Ultrasound of the Spleen, Abdominal Vessels and Lymph NodesAlain Giroux, DVM, MSc ,DACVR

Introduction:

The spleen is fairly easy to identify in dogs. In cats it can be more difficult to find and may require more of a search especially in the obese patient. As with other abdominal organs, identification and complete visualization is often influence by anatomy knowledge of the ultrasonographer.

The abdominal lymph nodes are generally not seen unless abnormal. Their location and number will vary in each animal. Having a general knowledge of their location and of the diseases that may alter their echogenecity and size is helpful in their identification.

Spleen Anatomy (Fig.1SN):

The spleen is located mostly on the left side of the abdomen. Although the body and tail can be in various locations, the head of the spleen is consistently located in the cranial left abdomen, just caudal to the left lobes of the liver. In addition, the head of the spleen is predominantly oriented parallel to the lateral body wall (Fig. 1 SNb). The body and tail of the spleen are parallel to the ventral abdominal wall (flat when patient laying on is back). The body and tail of the spleen vary in position and can extend to the right side and may be adjacent to the cranial aspect the urinary bladder (especially if spleen or bladder are enlarged). The spleen is identified easily by its vessels, which pierce through the capsule on its dorsal and medial aspect (surface facing the stomach and bowel).

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Figure 1.SNa: Spleen Anatomy:Ventral View

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Abdominal Vessels and Lymph Nodes Anatomy:

The abdominal lymph nodes are located throughout the entire abdomen. A general rule is they are in the hilar region of each organ, particularly, the liver, spleen and kidneys (Fig.1LN). Additionally the root of the mesentery contains the largest lymph nodes (Fig. 2 LN). They can be distributed in the central abdomen. These lymph nodes are often distributed on each side of the mesenteric vessels. Large numbers of lymph nodes are also located along the greater abdominal vessels. They are often referred to the sub-lumbar lymph nodes.

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Figure 1.SNb: Spleen Anatomy:

Left Lateral View :

Figure 1 LN: Cranial Abdominal Lymph Nodes: -Lymph nodes: purple structures

Figure 2 LN: Mesenteric Lymph Nodes: -Lymph nodes: purple structures

The major abdominal vessels (Fig.3 LN) run in an axis parallel to the spine. More precisely, the CVC, AO and portal vein are the easiest to identify. The main AO branches most often seen are from cranial to caudal:Celiac (1); Cranial mesenteric (1); Renal (2); caudal mesenteric (1); external iliac (2); internal iliac (2). Other branches that can be identified are the lumbar arteries, sacral median (1) and deep circumflex iliac (2)

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Figure 3 LN: Sub-Lumbar Lymph Nodes and Caudal Branches of the AO:

Spleen Ultrasound Anatomy:

The splenic capsule is a thin hyperechoic line that contains the splenic parenchyma (Fig. 2 SN).In some occasions, it may not be seen if low a frequency transducer is utilized or if parenchymal disease involving the capsule is present. The splenic parenchyma is smoother and hyperechoic in comparison to the liver or renal cortex. The splenic parenchyma contains anechoic vessels that can be trace in and out of the capsule. The vessels are numerous, and as mentioned above, are on the medial and dorsal surface. The most common beginner error in evaluating the spleen is not scanning its entirety. More precisely, the head is often not examined as the evaluator fails to scan the most lateral aspect of the head, which is parallel to the lateral abdominal wall. As an indication, when the head of the spleen is imaged it will fill most of the screen in comparison to when the body and tail of the spleen are imaged. This phenomenon is better explained with practical application during the lab. Here are some criteria utilized for splenic evaluation:

1-Overal size and contour (subjective, especially if mild or moderate enlargement).2-Overal echogenecity compared to liver and kidneys.3-Focal parenchymal lesions and capsule irregularities4-Integrity of the vessels (presence of thrombus, enlargement)5-Smoothness of the parenchyma (in diffuse fine disease)

Note that ultrasound of the feline spleen is often more difficult in obese patient. Ferret spleen are always prominent and it appears to be sp characteristic.

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Figure 2 SN: Normal Spleen

Abnormal Spleen:

Diffuse Splenic Disease:

Diffuse disease of the spleen is sometime harder to recognized. It is best to compare echogenecity and overall smoothness of the parenchyma to other organs. Evaluating the contour can be difficult if there is lots of gas in bowel. In cases of diffuse infiltrate, it is important to evaluate for adjacent lymph nodes enlargement or infiltration of other organs. The integrity of the vessels is rarely affected but a thrombus maybe present in some occasions.

Here is a Summarized List of Disease you may Encounter: Diffuse Hyperechoic Diseases:

Chronic CongestionChronic InflammationChronic TorsionDiffuse Neoplasm: Lymphosarcoma; Hystiocytoma

Diffuse Hypoechoic Diseases:Congestion (Table 1.SN)Diffuse Neoplasm: Lymphosarcoma; Hystiocytoma; Mast Cell TumorInflammation/Infection (Table 2 SN)

Diffuse isoechoic Diseases:Mast cell neoplasm, lymphoma.

As you will notice lymphosarcoma is the greatest chameleon, and it can have multiple different appearances. This is also true for most myeloproliferative neoplasms.

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Table 1.SN: Causes of Splenic CongestionHypertensionAnestheticVenous CongestionTorsionThrombosis of Splenic or Portal veinExtramedullary HematopoiesisErythrophagocytosis

Table 2.SN: Causes of Inflammation/Infection (often hypoechoic)

Systemic MycosisBlood ParasitesToxoplasmosisBacterial Splenitis

Focal Splenic Diseases:

Focal splenic disease is in general easier to identified. Focal isoechoic diseases are the most difficult and are identified based on the difference in smoothness of the parenchyma and possible capsule irregularity.

Here is a summarized list of disease you may encounter:

Focal Hyperechoic Diseases:Nodular Hyperplasia (fig.3.sn)Mesenteric Fat Surrounding Vessels EntryCalcificationInfarctsGranulomas Abscess With Gas

Focal Hypoechoic Diseases:Nodular HyperplasiaCystHematomaPrimary NeoplasmMetastatic Neoplasm

Focal Mixed Echogenic Diseases:NeoplasmsMultiple AbscessesHematoma

Focal isoechoic Diseases:Hemangioma (Fig. 4 SN)

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Figure 3 SN: Nodular Hyperplasia

Figure 4 SN: Hemangioma(Increased Image Contrast to Improve

Visualisation)Figure 5 SN: Hemangiosarcoma

Abdominal Lymph Nodes and Vessels Ultrasound Anatomy:

In general, normal lymph nodes are not seen ultrasonographically. They can be seen if a high frequency transducer is utilized (10 MHz). In immature patient, the lymph nodes can be seen without pathology.Knowing where to look is the best way to recognize what they are.

Here are some criteria utilized for screening of lymph nodes:

1-Size (fairly subjective)2-Echogenecity3-Number and diffuse or focal involvement4-Proximity to other organs that are diseased.

The vessels all have a similar appearance (Fig. 1VS). They have an anechoic lumen and their wall is slightly hyperechoic. However, their wall is not always seen. In some occasion, flowing blood can be seen inside their lumen. The aorta, and its caudal branches, and the CVC are best identified in the caudal abdominal region due to their shorter distance to the transducer and abdominal wall. The portal vein is also fairly easily identified, especially in cats. It is critical, in the presence of some diseases or suspected intrinsic disease (porto-systemic shunts), to evaluate the vessels carefully. Vessels evaluation takes time and lots of experience. Ideally, Doppler evaluation should be perform when disease is suspected. The CVC and AO are normally of similar diameter. It is important to realize that if too much pressure is applied with the transducer, the veins especially the CVC can appear collapse. The portal vein has a more tortuous appearance, it is lightly smaller that the CVC and aorta, and is seen in the cranial abdomen, ventral to the CVC.

Here are some criterias utilized for screening of vessels:

1-size 2- Lumen integrity3-Flow4-Location and orientation

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Figure 1VS: Caudal Aorta: Sagital View

Figure 2VS: Caudal Aorta and CVC: Transverse View

Figure 3VS: Caudal Aorta and Branches: Transverse View

External Iliac a. (EI)Median Sacral a.(MS)

Abnormal Lymph Nodes (Fig. 3LN and 4 LN):

Abnormal lymph nodes are often hypoechoic and enlarged. However they can also be hyperechoic and enlarged. In some cases, the lymph nodes are only hypoechoic and their size is still within normal limits. The lymph nodes at the root of the mesentery are often between 1 and 2 cm long by one cm wide by 0.5 to 1 cm thick. No definitive measurements for other abdominal lymph nodes size is well described.The bottom line is if you see the lymph nodes in an adult patient, if it is because they affected by disease. Abnormal lymph nodes are in general enlarged because of disease in organs or regions they drain. If you see an enlarged lymph node, you should carefully evaluate the surroundings organs for infiltrative diseases. For example, finding only the caudal mesenteric lymph nodes enlarged should prompt more careful evaluation of the bowel for carcinoma and focal lymphoma.

Here is a summarized list of cause for their identification:

Neoplastic infiltrate: Lymphoma Mast cell tumorMalignant hystiocytomaOther Myeloproliferative neoplasm

Infectious processes: AbscessIntestinal parasitesFungalViralInflammation: IBDDietary indiscretion and non-specific enteritis

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Figure 3 LN: Enlarged Mesenteric Lymph Nodes

Figure 4 LN: Enlarged Gastric Lymph Node (GLN)

Abnormal vessels: Vessels affected by disease will havea change in their diameter. You may be able to see an intraluminal

defect appearing as hyperechoic structure in the anechoic lumen. Rupture of a vessel is more difficult to see. Most occasions the rupture site may be demonstrated by a large hematoma. If the vessel pressure is high, no hematoma is present, and bleeding is active you may see blood flowing out of the vessels into an anechoic pocket of non-coagulated blood. Color flow Doppler is essential for complete evaluation of the vascular integrity, but valuable information regarding vascular pathology can be obtained without its availability. Abnormal, aberrant vessels are identified by their unusual orientation, size, shape and location.

Here is a summarized list of disease:Portosystemic shuntsArteriovenous fistulaPortal HypertensionVenous congestionThrombosis (Fig. 4 VS)Stenosis

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Figure 4VS: CVC thrombus