Elodie E. Huguet
DVM, DACVR (DI)
Dr. Huguet grew up in France before moving to South Carolina in 2001. She obtained her veterinary degree at the University of Georgia College of Veterinary Medicine, followed by radiology and small animal rotating internships in private practice and a radiology residency at Veterinary Specialty Hospital of the Carolinas and the University of Florida, respectively. Dr. Huguet is currently working part-time as a clinical assistant professor of diagnostic imaging at the University of Florida and is part of the IDEXX teleradiology team. When not working, she is an active long-distance runner and enjoys spending time with her dog, Arya, traveling, oil painting, and competing her horse, Stan, in the sport of dressage.Read Articles Written by Elodie E. Huguet
Clifford R. Berry
Dr. Berry is an adjunct professor of diagnostic imaging at the University of Florida and a clinical assistant professor of diagnostic imaging at North Carolina State University College of Veterinary Medicine. He received his DVM from University of Florida and completed a radiology residency at University of California–Davis. He has a specific interest in diagnostic imaging of the thorax.
Updated October 2022Read Articles Written by Clifford R. Berry
Robert C. Cole
DVM, DACVR (DI, EDI)
Dr. Cole is a professor of diagnostic imaging at Auburn University College of Veterinary Medicine. After obtaining his DVM degree from Auburn University, he spent 4 years in general mixed animal practice. He then completed a residency in diagnostic imaging at the University of Tennessee and spent 7 years in Texas in both academia and private practice before returning to Auburn University as a faculty member in the department of clinical sciences.Read Articles Written by Robert C. Cole
Chronic kidney disease (CKD) is a common diagnosis for dogs and cats; estimated prevalence in small animal general practice is approximately 0.5% to 1.5% among dogs and 1% to 3% among cats.1 CKD is defined as a progressive and irreversible loss of structural renal integrity and function.2 Although most prevalent among older patients, CKD can be identified in patients of all ages, from 9 months to 22 years.2 Although for most patients a cause for CKD is often not identified, various inciting etiologies result in either tubulointerstitial nephritis, glomerulonephropathy, or amyloidosis. Of these, tubulointerstitial nephritis is most commonly reported in both dogs and cats.1,3
During the early stages of CKD, patients are often asymptomatic and biochemical testing may not indicate renal abnormalities. Over time, the progressive loss of functional nephrons is accompanied by azotemia and elevated levels of SDMA (symmetric dimethylarginine). Clinical signs become apparent after normal kidney function is reduced by two thirds.4 End-stage kidney disease is defined as the critical loss of kidney structure and function.3 Kidneys play a major role in endocrine and metabolic functions; loss of kidney function results in generalized systemic disturbances with possible detrimental clinical implications.5 Ensuing uremic syndrome may result in critical decline of clinical status.4
When nephrons are injured and the interstitium is concurrently damaged, the tissues are eventually replaced with fibrosis and mineralization. The remaining nephrons will hypertrophy to compensate for the loss of function.3 The goal with therapy is to slow the progressive loss of nephrons and correct any sequelae (e.g., systemic hypertension, hyperphosphatemia, anemia).4 Successful long-term CKD therapy depends on early recognition and staging.6 For further evaluation of the kidneys in cats and dogs, the imaging modality of choice is ultrasonography due to its high resolution of the soft tissues, cost, and availability. Continual technologic advancements in ultrasonography systems enable detection of more subtle tissue alterations, and recognizing these changes is helpful for diagnosing and monitoring CKD in dogs and cats.
Patient Preparation and Machine Settings
Patient preparation is key for successful ultrasonography evaluation. For noncompliant patients, sedation may be indicated. Any fine needle or biopsy sampling should be performed with the patient under heavy sedation or anesthesia. The area of interest should be widely shaved, with alcohol and gel used as contact media to optimize image quality. The choice of probe is one with the highest frequency and enough penetration to enable visualization of the kidneys, which is best achieved with a linear transducer; however, a microconvex transducer may also be used if a smaller footprint or greater penetration is needed, such as in larger patients.
To optimize image quality, the following adjustments are recommended:
- Depth: Fill the field of view with the organ of interest.
- Focal zone: Place the probe at or just below the organ of interest. The focal zone represents the narrowest part of the ultrasonography beam, hence the region of highest spatial resolution and image quality.
- Gain: Adjust the overall brightness of the image so that there is good contrast between structures. In general, gain should be set lower to provide good contrast. Turning the gain up to compensate for a brightly lit room will make subtle lesions harder to find.
- Time gain compensation: Set the brightness at various depths within the field of view so that the overall image on the screen is uniform throughout, from near field to far field.
Normal Ultrasonographic Appearance
The normal ultrasonographic appearance of the kidney is well reviewed elsewhere,7 and this article focuses on CKD in cats and dogs. The kidneys should be imaged in 3 planes: sagittal, dorsal, and transverse (FIGURE 1).7
Figure 1. Ultrasonographic images of a normal kidney of a cat in (A) sagittal, (B) dorsal, and (C) transverse planes. Associated illustrations show the imaging planes. Illustrations: Kip Carter
A hyperechoic outer medullary band in dogs, especially small-breed dogs, has been described and is seen with moderate prevalence in patients without renal disease (FIGURE 2).8 In cats, renal cortices may be homogeneously strongly hyperechoic with subsequently increased corticomedullary distinction, secondary to proximal tubular lipidosis (FIGURE 3).9 Fat deposition within the renal tubules is a consequence of obesity without impairment of renal function.10 In addition, an interrogation angle–dependent artefactual increase in renal cortical and medullary hyperechogenicity secondary to anisotropy is routinely seen in regions of the kidneys where the renal tubules are oriented perpendicular to the plane of sound. For example, when imaging the kidney in a sagittal plane, echogenicity of the cranial and caudal poles will be focally and artefactually increased (FIGURE 4).11 In the authors’ experience, mild irregularity of the renal margins, especially in cats, can be seen where the renal vessels track along the renal capsule and should not be misinterpreted as a degenerative change (FIGURE 5). Color or power Doppler ultrasonography is useful for localizing these vessels.
Ultrasonographic Features of CKD
Ultrasonographic features of CKD have been reported with limited histopathologic correlation.12 In addition, many ultrasonographic findings associated with CKD are nonspecific and are reported with other infectious, inflammatory, and neoplastic etiologies. Unfortunately, not all dogs and cats with CKD will exhibit renal ultrasonographic changes. However, ultrasonography is still valuable for supporting a diagnosis of CKD established with other routine diagnostics, evaluating CKD sequelae, and ruling out other diseases.
Ultrasonographic features of CKD are as follows:
Renal Cortical Hyperechogenicity
The most common abnormality seen in dogs and cats with CKD is a diffuse increase in renal cortical hyperechogenicity (FIGURE 6).12,13 Degeneration of the renal cortex has been speculated as an etiology; however, a distinct cause remains uncertain, mostly due to replacement of the tissues with fibrous tissues with/without mineralization resulting in nonspecific histopathologic changes, especially in more advanced cases.13 In studies by Banzato et al and Zotti et al, increased cortical echogenicity in dogs was associated with glomerulosclerosis, tubular atrophy, and fibrosis; in cats, interstitial nephritis/necrosis and fibrosis were most prevalent.10,13 Alterations in renal cortical echogenicity were more commonly correlated with histopathologic abnormalities in cats than in dogs.13 In the human literature, dehydration is a reported cause of increased renal cortical hyperechogenicity.14 A similar change could be expected in dogs and cats with CKD caused by loss of renal fluid retention as a result of diuresis, which may lead to increased renal cortical echogenicity resulting from collapse of the Bowman’s space and tubule. Unfortunately, renal hyperechogenicity is a very nonspecific etiology with numerous other causes that include but are not limited to acute nephropathies (e.g., lily toxicosis in cats, leptospirosis in dogs).10
Reduced Corticomedullary Distinction
Good contrast is seen between the renal cortex and medulla due to higher cellular density within the renal cortex and fluid content within the medulla (FIGURE 7).15 With degeneration and loss of fluid retention in the medulla, medullary echogenicity progressively increases, leading to loss of corticomedullary distinction.15
Along with other processes, such as intravenous fluid administration or pyelonephritis, increased urinary secretion in patients with CKD is commonly associated with pyelectasia.16 Pyelectasia is the collection and subsequent dilation of the renal pelvis with fluid secondary to a nonobstructive process (FIGURE 8).8 In contrast, hydronephrosis is renal pelvic dilation secondary to urinary obstruction. Pyelectasia is most often symmetrical and bilateral; however, it may be unilateral or asymmetrically affect both kidneys.3,16 In patients with pyelectasia with renal insufficiency, renal pelvis height usually does not exceed 3.4 mm in dogs and 3 mm in cats.16 If renal pelvis dilation is higher, the urinary tract should be thoroughly evaluated for abnormalities to support the presence of other disease processes, such as pyelonephritis or hydronephrosis secondary to urinary tract obstruction.16
Irregular Renal Contour
Irregular renal margins (FIGURE 9) may be associated with one or more, often a combination, of fibrosis, sclerosis, atrophy, cyst formation, and infarction.12 Irregular renal margins are commonly seen with more advanced stages of CKD and may be a poor diagnostic indicator.8,15,17
Reduced Kidney Size
Ultrasonographic measurements of kidney size have been correlated with gross measurements at necropsy, although some underestimation of kidney length may be expected.18 Decreased kidney size (FIGURE 10) is expected with fibrosis; however, size may not directly correlate with the degree of fibrosis in the presence of concurrent compensatory hyperplasia, renal cysts, and/or pyelectasia.8,17 Small kidneys may indicate a higher degree of fibrosis than compensatory hypertrophy, therefore indicating more advanced stages of CKD. Wide variability is seen with dog size; differences between breeds limit evaluation of kidney measurements.18 A kidney-to-aorta ratio was previously proposed in dogs, with normal kidney size ranging from 5.5 to 9.5 cm19; however, this measurement is not commonly used clinically. Variation is less among cats of different breeds, and normal kidney lengths have previously been described as ranging from 3 to 4.5 cm.8
Dystrophic mineral deposition within the nephrons and interstitium can occur secondary to renal damage and can be difficult to differentiate from fibrosis. Histologic correlation is limited; however, mineralization (FIGURE 11) is speculated to primarily represent punctate and/or linear hyperechoic foci within the renal cortex, which are often too small to have distal acoustic shadowing.8 Hyperechoic streaks within the renal medulla, also thought to represent mineralization and/or fibrosis, are observed with more progressive renal disease.3 Although seen in dogs with and without CKD, a medullary rim sign has previously been correlated with possible mineralization of the renal tubular epithelium and basement membrane in dogs with a hypercalcemic nephropathy.9,20 Mineralization within the kidneys may be exacerbated by chronic hypercalcemia, resulting from parathyroid hormone elevation in response to decreased renal function. Although renal mineralization may be a response to CKD, it may also be a response to an underlying endocrinopathy or other metabolic disturbances.8,17 In addition, mineralization of the renal diverticula may represent a chronic sequela to prior and/or chronic pyelonephritis.
Renal cysts (FIGURE 12) occur more commonly in the cortex and can be acquired or congenital; therefore, without prior ultrasonographic examination for comparison, they are not a sensitive indicator of CKD in dogs.8,17 Renal cysts are common in dogs with CKD and most often do not affect renal function. They can distort the renal margins and may be a contributing cause of renal margin irregularity.8,12,21
Renal infarcts (FIGURE 13) most commonly affect the renal cortex and can vary in number and size.8,17 They are associated not only with CKD but with other causes, including underlying coagulopathy, sepsis, or neoplasia.8 Correlation with serologic testing is often warranted.3 Renal cortical infarcts also contribute to renal margin irregularity and rarely affect more than two-thirds of the renal parenchyma; therefore, they are often not the only clinically significant cause of renal function loss.17 Ultrasonographic features of renal infarcts may vary depending on whether acute or chronic.22 After experimental focal occlusion of a branch of the renal artery, focal swelling and hypoechogenicity of the renal cortex occurred within 24 hours and persisted for 5 to 7 days.21 In that study, after 7 days, echogenicity of the affected region progressively increased, and a gradually shrinking wedge-shaped lesion with a concave defect along the capsular margin was seen by day 17.
Additional Uses of Ultrasonography for Evaluation of CKD
In dogs with more advanced stages of CKD, increased severity and numbers of ultrasonographic abnormalities are seen.12 However, dogs and cats may have CKD without apparent ultrasonographic renal abnormalities.12 Although currently of limited use for diagnosing CKD, evaluating the kidneys with color and/or power Doppler ultrasonography, measuring resistive and pulsatility indices, shear wave elastography, and contrast-enhanced ultrasonography may help support a diagnosis in the absence of other ultrasonographic findings.
Color/Power Doppler Ultrasonography
Color/power Doppler ultrasonography is used to demonstrate the presence and/or reduction of intrarenal arterial flow to assess renal function. Hypovascularization has been described for all stages of CKD in cats but is reported to be a more severe and consistent change in cats with more progressed stages of CKD.22 In addition, color/power Doppler ultrasonography is useful for identifying renal infarcts, especially in the acute stages as swelling of the infarcted cortical region may resemble a nodule.8,17
Resistive and Pulsatility Indices
Resistive and pulsatility indices may be valuable for evaluating renal parenchymal perfusion.23,24
- Resistive index = (peak systolic velocity – end-diastolic velocity) / peak systolic velocity
- Pulsatility index = (peak systolic velocity – end-diastolic velocity) / mean systolic velocity
These indices are unitless measurements of the pulsed-wave Doppler signal used to assess vascular resistance. Measurements are obtained near the hilus to evaluate the segmental or interlobar arteries or the corticomedullary junction to evaluate the evaluation of the arcuate arteries.23,24
As described in the literature for human medicine, these measurements are affected by sedation, underlying heart disease, and/or abnormal blood pressure.25 Veterinary studies have found no correlation between resistive index and creatinine levels in cats and dogs, making this index of uncertain clinical reliability.25,26 In addition, a large overlap in resistivity index is reported for dogs with and without CKD.25 On the other hand, an elevated pulsatility index may represent a more dependable measurement of renal perfusion in dogs and cats when elevated renal markers in blood support a diagnosis of CKD.23 Normal reference ranges are shown in TABLE 1.15,20
Shear Wave Elastography
Shear wave elastography qualitatively and quantitatively measures tissue stiffness. Elevated renal stiffness in human patients with CKD, possibly resulting from fibrosis, has been reported.27 A similar change has been observed in the renal cortex of cats and dogs with CKD.28 However, using renal cortical stiffness measurement to diagnose CKD remains of limited reliability, and the need for special software restricts the availability of shear wave elastography in everyday veterinary practice.29
In the United States, availability of contrast-enhanced ultrasonography is limited and cost prohibitive. To evaluate renal perfusion, a contrast agent composed of microbubbles is injected intravenously. The most repeatable measurements of renal perfusion have been reported to be time to peak enhancement and rise and fall enhancement times.30 Long time to peak and shorter mean transit times may be observed for cats and dogs with CKD; however, interpretation may be affected by variations in ultrasonography settings, stability of the contrast media, and inherent patient factors (e.g., abnormal blood pressure, presence of heart disease, motion).28,30,31
Ultrasonographic Features of Advanced CKD
As CKD progresses, normal renal architecture may be lost. The kidneys become severely small and irregular. The kidneys appear exceedingly hypovascular on color and power Doppler ultrasonography interrogation and may be hard to identify with standard B-mode ultrasonography. If one of the kidneys is more severely affected, the contralateral kidney may enlarge as its nephrons enlarge as a compensatory response to increase functional capacity of remaining nephrons.3 As a consequence of severe CKD in dogs and cats, multisystemic complications of chronic uremia may develop,3 most commonly soft tissue mineralization. Proposed underlying etiologies, which may occur in combination, are metastatic mineralization caused by hypercalcemia from secondary hyperthyroidism and dystrophic mineralization caused by replacement of inflamed tissues with minerals in response to circulating toxins in uremic patients.32,33 A recent study by Cardoso et al identified the kidneys as the most common site of mineralization; however, other affected sites evaluated during abdominal ultrasonography are gastric mucosa, aorta, and intestine.32 Nephrotic syndrome may result from excessive urinary loss of proteins, with ensuing hypercoagulability and thromboembolization. Multisystemic evidence of infarction observed with ultrasonography should be correlated with serologic testing to support a diagnosis.3 Other possible observations are peritoneal effusion secondary to portal hypertension from underlying thromboembolism or third spacing of fluid as a direct response to the hypoproteinemia. These syndromes are serious conditions that should be recognized to direct therapeutic recommendations and improve prognosis.
Ultrasonography plays a key supportive role in identifying, staging, and monitoring CKD in dogs and cats. Ultrasonographic abnormalities may be observed before serologic abnormalities are evident, therefore prompting early management of CKD in affected patients. However, ultrasonographic appearance of the kidneys may remain normal in patients with CKD. Therefore, a diagnostic work-up of CKD is most complete if combined with serologic testing. In addition, ultrasonography may guide further diagnostic and treatment recommendations if other or non–CKD-related lesions are seen, such as neoplasia, pyelonephritis, or evidence of proximal urinary obstruction. Ultrasonography is also useful for screening for the clinical manifestations of CKD sequelae, such as uremic or nephrotic syndrome, which are signs of more advanced CKD, requiring more extensive medical support.
- Polzin DJ. Chronic kidney disease In: Bartges J, Polzin DJ, eds. Nephrology and Urology of Small Animals. Wiley Blackwell; 2011:433-471.
- DiBartola, SP, Rutgers HC, Zack PM, Tarr MJ. Clinicopathologic findings associated with chronic renal disease in cats: 74 cases (1973-1984). JAVMA. 1987;190(9):1196-1202.
- Zachary JF, McGavin MD. Pathologic Basis of Veterinary Disease. 5th edition. Elsevier Health Sciences; 2012.
- McGrotty Y. Diagnosis and management of chronic kidney disease in dogs and cats. In Practice. 2008;30(9):502-507. doi:10.1136/inpract.30.9.502
- Bartges JW. Chronic kidney disease in dogs and cats. Vet Clin North Am Small Anim Pract. 2012;42(4):669-692. doi:10.1016/j.cvsm.2012.04.008
- International Renal Interest Society. IRIS staging of CKD. Accessed January 2023. http://www.iris-kidney.com/guidelines/staging.html
- Huynh E, Berry CR. Ultrasonography of the urinary tract: kidneys and ureters. Todays Vet Pract. 2017;7(6):31-45.
- Penninck D, d’Anjou M-A. Atlas of Small Animal Ultrasonography. Wiley Blackwell; 1991.
- Yeager AE, Anderson WI. Study of association between histologic features and echogenicity of architecturally normal cat kidneys. Am J Vet Res. 1989;50(6):860-863.
- Banzato T, Bonsembiante F, Aresu L, Zotti A. Relationship of diagnostic accuracy of renal cortical echogenicity with renal histopathology in dogs and cats, a quantitative study. BMC Vet Res. 2016;13(1):1-8. doi:10.1186/s12917-016-0941-z
- Ruth JD, Heng HG, Miller MA, Constable PD. Effect of anisotropy and spatial compound imaging on renal cortical echogenicity in dogs. Vet Radiol Ultrasound. 2013;54(6):659–665. doi:10.1111/vru.12056
- Perondi F, Lippi I, Marchetti V, Bruno B, Borrelli A, Citi S. How ultrasound can be useful for staging chronic kidney disease in dogs: ultrasound findings in 855 cases. Vet Sci. 2020;7(4):147. doi:10.3390/vetsci7040147
- Zotti A, Banzato T, Gelain ME, Centelleghe C, Vaccaro C, Aresu L. Correlation of renal histopathology with renal echogenicity in dogs and cats: an ex-vivo quantitative study. BMC Vet Res. 2015;11:1-8. doi:10.1186/s12917-015-0415-8
- Chien CC, Chou YH, Tiu CM, et al. Transient hyperechoic renal cortex caused by dehydration and induced acute renal failure in two patients with intra-abdominal infection. J Med Ultrasound. 2012;20(1):43-46. doi:10.1016/j.jmu.2012.01.004
- Bragato N, Borges NC, Fioravanti MCS. B-mode and Doppler ultrasound of chronic kidney disease in dogs and cats. Vet Res Commun. 2017;41(4):307-315. doi:10.1007/s11259-017-9694-9
- D’Anjou MA, Bedard A, Dunn ME. Clinical significance of renal pelvic dilatation on ultrasound in dogs and cats. Vet Radiol Ultrasound. 2011;52(1):88–94. doi:10.1111/j.1740-8261.2010.01729.x
- Mattoon JS, Sellon RK, Berry CR. Small Animal Diagnostic Ultrasound. 4th edition. Elsevier Health Sciences; 2020.
- Barr FJ, Holt PE, Gibbs C. Ultrasonographic measurement of normal renal parameters. J Small Anim Pract. 1990;31(4):180-184. doi:10.1111/j.1748-5827.1990.tb00764.x
- Mareschal A, D’Anjou MA, Moreau M, Alexander K, Beauregard G. Ultrasonographic measurement of kidney-to-aorta ratio as a method of estimating renal size in dogs. Vet Radiol Ultrasound. 2007;48(5):434-438. doi:10.1111/j.1740-8261.2007.00274.x
- Biller DS, Bradley GA, Partington BP. Renal medullary rim sign: ultrasonographic evidence of renal disease. Vet Radiol Ultrasound. 1992;33(5):286-290. doi:10.1111/j.1740-8261.1992.tb00145.x
- Spies JB, Hricak H, Slemmer TM, et al. Sonographic evaluation of experimental acute renal arterial occlusion in dogs. Am J Roentgenol. 1984:142(2):341-346. doi:10.2214/ajr.142.2.341
- Saraiva FH. O exame ultrassonográfico modo B, Doppler colorido e pulsado na avaliação da doença renal crônica em felinos. [Masters dissertation]. São Paulo, Brazil: Universidade de São Paulo; 2010.
- Novellas R, de Gopegui RR, Espada Y. Assessment of renal vascular resistance and blood pressure in dogs and cats with renal disease. Vet Rec. 2010;166(20):618-623. doi:10.1136/vr.b4820
- Rivers BJ, Walter PA, Letourneau JG, et al. Duplex Doppler estimation of resistive index in arcuate arteries of sedated, normal female dogs: implications for use in the diagnosis of renal failure. JAAHA. 1997;33(1):69-76. doi:10.5326/15473317-33-1-69
- Heine GH, Reichart B, Ulrich C, Köhler H, Girndt M. Do ultrasound renal resistance indices reflect systemic rather than renal vascular damage in chronic kidney disease? Nephrol Dial Transplant. 2006;22(1):163-170. doi:10.1093/ndt/gfl484
- Rivers BJ, Walter PA, Polzin DJ, King VL. Duplex Doppler estimation of intrarenal Pourcelot resistive index in dogs and cats with renal disease. J Vet Intern Med. 1997;11(4):250-260. doi:10.1111/j.1939-1676.1997.tb00099.x
- Asano K, Ogata A, Tanaka K, et al. Acoustic radiation force impulse elastography of the kidneys: is shear wave velocity affected by tissue fibrosis or renal blood flow? J Ultrasound Med. 2014;33(5):793-801. doi:10.7863/ultra.33.5.793
- da Cruz ICK, Gasser B, Maronezi MC, et al. Applicability of B-mode ultrasonography, ARFI elastography and contrast-enhanced ultrasound in the evaluation of chronic kidney disease in dogs. Pesqui Vet Brasil. 2021;411:e06785. doi:10.1590/1678-5150-PVB-6785
- Holdsworth A, Bradley K, Birch S, Browne WJ, Barberet V. Elastography of the normal canine liver, spleen and kidneys. Vet Radiol Ultrasound. 2014;55(6):620-627. doi:10.1111/vru.12169
- Liu DJX, Hesta M, Stock E, et al. Renal perfusion parameters measured by contrast-enhanced ultrasound in healthy dogs demonstrate a wide range of variability in the long-term. Vet Radiol Ultrasound. 2019;60(2):201-209. doi:10.1111/vru.12690
- Stock E, Paepe D, Daminet S, et al. Contrast-enhanced ultrasound examination for the assessment of renal perfusion in cats with chronic kidney disease. J Vet Intern Med. 2018;32(1):260-266. doi: 10.1111/jvim.14869
- Cardoso PGS, Pinto MPR, Moroz LR, et al. Dystrophic mineralization
in uremic dogs: an update. Pesqui Vet Brasil. 2019;39(11):889-899. doi:10.1590/1678-5150-PVB-6250
- Cheville NF. Uremic gastropathy in the dog. Vet Pathol. 1979;16(3):292-309. doi:10.1177/030098587901600303