Ultrasonography of the canine kidneys

Introduction

The main objective of an ultrasonography examination of the kidneys is to discover the nature of renal changes in order to propose the most appropriate treatment. The key to a good ultrasonography assessment of the kidneys lies in a complete and rigorous examination, a list of suitable differential diagnoses, good correlation with clinical data, and sampling when necessary. Renal ultrasonography requires not only a mastery of the anatomy and topography of the kidneys, but also an adequate knowledge of individual variations (according to breed and age, for example) and the main ultrasonographic changes associated with the most frequent pathologies. This article reviews the major factors involved when imaging the canine renal system, and discusses common findings.

General principles and technique

When performing an ultrasound examination, the clinician must pay particular attention to a number of fundamental features, such as the size of both kidneys, their contours, the preservation or otherwise of their internal architecture, their relative echogenicity (cortex and medulla) and finally the appearance of the collecting system (1). An assessment of the adjacent retroperitoneal space often provides additional information to help interpret any observed renal changes. Although certain lesions may be specific to a condition (e.g., renal cysts, obstructive calculi or renal masses), it is often the case that the ultrasonic appearance of the kidneys is rather non-specific, as the same lesion may correspond to different conditions, or the same condition may produce different lesions in different individuals, justifying the need for sampling (ultrasound-guided cytopuncture or biopsies) (2). 

The kidneys can be examined with the patient in the supine or lateral position. The left kidney, which is more accessible, is generally well visualized by a ventrolateral approach. The right kidney is more difficult to visualize, due to its craniodorsal position, particularly in deep-chested dogs, and whilst a right subcostal approach is often sufficient, an intercostal approach to the 11^th or 12^th space is sometimes necessary. The contents of the digestive tract (food, gas or stool in the colon) can also interfere with visualization of the kidneys.

The choice of ultrasonic probe depends on the size of the animal and the depth of the kidneys. Microconvex probes are particularly useful for visualizing structures below the costal arch or for an intercostal approach. Multi-frequency probes (> 5 MHz) are versatile and suitable for most dogs, while linear probes offer excellent resolution for more superficial or easily accessible kidneys, especially in smaller patients.

Normal ultrasonographic anatomy

The kidneys, which are generally more or less egg-shaped, should be symmetrical, identical in shape and regular in outline (Figure 1). However, kidney size varies widely in dogs depending on breed, weight and conformation, making estimation difficult and rather subjective. Some authors have proposed more objective methods for measuring organ size, such as the ratio of kidney length to aortic diameter (normal values between 5.5-9.1) (3) or the ratio of kidney length to the length of the L5 or L6 vertebra (normal values between 1.3-2.7), but the extent of physiological variation limits their use in daily practice.

Figure 1. Normal ultrasonic anatomy of the kidney in sagittal (a) and transverse (b) planes. 1) cortex, 2) medulla, 3) renal crest, 4) renal pelvis, 5) renal sinus.
© Eric Norman Carmel

For clinicians with less ultrasound experience, it is often easier to compare the echogenicity of the renal cortex with that of adjacent organs (4). The cortex should normally be hypoechogenic or isoechogenic in relation to the liver (for the right kidney) and typically hypoechogenic in relation to the spleen (for the left kidney). However, some healthy dogs may show a hyperechogenic cortex in relation to the liver, thus challenging this commonly accepted rule. The angle of the ultrasound beam can also modify the perceived echogenicity of the cortex, and the renal poles sometimes appear focally more echogenic due to cortical anisotropy, an artefactual phenomenon not to be confused with real lesions such as cortical infarcts.

The normal renal medulla is hypoechogenic compared to the cortex, and the corticomedullary junction should be easily perceptible (5). The medulla is segmented into lobes, separated by interlobar vessels and renal diverticula, which appear as hyperechogenic linear structures. If settings are highly contrasted, the medulla can sometimes appear virtually anechogenic, and this should not be confused with pathological dilatation of the collecting system. Occasionally, the outer part of the medulla is more echogenic and sometimes even slightly hyperechogenic compared with the adjacent cortex, particularly in younger patients or smaller breeds, which can give a false impression of cortical thickening or a hyperechogenic band running parallel to the cortex (Figure 2).

Figure 2. Hyperechogenicity of the external medulla in a 9-year-old Chihuahua with no other renal abnormalities.
© Eric Norman Carmel

The renal sinus corresponds to the central cavity of the kidney, which is mainly filled with fat and appears in cross-section as a hyperechogenic “C” – or “Y”– shaped area in the center of the kidney. The renal pelvis is located in the renal sinus, bordered laterally by the renal crest, an extension of the deep medulla, and extended medially by the ureter at the level of the renal hilum (Figure 1). The height of the renal pelvis is generally less than 2 mm in normal dogs, and sometimes not measurable. Conversely, renal diverticula and the ureter are not normally visible in dogs.

Renal arteries and veins are visible at the level of the renal hilum, and Doppler can be useful in distinguishing them from a dilated ureter. The walls of arched arteries can be seen at the corticomedullary junction as short, paired hyperechogenic lines; it is important not to confuse this finding with peridiverticular mineralization. 

Congenital anomalies

Congenital renal malformations are rare in dogs. Complete absence (agenesis) or incomplete development (hypoplasia) of a kidney often results in compensatory contralateral nephromegaly. However, renal dysplasia is a hereditary disease that can affect several breeds; if present, the kidneys are generally small and/or irregularly contoured, with cortical hyperechogenicity and attenuation of the corticomedullary demarcation. The consequences for renal function are variable, and the age at which clinical signs may appear also varies, but the condition should be suspected in any young dog with renal failure and altered kidney architecture.

Diffuse parenchymal changes

Increased renal echogenicity is a frequent sonographic finding in dogs with renal failure, and may affect the cortex, medulla or both. It is, however, a non-specific change that can be observed in both acute and chronic phases, and it has been described in a wide range of conditions, including interstitial and glomerular nephritis, toxic nephropathy (e.g. ethylene glycol or grape nephropathy), acute tubular necrosis and nephrocalcinosis. 

The distribution of this hyperechogenicity affects the perception of the ultrasonic distinction between cortex and medulla. For example, the echogenicity of the renal cortex can be greatly increased in ethylene glycol intoxication due to calcium oxalate deposits, resulting in accentuated corticomedullary demarcation in affected dogs (Figure 3). In contrast, the more diffuse increase in echogenicity affecting both cortex and medulla that is generally observed in chronic kidney disease attenuates the corticomedullary demarcation.

Figure 3. Marked cortical hyperechogenicity in a dog with ethylene glycol intoxication (a) and a dog with leptospirosis (b). A perirenal effusion accompanies the cortical changes in the case of leptospirosis (white asterisk).
© Eric Norman Carmel

A distinct hyperechogenic line is also sometimes observed within the medulla parallel to the corticomedullary junction, often described as a medullary “rim sign” in the literature (6). This should not be confused with the normal hyperechogenicity of the outer medulla, particularly common in small breeds. This more or less thickened medullary line has been described in dogs with various acute and chronic renal disorders (such as hypercalcemic nephropathy, chronic interstitial nephritis and acute tubular necrosis), possibly reflecting damage to metabolically active deep tubules that would be more vulnerable to ischemia. The clinical relevance of this sign remains uncertain, however, as healthy dogs also show this ultrasonographic feature (Figure 4). Distinguishing between different types of medullary rim sign could, however, be an interesting future avenue, as a thick, ill-defined band seems to correlate more strongly with renal disease in cats.

Figure 4. A defined hyperechogenic band or “rim sign” (white arrows) in the medulla of a 6-year-old Border Collie with Lyme nephritis (a) and in a female Siberian Husky with blood tests and urinalysis that were within normal limits (b).
© Eric Norman Carmel

Acute nephropathy of an infectious (e.g., pyelonephritis, leptospirosis (7), Lyme nephritis) or toxic nature is usually distinguished by concomitant smooth-edged renomegaly, and is often accompanied by signs of perirenal inflammation (Figure 5) (8).

Figure 5. Signs of perirenal inflammation in a dog with pyelonephritis. The perirenal fat is diffusely hyperechogenic (white arrowheads) and an effusion is visible around the kidney (white asterisks). Echogenic content can also be seen in the dilated renal pelvis (white arrow).
© Eric Norman Carmel

In older dogs, the kidneys may show several structural changes without any underlying clinical renal disease. Chronic interstitial nephritis often manifests as small, irregular, diffusely hyperechogenic kidneys and somewhat more heterogeneous cortices, reflecting fibrotic remodeling (Figure 6). 

Figure 6. Chronic age-related changes in the left (a) and right kidney (b) of a 12-year-old male Bichon Frise with normal renal function. The cortices are hyperechogenic, and there are small degenerative cortical cysts (white asterisks) and a few foci of peridiverticular mineralization (white arrow).
© Eric Norman Carmel

Focal lesions

Cystic lesions

Benign renal cysts typically present as well-defined round or oval anechogenic structures with a thin hyperechogenic wall and distal acoustic enhancement. They may be solitary or multiple, and vary in size (Figure 7). Renal cysts may be primary or congenital, but (unlike cats) polycystic kidney disease is rare in dogs, and is limited to a few predisposed breeds. Cysts are more frequently degenerative in nature and secondary to another renal condition, usually chronic kidney disease, and are most often an incidental finding with no clinical impact. 
Some more complex cysts may show internal septations or echogenic debris of a hemorrhagic or necrotic nature; these may mimic other lesions such as abscesses, hematomas or solid masses. Superinfected cysts may have a similar echogenic appearance. Renal abscesses are rare and tend to present with internal echoes, sedimentation, and less well-defined contours. Distal acoustic enhancement may, however, persist in more echogenic lesions if cell/protein content is low. Cysts that become particularly large and distort the renal capsule may become a source of abdominal discomfort. 

A few uncommon conditions may present with a cystic appearance; it is important not to confuse these with benign renal cysts. Some malignant tumors may show areas of cavitation secondary to necrosis, or a cystic component infiltrating the kidney. Perirenal pseudocysts are subcapsular or perirenal fluid accumulations that manifest as an anechogenic cavity around a mostly abnormal kidney. 

Figure 7. (a) Multiple small degenerative cysts (white arrowheads) secondary to chronic nephropathy in the left kidney of a 13-year-old Miniature Greyhound. (b) Large benign cyst (white arrows) in the caudal pole of the left kidney (white asterisk) of a 12-year-old female Dachshund presenting with abdominal discomfort, with no other ultrasound abnormalities.
© Eric Norman Carmel

Renal tumors

Primary renal tumors are rare in dogs (9), with carcinomas being the most common (Figure 8a). Apart from lymphoma and histiocytic sarcoma, which are often associated with the presence of hypoechogenic nodules or masses (Figure 8b), the sonographic appearance of other primary (hemangiomas, nephroblastomas and various sarcomas) or metastatic neoplastic processes is rather non-specific and varies widely. 

Renal masses can vary in size, regularity of shape, clarity of contour and echotexture, depending on cellular origin, vascularity and the presence of necrosis, fibrosis, mineralization or hemorrhagic zones. Occasionally the kidney architecture is so altered that it becomes difficult to recognize the affected organ, with the renal vessels being the only anatomical sign to confirm a renal origin (Figure 8a).

Figure 8. (a) Renal cell carcinoma in the right kidney of a 5-year-old American Cocker Spaniel. A large heterogeneous mass (dotted line) almost completely replaces the normal architecture of the kidney, with a small band of cortex remaining visible on the caudal aspect of the kidney (white arrows). (b) Renal lymphoma in a 7-year-old Golden Retriever; both kidneys contain multiple hypoechogenic nodules (white asterisks) that deform the renal capsule in several places.
© Eric Norman Carmel

Other focal lesions

Renal mineralization is common, particularly in older dogs and small breeds. They are frequently localized in the peridiverticular region and appear as hyperechogenic foci with more or less obvious distal acoustic shadowing. It can be difficult to distinguish between dystrophic soft tissue mineralization and true small nephroliths, both of which can accompany chronic kidney disease.

Chronic renal infarcts are typically identified in the cortex as triangular or linear hyperechogenic lesions perpendicular to the capsule, and are usually associated with focal indentation or depression of the cortex.

Collecting system anomalies

The renal pelvis and diverticula are not usually distended in normal dogs, and often cannot be measured. However, dilatation of the renal pelvis (pyelectasis) is a frequent sonographic finding (10). If present, it is best visualized on a cross-section through the renal hilum, where it appears as a more or less flared anechogenic crescent within the renal sinus, medial to the renal crest. Renal pelvis height should always be measured in the transverse plane, to avoid over/underestimation of the measurement, even when dilatation is evident in the sagittal plane.

Mild pyelectasis may be observed in normal animals and may even exceed 3 mm in patients with increased diuresis (e.g. intravenous fluid therapy, diuretic treatment or chronic kidney disease). Larger (> 4 mm) pyelectasis usually develops in cases of congenital malformation involving the ureters (ectopic ureter or other), pyelonephritis or urinary tract obstruction (Figure 9). In more advanced stages of pyelectasis, or when hydronephrosis is present as a result of obstruction of urinary flow, diverticula dilate to form rounded anechogenic projections at the margins of the dilated pelvis, visible between the lobes of the medulla.

Figure 9. Different degrees of renal pelvis dilatation. (a) Mild pyelectasis in a 10-year-old Miniature Schnauzer. (b) Marked pyelectasis in a young French Bulldog with infectious medial iliac lymphadenitis compressing the terminal portion of the right ureter. (c) Bilateral hydronephrosis (here the right kidney) in a 9-year-old Labernese with a prostatic mass invading the region of the bladder trigone. The diverticula are severely dilated (white asterisks) and only a thin band of cortex remains visible at the periphery of the kidney (white arrowheads).
© Eric Norman Carmel

Pyelonephritis is usually associated with significant pyelectasis, although the degree of pelvic dilatation may vary, particularly in the early stages. The presence of proteins and/or cells (pus or blood) in the renal pelvis usually results in increased echogenicity of its contents (Figure 10). In more chronic cases, the pelvis and diverticula may become permanently deformed, with a hyperechogenic rim due to fibrous remodeling. Pyonephrosis is an accumulation of pus in the renal pelvis, usually as the result of obstructive pyelonephritis. A characteristic sign of this condition is the formation of a liquid-debris interface that varies with severity.

Unlike pyelectasis, the term hydronephrosis refers more specifically to an obstructive phenomenon, whether caused by the migration of a nephrolith, by a neoplastic process infiltrating a portion of the urinary tract (urothelial carcinoma infiltrating the ureterovesical junctions) or by a retroperitoneal mass effect compressing a portion of the urinary tract. Renal pelvis dilatation of 13 mm or more is strongly suggestive of obstruction. If the obstruction is severe or becomes more chronic, the pressure exerted by the progressive accumulation of urine can lead to thinning of the renal cortex, eventually transforming the kidney into an anechogenic cavity surrounded by a thin wall.

Figure 10. Pyelonephritis in a 9-year-old Australian Shepherd with azotemia and pyuria. The right renal pelvis and proximal ureter are dilated (white asterisks) and echogenic material is present in the lumen (white arrow). The renal sinus fat is hyperechogenic (black asterisks). The renal cortex is hyperechogenic and the corticomedullary demarcation is reduced.
© Eric Norman Carmel

Ureteral dilatation is generally observed in association with hydronephrosis in cases of urinary tract obstruction distal to the renal pelvis. The path of a dilated ureter is easier to follow with little experience and the use of a high-frequency probe. Doppler can also be useful to help distinguish between a dilated ureter and an abdominal vessel. As in the rest of the urinary tract, ureteroliths are hyperechogenic and most often associated with acoustic shadowing (Figure 11). In the case of complete obstruction, the ureter is usually dilated to the site of obstruction and then abruptly thin and collapsed distally, but when performing the examination it is important to try to follow the entire ureteral pathway, from the kidney outlet to the ureterovesical junction, since several ureteroliths may be present.

Figure 11. Ureteral obstruction in a 10-year-old Chihuahua with a calculus generating acoustic shadowing and localized in the proximal third of the right ureter (dotted circle). The ureter is dilated proximal to the ureterolith, with a thickened wall (white arrows). The renal pelvis is also dilated (white asterisk). The fatty areas of the renal sinus and along the dilated ureter are hyperechogenic.
© Eric Norman Carmel

Ultrasound-guided procedures

Ultrasound-guided cytopuncture of the kidney is performed using a technique similar to that used for other abdominal organs, although care must be taken to avoid the renal hilum to limit the risk of life-threatening hemorrhage. When changes are bilateral and/or diffuse, the caudal pole of the left kidney is preferred for sampling because of its easier access. Ultrasound-guided cytopunctures are particularly useful in cases of suspected lymphoma, as they often enable a definitive cytological diagnosis to be made. Renal cysts are not usually sampled if they appear benign and asymptomatic on ultrasound, but cytopuncture may be performed if the cyst has atypical features (such as a thick wall, echogenic contents or tissue component protruding into the cyst). Some larger cysts distort the renal capsule and can become a source of abdominal discomfort, requiring echo-guided drainage.

Conclusion

Ultrasonography is an invaluable tool to assess all patients with suspect kidney pathologies, as it is painless, non-invasive and often diagnostic. The time required to master the interpretation of scans will be time well spent, as even if imaging does not always provide a definitive answer, it will often direct the clinician towards further appropriate tests. However, it is important to recognize that some artifacts can be found at imaging that may lead to an erroneous diagnosis, and an appreciation of normal variations and incidental findings is essential.

References

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