IVC, a good demonstration of normal acoustic enhancement

IVC, Aorta (transverse)

Image #15 is a transverse view through both the inferior vena cava (IVC) and the aorta.

Annotated Image

Observations

The image demonstrates good posterior acoustic enhancement for each of the structures (IVC and the aorta) and a larger caliber for the IVC in this patient.

Retroperitoneal lymphadenopathy can frequently be well demonstrated in thin patients at this level, with the enlarged lymph nodes appearing as hypoechoic round masses, often with posterior acoustic enhancement as well (reflecting their uniform internal texture with a lack of acoustic interfaces).

Spleen (sagittal)

Image #16 is a coronal view of the left upper quadrant demonstrating along the upper most portion the curving echogenic diaphragm and air filled echogenic lung superiorly.

Annotated Image

Observations

Beneath the diaphragm is an isoechoic structure, the spleen, and beneath that, the left kidney. The normal spleen generally spans up to 13 cm, and is uniform in echotexture. Unlike the liver, there are no tubular structures (i.e.,portal triads) coursing through it.

Note that the left kidney is incompletely seen in this image and that the lower pole is being shadowed out by bowel gas. The left kidney is frequently more difficult to image due to the smaller size of the spleen relative to the kidney (unlike the situation in the right upper quadrant where the inferior edge of the liver frequently extends lateral and inferior to the right kidney). This illustrates how we use solid superficial abdominal organs as acoustic windows for visualizing deeper organs. We have already seen an example of this when the left lobe of the liver is used as an acoustic window for the pancreas (Image #11).

Comment

Whenever bowel gas interposes itself between the ultrasound transducer and the organ of interest, all of the sound waves get either reflected or scattered and the deeper organs cannot be visualized; hence it is necessary to use multiple sonographic imaging planes in order to see the lower pole of the left kidney as well as the pancreas.

Right Kidney (sagittal)

Image #17 is a sagittal view through the right kidney.

Annotated Image

Observations

Again note the three differing areas of echogenicity, namely the renal sinus, the medullary pyramids,and the cortex. In the centermost portion of the kidney is the echogenic renal sinus (containing fat and the collapsed renal pelvis and calyces).

The renal parenchyma contains hypoehoic areas representing the medullary pyramids and the more peripheral isoechoic tissue representing the cortex. Note that the central renal sinus is normally uniformly echogenic. In the presence of hydronephrosis the distention of the renal pelvis and calyces results in the appearance of tubular interconnecting structures in the renal sinus.

Right Kidney (transverse)

Image #18 is a transverse view of the right kidney.

Annotated Image

Observations

Extending medially from the mid-portion of the kidney is a tubular anechoic structure representing the right renal vein. The right renal artery, being smaller and lying posterior to the renal vein, is not visualized on this image.

Superior to the right kidney is the liver which normally appears more echogenic than the renal cortex. The spine is seen posteriorly and centrally.

Kidney - Columns (sagittal)

Image #19 is a sagittal view of the right kidney in a different patient.

Annotated Image

Observations

The image nicely demonstrates the normal hypoechoic appearance of the medullary pyramids. These may occasionally be mistaken for hydronephrosis, but their uniform spacing and size coupled with their characteristic location just beyond the renal sinus fat confirm that these are normal medullary pyramids and not dilated calyces.

When the pyramids become hyperechoic and demonstrate acoustic shadowing, the diagnosis of medullary nephrocalcinosis is made.

Left Kidney (sagittal)

Image #20 is as sagittal view through the left kidney.

Annotated Image

Observations

Note that the medullary pyramids are not visualized on this image, which is still considered within the normal range for the sonographic appearance of a kidney.

As mentioned before the left kidney is more difficult to visualize that the right due to a smaller sonographic window. This most likely accounts for the failure of visualization of the renal pyramids in this case.