Mouse Abdominal Imaging Guide: Imaging the Liver, Hepatic Vessels, Portal Veins and Gallbladder

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Liver and Gallbladder

Anatomy

The mouse liver weighs approximately 2g [1], spans the entire subdiaphragmatic region, and is divided into four main lobes: caudal lobe (located dorsal and ventral to the esophagus), right lateral lobe (subdivided horizontally into anterior and posterior portions), median lobe (subdivided into left and right portions), and the left lateral lobe [1]. The right lateral lobe has a transverse septum that approximately bisects it. The median lobe sits most ventrally and is the most prominent lobe when the abdominal cavity is viewed from the usual ventral aspect. Its two pyriform wings connected by a narrow central isthmus are present beneath the median lobe, and these produce the ventral sac of the gallbladder. The left lobe is the largest and is a frequent site of sampling for histology. The caudate lobe is small and has two distinct ear-shaped segments [2][3]

Liver

Transducer Position and Sonography Procedures

When imaging the liver the transducer scan head should be moved across the processes xiphoideus gradually toward hind limbs of the mouse (Fig. 1(A)). Because the liver lies just beneath the skin surface, the transducer scan head should be positioned away from the skin surface in order to obtain a focused liver image. A B-mode image is a two-dimensional(2-D) presentation of echo-producing interfaces in a single plane, and it displays shapes, texture, and contours of the body. B-mode imaging is popular for identifying anatomical structure sand diagnosing lesions, especially of the abdomen. The liver parenchyma appears echogenic, homogeneous, and of medium texture in a B-mode image. The margin of the liver is bounded by an intensely echogenic curvilinear structure (Fig. 1(B)). The sonographer should examine not only the appearance of the parenchyma but also the margins of the liver, the porta hepatis, and the contour and appearance of the diaphragm. Liver margin evaluation allows detection of protruding nodules and surface irregularities as well as assessment of rounded margins that appear on an enlarged liver.

Hepatic Vessels and Portal Veins

When the cross section of the liver is found, anechoic round and tubular vascular structures will simultaneously be noted; these represent hepatic vessels (Fig. 1(B)) and portal veins. The walls of the portal veins are more echogenic due to the more organized connective tissue in the vessel walls scattering more of the ultrasound beam than those of the hepatic vessels. The probe can be moved toward hind limbs to scan all of the liver in an attempt to find more notable vessels or rotated in combination with moving the platform slightly in different directions to obtain a longitudinal-section view of specific vessels (Fig. 2(A)and Fig.3(A)). Liver vessels can also be identified by using Doppler modes. Two kinds of Doppler modes are provided: color/power Doppler-mode and pulsed-wave (PW) Doppler-mode. The color/power Doppler-mode provides a 2-D color velocity map superimposed on the B-mode image, which facilitates the positioning of the vessels of interest. PW Doppler-mode, on the other hand, represents the spectrum (i.e., the velocity distribution) as a function of time. Employing the triplex function in PW Doppler-mode (i.e., B-mode, color Doppler-mode, and spectral Doppler-mode images displayed simultaneously in the same window) makes it possible to locate the vessels and conduct quantitative flow measurements, as shown in Fig. 2(C)(D) and Fig.3 (C) (D).

 

Fig.2. Imaging a hepatic vein: (A) transducer position, (B) B-mode image, (C) Color-mode image, and PW-mode image (the artificial color around the edge of the region of interest edge is due to respiration vibration; this is also evident as regular interruptions in spectral Doppler images).

Fig.3. Portal vein imaging of the mouse:(A) transducer position, (B) B-mode image,(C) Color-mode image, and (D) PW-mode image.

Gallbladder

As the transduce is moved across the left of the midline (Fig. 4(A)), the gallbladder will appear in the B-mode image as a round or oval anechoic structure with a thin, echogenic wall surrounded by the liver parenchyma (Fig. 4(B)). The normal gallbladder should contain bile and have no space-occupying lesions. The gallbladder should therefore appear anechoic (i.e., be completely black) and be devoid of internal echoes.

Fig.4.Imaging the gallbladder: (A) transducer position and (B) B-mode image.

Inferior Vena Cava and Abdominal Aorta

   Anatomy

In many instances the blood vessels constituting the venous system are comparable to those of the arterial system. The aortic arch continues as the abdominal aorta, and the aorta passes through the diaphragm and supplies blood to the lower extremities. The inferior vena cava and abdominal aorta are adjacent anatomical structures. The inferior vena cava carries blood from the lower part of the body to the right atrium. The abdominal aorta branches to the hepatic artery near the liver, and the inferior vena cava passes through the liver where the hepatic vein enters it [6].

  Transducer Position and Sonography Procedures

B-mode imaging is first used to find the cross section of the liver, and then the probe is moved closer to the surface and adjusted to the appropriate position to display the major abdominal vessels (Fig. 5(A)). The inferior vena cava normally appears beside the abdominal aorta and has walls that are more echogenic than those of the aorta. The portal vein is above both the inferior vena cava and the abdominal aorta, and under the edge of the liver (Fig. 5(B)). Furthermore, the pulse movement is larger for the abdominal aorta than for the portal vein and the inferior vena cava in the B-mode, it is the basis for preliminary identification of an artery and a vein. The longitudinal section of a vessel can be viewed by moving the probe so that the cross section of the vessel appears in the middle of image and then rotating the probe to the appropriate angle, such as Fig. 6(B) and Fig. 7(B).

However, it remains difficult to distinguish the kind of blood vessel using only B-mode imaging; color/power Doppler-mode and PW-mode imaging are useful for identifying the vessel position and distinguishing between an artery and a vein. Color/power Doppler imaging is normally used to confirm whether or not a cavity is a vessel. This is possible since color/power Doppler imaging can detect moving blood cells or other moving structures, with their direction and speed of movement being indicated by the color (Fig. 5(C), Fig. 6 (C) and Fig.7 (C)). Therefore, no color will appear in a color/power Doppler-mode image when the cavity is filled with liquid that is not flowing. After a specific vessel has been selected, the imaging is switched to PW-mode and the obtained PW Doppler waveforms represent the velocity of the blood flowing in a vessel over time. The velocity in a high-resistance arterial vessel will decrease rapidly following the heart systole and will exhibit a low-velocity forward-flow phase in late heart diastole caused by the recoil of the vessel wall(Fig. 6(D)).On the other hand, allow-resistance vein is characterized by a low flow velocity and minimal velocity fluctuations (Fig. 7(D)).Furthermore, the velocity is measured using the “Measure tool “tag(  )by clicking the point icons of the distance type (  )and then dragging and clicking on the PW-mode ultrasound image window to display the flow velocity data, as shown in Fig. 6(D).It is noteworthy that an appropriate estimate of the angle correction is essential for accurately determining the blood flow velocity. The angle should be less than 60 degrees at all times, since errors in the angle correction will be substantially greater for larger angles.

Fig.5.Ultrasound cross-sectional imaging of the mouse portal vein, abdominal aorta, and inferior vena cava: (A) transducer position, (B) B-mode image, and(C)color-mode image.


Prospect T1: High-Frequency Ultrasound