Ultrasonography of the Abdominal Vessels

Assessment of the abdominal vessels should form part of a routine abdominal ultrasound examination.

Abdominal vessels and their major branches are important ultrasonographic landmarks for other abdominal structures. Specific indications for ultrasonography of the abdominal vessels include:

• Identification of emboli/thrombi 
• Rule out vascular invasion by neoplastic lesions 

Normal Anatomy

The aorta supplies the arterial flow for all abdominal structures. Whilst the venous flow is divided, with some organs draining directly into the caudal vena cava (CVC) and others draining via the portal vein. 

 An illustration of the major abdominal blood vessels. Portal circulation (Left): 1. Portal vein 2. Splenic vein 3. Left gastric vein 4. cranial mesenteric vein. Systemic circulation (right): 5. Aorta 6. Caudal vena cava 7. Renal artery 8. Renal vein 9. Phrenicoabdominal artery 10. Coeliac artery 11. Cranial mesenteric artery 12. Circumflex artery 13. External iliac artery 14. Internal iliac artery 15. Circumflex vein 16. External iliac vein 17. Internal iliac vein. 

Examination Technique 

The aorta and the caudal vena cava are situated in the dorsal abdomen. Therefore, it is important to ensure that the patient is adequately clipped. Extend the clipped area to the edge of the lumbar musculature dorsally and, in deep chested or large breed patients, include the caudal 2-3 intercostal spaces in the clipped area. This will allow the intercostal spaces to be used for the examination of more cranial and dorsal areas of the abdomen. 

The highest frequency setting that gives adequate depth of penetration to view the area of the vessels should be used. In most patients, a frequency of 5-7.5Mhz is adequate. However, higher frequencies (8-15Mhz) will improve image resolution and can be utilised in smaller patients. Either a micro-convex or linear transducer can be used for the examination. Linear transducers are capable of higher frequency settings and are ideal for smaller patients. However, their larger footprint makes them harder to manoeuvre around the body. In comparison, micro-convex transducers produce a lower frequency range though their smaller, curved surface make them ideal in larger patients and for imaging using the intercostal spaces. 

Aorta 

The aorta is visible in the dorsal abdomen, ventral to the epaxial musculature and the vertebral column and to the left of the midline.  

When scanning the dorsal abdomen with the animal in right lateral recumbency, the aorta will be visible in the near field of the ultrasound image. The aorta appears pulsatile and cannot be compressed by increasing the transducer pressure on the patient in the same manner as the vena cava. 

The left kidney can be used as a landmark for the aorta. By identifying the left kidney in a dorsal or sagittal plane, the transducer can be fanned ventrally towards the patient’s midline to identify the aorta in the sagittal plane. 

Caudal Vena Cava 

The vena cava is visible in the dorsal abdomen, ventral to the vertebral column. It is present in the near field when imaged with the patient in left lateral recumbency and it can be easily compressed compared to the adjacent aorta. The vena cava may also appear pulsatile as a result of referred movement from the aorta. 

Like the technique described for the aorta, the right kidney can be used as a landmark. By identifying the right kidney in a dorsal or sagittal orientation and fanning the transducer ventrally towards the patient’s midline the caudal vena cava can be identified in a sagittal or longitudinal orientation. 

Normal appearance

The lumen of vascular structures appears anechoic. Slice thickness artefact, whereby the anechoic lumen and echogenic tissue adjacent to the blood vessel are superimposed on the two-dimensional screen image, can create the appearance of echogenic tissue within the vessel lumen. Imaging the vessels in two orthogonal planes, as well as utilising Doppler ultrasound modes, can help to distinguish artefacts from pathological conditions. 

Caudal vena cava in a dog. In this dorsal plane image of the caudal abdomen of a dog in left lateral recumbency, the caudal vena cava is visible in the near field (blue arrow) whilst the aorta is visible in the far field (red arrow). 

Doppler Ultrasound 

Doppler ultrasound modes can be used to visualise and document the blood flow within vascular structures. 

Colour Doppler 

In this display mode, Doppler Shift data received by the ultrasound transducer is displayed as colour pixels within the sample gate shown on the B-mode image. This gives information about direction and a semi-quantitative assessment of blood flow velocities. 

Conventionally, echoes representing flow away from the ultrasound transducer are displayed as shades of blue and echoes representing flow towards the ultrasound transducer are displayed as shades of red. Note that if flow occurs perpendicular to the ultrasound beam, no colour will be displayed despite the presence of flow. Therefore, it is important to orientate the ultrasound beam as close to parallel to the direction of blood flow as possible. 

Power Doppler 

Power Doppler ignores the directional information provided by Doppler shift and displays the total Doppler signal strength as shades of one colour. While it does not display any data on flow direction, Power Doppler is a useful tool for examining low velocity blood flow and is more sensitive to flow than colour doppler. Therefore, it can detect flow in smaller blood vessels. 

Spectral Doppler 

Spectral Doppler displays Doppler shift in a graphical format. Velocity is plotted on the Y – axis and time is plotted on the X – axis. Flow towards the transducer is plotted above the baseline and flow away from the transducer is plotted below the baseline. 

Colour Doppler image of the caudal vena cava. In this dorsal plane image of the caudal abdomen of a dog in left lateral recumbency, the caudal vena cava is visible in the near field. Coloured pixels representing Doppler shift caused by vascular flow are visible within the Doppler sample gate on the ultrasound image. Note that because the coloured pixels represent flow relative to the transducer, there are areas within the caudal vena cava where the blood flow is towards (coloured red) and away (coloured blue) from the transducer. Similarly, there is section of the vena cava which lacks colour. At this point the transducer surface is parallel to the vessel and there is no blood flow angled towards or away from the transducer. Therefore, there is no Doppler shift detected. 

Power Doppler image. In this image power Doppler imaging is being used to assess the blood flow within a soft tissue structure. The blood flow within the sample volume gate (green box) is shown as a single colour ignoring directional data. 

Spectral Doppler Trace. This image shows a graphical representation of Doppler flow. Flow towards the transducer is displayed above the baseline and flow away from the transducer is displayed below the baseline. The B-mode image showing positioning of the sample gate is shown above the spectral trace. 

Ultrasonographic appearance of vessel pathology

Vascular Thrombosis 

Thrombi can occur secondary to cardiac disease, systemic conditions such as those causing a hypercoagulable state, or localised pathology such as neoplasia. 

Thrombi appear as echogenic tissue within the vascular lumen1. The echogenicity can vary and may appear homogenous or heterogenous2. In the acute phase, thrombi can be poorly echogenic, making them harder to identify. At this stage, Doppler can be used to document the lack of flow within the vessel. 

Aortic thrombus in a dog. A) In this sagittal plane image of the dorsal abdomen, an echogenic thrombus is present within the lumen of the distal aorta. B) In the same patient, colour Doppler mode is used to show the lack of blood flow caused by the thrombus occluding the aortic lumen. 

Neoplastic Invasion 

Neoplastic lesions can invade regional vascular structures. The caudal vena cava can be invaded by neoplastic lesions arising from the liver, local lymph nodes, kidneys and, most commonly3, the right adrenal gland.  

As neoplastic invasion also appears as echogenic tissue within the vascular lumen, it can be difficult to differentiate from thrombus formation. In these cases, computed tomography (CT) or magnetic resonance imaging (MRI) may be useful for any surgical planning and to identify more subtle vascular invasion. 

References

1. Rogers C., O’Toole T., Keating J., Penninck D., Webster, C. (2008) Portal Vein Thrombosis in Cats: 6 Cases (2001–2006). Journal of Veterinary Internal Medicine 22: 282-287. 
2. Respess M., O’Toole T., Taeyman, O., Rogers C., Johnston A. Webster, C. (2012) Portal Vein Thrombosis in 33 Dogs: 1998–2011. Journal of Veterinary Internal Medicine 26: 230-237. 
3. Kinns J. (2012) Abdomen. In: BSAVA Manual of Canine and Feline Ultrasonography. Eds. Barr F., Gaschen L. Gloucestershire: British Small Animal Veterinary Association, pp 72–84. 
4. Davis M.K., Schochet, R.A., Wrigley, R. (2012) Ultrasonographic Identification of Vascular Invasion by Adrenal Tumors in Dogs. Veterinary Radiology and Ultrasound 53: 442-445. 

Further Reading 

d’Anjou M.A., Carmel E.N. (2015) Abdominal Cavity, Lymph Nodes, and Great Vessels, In: Atlas of Small Animal Ultrasonography 2nd edn., Eds: Penninck D., d’Anjou M.A., John Wiley & Sons, Chichester, pp 455-479. 

Specchi S., d’Anjou, M.A. (2019) Diagnostic Imaging for the Assessment of Acquired Abdominal Vascular Diseases in Small Animals: A Pictorial Review. pp 613– 632. 

Widmer W.R., Mattoon J.S., Nyland T.G. (2015) Peritoneal Fluid, Lymph Nodes, Masses, Peritoneal Cavity, Great Vessel Thrombosis, and Focused Examinations, In: Small Animal Diagnostic Ultrasound 3rd edn., Eds. Mattoon J.S., Nyland T.G., Elsevier, St. Louis, pp 501-516. 

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