Ultrasonography of the Kidneys
Ultrasonographic evaluation of the kidneys should be included in every routine abdominal ultrasound examination.
Specific indications for renal ultrasonography include:
- Palpable renal masses (unilateral or bilateral)
- Acute or chronic renal failure
- Congenital renal conditions
- Investigation into lower urinary tract disease
The kidneys are paired, ovoid structures situated dorsally in the abdomen within the retroperitoneal space. They are surrounded by retroperitoneal fat and each kidney has an indented medial hilus where the renal artery enters, and the renal vein and ureter exit.
The left kidney is just medial and caudal to the head of the spleen. The right kidney is situated relatively more cranially and rests close to, or within the renal fossa of the caudate lobe of the liver.
Renal size varies greatly in dogs due to a wide range in body weight and conformation. In cats, the kidneys are shorter and rounder with normal renal length being between 3 - 4.5cm1.
Ultrasonographic Examination Technique
The kidneys are imaged with the patient in lateral recumbency – the uppermost kidney is examined in each case i.e. the left kidney is examined with the animal in right lateral recumbency and the right kidney is examined with the animal in left lateral recumbency. Imaging with the animal in lateral rather than dorsal recumbency can aid visualisation by causing the neighbouring parts of the gastro-intestinal tract to move ventrally.
It is important to consider the dorsal position of the kidneys when preparing the patient and the clipped area should be continued dorsally to the edge of the lumbar musculature. Because the right kidney is more cranial relative to the left, the last 2-3 intercostal spaces are often required to be included in any clip, especially in larger dogs.
For cats and small dogs, linear ultrasound transducers that are capable of higher frequency ranges can provide higher resolution images of the kidneys, although the larger footprint of the probe can make imaging the kidney utilising an intercostal approach much more difficult. For this reason, micro-convex transducers are the author’s probe of choice for renal imaging in larger patients. A frequency range of 7-10Mhz is commonly used; large or giant breed dogs may require the use of a lower frequency setting for increased depth of ultrasound penetration.
The left kidney can be imaged by placing the probe at the left paralumbar fossa. The right is imaged by placing the probe caudal to the last rib in the dorsal abdomen or utilising the last 2-3 intercostal spaces. In cats, the kidneys often sit more caudally compared with dogs, and an intercostal approach is rarely necessary.
The kidneys should be examined thoroughly in at least two orthogonal planes (Figure 1):
Each kidney is surrounded by a renal capsule which appears as a thin, hyperechoic line when observed in perpendicular orientation relative to the ultrasound beam.
The outer renal cortex has a fine and uniform echotexture. The renal cortex is often hypo- or isoechoic relative to the liver parenchyma (Figure 2) and hypoechoic relative to the splenic parenchyma. As for other organs, comparisons of echogenicity should be interpreted with care because both the ultrasound machine settings and any pathological conditions can greatly affect the appearance of the abdominal anatomy.
Figure 2. In this transverse plane image of the right kidney (arrow) of a normal dog, the adjacent liver parenchyma of the caudate lobe of the liver can be seen (arrowhead). Note the isoechoic renal cortex and liver parenchyma.
The inner renal medulla is hypoechoic to anechoic in comparison to the outer cortex (Figures 3 and 4). The medulla is divided by echogenic lines which represent the renal diverticula and interlobar vessels (Figure 3). The renal arcuate vessels which branch from the interlobar vasculature can be identified as hyperechoic structures at the cortico-medullary junction.
The renal sinus is composed of the renal pelvis, peri-pelvic fat, proximal ureter and renal vasculature, appearing as a hyperechoic structure situated centrally in the kidney (Figure 4). The renal pelvic lumen and proximal ureter are not visible as distinct structures under normal circumstances.
Figure 3 (on left). Sagittal view of the kidney. The outer cortex (C) and inner medulla (M) can be identified. The echogenic renal diverticula and interlobar vessels can also be seen (arrows).
Figure 4 (on right). Dorsal view of the kidney. The outer cortex (C), inner medulla (M) and hyperechoic central renal sinus (S) can be identified.
Ultrasonographic Appearance of Renal Disorders
It is important to consider that renal ultrasonography does not provide data on renal function – although changes in ultrasonographic appearance could precede biochemical changes relating to renal compromise. It is also possible for the ultrasonographic appearance of the kidneys to be normal despite advanced renal compromise. For this reason, any findings should be considered along with biochemistry, haematology and urinalysis.
Uroliths can appear as hyperechoic interfaces with distal acoustic shadowing (Figure 5). Smaller uroliths may not cause a typical acoustic shadowing artefact.
Figure 5. Sagittal plane image of the left kidney. Two hyperechoic uroliths can be seen (arrowheads) with distal acoustic shadowing (arrows).
In cases of suspected urolithiasis, it is important to use ultrasonography in combination with radiography because this aids the detection of mineralised areas or uroliths.
Dilation of the renal pelvis or pyelectasia can be seen under several circumstances. It has been identified in normal animals and those undergoing diuresis via intravenous fluid administration or diuretic medications2. It can also be seen in pathologic conditions such as chronic renal failure, pyelonephritis and ureteral obstruction (possible causes of obstruction include urolithiasis and neoplasia). The greater the degree of pyelectasia, the more suspicion there would be of a pathological cause.
Renal pelvis dilatation can appear as a hypoechoic to anechoic area in the inner medulla when the kidney is viewed in a sagittal or dorsal plane. In a transverse plane, pyelectasia may appear as a semilunar hypoechoic to anechoic area outlining the renal papillae (Figure 6).
Figure 6. Renal Pelvis Dilation. This schematic diagram of a kidney in a transverse plane shows the semilunar appearance of a dilated renal pelvis (black section). Renal pelvic width is measured from the renal crest (C) to the origin of the ureter. In cats and dogs, a pelvic width of >13mm was predictive of obstruction2.
Secondary to obstruction, hydronephrosis can develop. Here, urine accumulation builds pressure leading to atrophy of the renal parenchyma (Figure 7.). If chronic, this can eventually lead to the affected kidney becoming a thin-walled, fluid-filled cavity.
Figure 7. Hydronephrosis – in this dorsal plane image obstruction of the ureter has led to dilation of the renal pelvis (P) and the proximal ureter (*). (Image courtesy of Dr Sally Griffin, Willows Referrals. Not for reproduction).
In cases of hydronephrosis or where there is a suspicion of ureteric obstruction, it is important to examine both kidneys thoroughly. Establishing if there is bilateral involvement can have a profound effect on the patient’s prognosis.
Diffuse Renal Parenchymal Disease
Diffuse renal conditions will usually present as a combination of changes in renal echogenicity, corticomedullary definition and renal size. The extent to which these changes occur in the kidney can be variable.
Congenital conditions can make the kidneys appear small and hyperechoic, with poor corticomedullary definition. In some cases, it can be difficult to identify the kidney on ultrasound examination due to the extent of the changes.
Chronic kidney disease may elicit more subtle ultrasonographic changes. Dogs and cats with chronic kidney disease showed increased cortical echogenicity, loss of cortico-medullary definition and reduced renal volume on ultrasound3.
Medullary rim sign: a hyperechoic line in the medulla parallel to the corticomedullary junction has been reported in a variety of renal diseases including hypercalcaemic nephropathies (Figure 8). However, it has been identified in patients with no evidence of renal disease4 and should be interpreted alongside other ultrasonographic findings and serum biochemistry results.
Ethylene glycol toxicity causes an increase in renal cortical echogenicity5 caused by renal tubular necrosis and calcium oxalate crystal deposition within the renal tubules.
Figure 8. Medullary rim sign – in this sagittal plane image of the right kidney, hyperechoic lines paralleling the corticomedullary junction can be seen (arrows). (Image courtesy of Dr Sally Griffin, Willows Referrals. Not for reproduction).
Benign renal cysts will appear as well defined, round, anechoic areas, often with associated distal acoustic enhancement artefact (Figure 9). They can be solitary or multifocal.
Figure 9. Solitary renal cyst – In this dorsal plane image a renal cyst can be seen in the medial part of the left kidney of a dog, measuring 0.92 by 1.2 cm. Acoustic enhancement artefact can be seen distal to the cyst (*).
Polycystic kidney disease is a heritable condition seen in Persian cats and Cairn Terriers. It results in multiple cysts that enlarge over time. This causes destruction of the normal renal parenchyma, leading to renomegaly and renal insufficiency.
Renal infarction can be an incidental finding, although infarctions may also be seen with systemic illness. Caused by occlusion of the arcuate artery by an embolus, the lesions affect the renal cortex and appear as a wedge or “V” shaped area within the renal cortex, with the apex originating at the cortico-medullary junction. In the acute phase the lesions appear hypoechoic but over time the affected renal cortex will atrophy, causing an area of depression within the renal capsule and a hyperechoic triangular area of renal cortex.
The kidneys can be the site of both primary and secondary neoplasia. The appearance of neoplastic lesions varies from small lesions to larger tumours which can completely efface the normal renal tissue (Figure 10).
Primary renal tumours are uncommon and renal carcinoma is the most common primary malignant neoplasm in dogs6.
Lymphoma is the most common renal tumour in cats7. A perinephric/subcapsular infiltrate is seen in some lymphoma cases causing a hypoechoic rim to form around the renal cortex.
Due to the variable appearance of renal neoplasia on ultrasound examination, fine needle aspiration (FNA) or biopsy is needed for definitive diagnosis. Where neoplasia is suspected it is important to evaluate the vasculature adjacent to the kidney because some tumour types can invade the neighbouring blood vessels.
Figure 10. Examples of renal neoplasia – A) Left kidney of a cat. There is complete loss of corticomedullary definition, and the kidney appears hypoechoic. B) and C) Transverse plane images of the left kidney of a cat. In image B) a hypoechoic to anechoic rim can be seen surrounding the renal cortex. In image C), which is at the level of the caudal pole of the left kidney, there is loss of normal renal architecture with mixed echogenic tissue replacing the normal renal parenchyma. Following nephrectomy this cat was diagnosed with renal lymphosarcoma and perinephric haemorrhage.
Renal Tissue Sampling
Fine needle aspiration of the kidney can be performed under ultrasound guidance and is used in the detection of highly cellular neoplasms such as renal carcinoma or lymphoma8.
A 22 - 25 gauge, 1.5 - 5cm needle can be used to aspirate the renal tissue. Care should be taken to avoid the renal hilus and surrounding vasculature. Using an acute angle between the needle and the skin can help to achieve this. The caudal pole of the left kidney is the easiest to access though the cranial pole of the left kidney and/or the caudal pole of the right kidney are usually accessible.
- Walter P.A., Feeney D.A., Johnston G.R., Fletcher T.F. (1987) Feline renal ultrasonography: Quantitative Analyses of Imaged Anatomy. American Journal of Veterinary Research 48: 596–599.
- d’Anjou M.A., Bédard A., Dunn M.E. (2011) Clinical Significance of Renal Pelvic Dilatation on Ultrasound in Dogs and Cats. Veterinary Radiology and Ultrasound 52: 88-94.
- Bragato N., Borges N.C., Fioravanti M.C.S. (2017) B-Mode and Doppler Ultrasound of Chronic Kidney Disease in Dogs and Cats. Veterinary Research Communications 41: 307-315.
- Mantis P., Lamb C.R. (2000) Most Dogs with Medullary Rim Sign on Ultrasonography Have No Demonstrable Renal Dysfunction. Veterinary Radiology and Ultrasound 41: 164-166.
- Adams W.H., Toal R.L., Breider M.A. (1991) Ultrasonographic Findings in Dogs and Cats with Oxalate Nephrosis Attributable to Ethylene Glycol Intoxication: 15 Cases (1984-1988). Journal of the American Veterinary Medicine Association 199: 492-496.
- Bryan J.N., Henry C.J., Turnquist S.E., Tyler J.W., Liptak J.M., Rizzo S.A., Sfiligoi G., Steinberg S.J, Smith A.N., Jackson T. (2006) Primary Renal Neoplasia of Dogs. Journal of Veterinary Internal Medicine 20: 1155-1160.
- Henry C.J., Turnquist S.E., Smith A., Graham J.C., Thamm D.H., O’Brien M., Clifford C.A. (1999) Primary Renal Tumours in Cats: 19 Cases (1992-1998). Journal of Feline Medicine and Surgery 1: 165-170.
- Liffman R., Courtman N. (2017) Fine Needle Aspiration of Abdominal Organs: A Review of Current Recommendations for Achieving a Diagnostic Sample. Journal of Small Animal Practice 58: 599–609.
d’Anjou M.A., Penninck D. (2015) Kidneys and Ureters, In: Atlas of Small Animal Ultrasonography 2nd edn., Eds. Penninck D., d’Anjou M.A., John Wiley & Sons, Inc. Chichester, pp 331-362.
Debruyn K., Haers H., Combes A., Paepe D., Peremans K., Vanderperren K., Saunders J.H. (2012) Ultrasonography of the Feline Kidney. Journal of Feline Medicine and Surgery 14: 794-803.
Graham J.P. (2011) Kidney and Proximal Ureters, In: BSAVA Manual of Canine and Feline Ultrasonography, Eds. Barr F., Gaschen L., BSAVA publications, Gloucester, pp 110-123.
Nyland T. G., Widmer W.R., Mattoon J.S. (2015) Urinary Tract, In: Small Animal Diagnostic Ultrasound 3rd edn., Eds. Mattoon J.S., Nyland T.G., Elsevier, St. Louis, pp 557-607.
Seiler G.S. (2018) Kidneys and Ureters, In: Textbook of Veterinary Diagnostic Radiology 7th edn., Ed. Thrall D., Elsevier, St. Louis, pp 823-845.
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