# Ultrasound of the Canine and Feline Urinary Tract: Kidneys, Ureters, and Bladder ## Key Takeaways - Ultrasonography is the primary imaging modality for the canine and feline urinary tract, offering real-time assessment of renal size, contour, parenchymal architecture, ureters, and bladder without ionizing radiation. - Normal renal length varies significantly between species, with canine adult kidneys measuring 6.5-9.5 cm and feline kidneys 3.0-4.3 cm; a renal length to aortic diameter ratio below 5.0 suggests reduced renal size in both species. - The resistive index (RI) of arcuate arteries, typically 0.62-0.73 in normal dogs and 0.52-0.63 in normal cats, quantifies renal vascular resistance and elevated values suggest increased resistance but are not specific to etiology. - Chronic kidney disease in both species manifests sonographically as reduced renal size, increased cortical echogenicity, loss of corticomedullary distinction, and irregular contours, though normal ultrasonography does not exclude early disease. - Feline polycystic kidney disease, an autosomal dominant inherited condition, is characterized by multiple anechoic cysts throughout the renal parenchyma and can be detected ultrasonographically from as early as 13 weeks of age, particularly in predisposed breeds like Persians. - Ureteral obstruction is indicated by dilation of the renal pelvis and ureter, with ureteroliths being a common cause in cats, appearing as hyperechoic foci with distal shadowing within the dilated ureter. --- Ultrasonography is the first-line imaging modality for the canine and feline urinary tract in most practice settings. It provides real-time assessment of renal size, contour, and parenchymal architecture, permits evaluation of the ureters and bladder without ionizing radiation, and guides interventional procedures such as cystocentesis, renal biopsy, and nephrostomy tube placement. This article reviews the technical approach, normal sonographic anatomy, and abnormal findings for the kidneys, ureters, and urinary bladder in dogs and cats, with emphasis on diagnostic reasoning and pattern recognition. The intended reader is the practicing veterinarian who performs or interprets abdominal ultrasound in small animal patients. The content assumes familiarity with basic ultrasound physics, transducer selection, and image optimization. Clinical questions addressed include how to distinguish incidental findings from clinically significant disease, how to characterize renal parenchymal disease, when ureteral dilation is meaningful, and how to differentiate cystic from solid bladder masses. Species differences between dogs and cats are highlighted throughout, as normal values and disease prevalence differ substantially. ## At a Glance | Parameter | Canine | Feline | Clinical Decision Point | |---|---|---|---| | Renal length (adult) | 6.5 to 9.5 cm, varies with body size | 3.0 to 4.3 cm | Length below reference range supports chronic kidney disease when other criteria present | | Renal length to aortic diameter ratio | 5.5 to 9.1 | 5.0 to 8.0 | Ratio below 5.0 suggests reduced renal size | | Cortical echogenicity | Isoechoic to hypoechoic relative to liver, hypoechoic to spleen | Isoechoic to hypoechoic relative to liver | Increased cortical echogenicity is nonspecific but supports parenchymal disease | | Medullary rim sign | Incidental in many dogs | Incidental in many cats, more common with age | Not diagnostic of disease in isolation | | Resistive index (arcuate artery) | 0.62 to 0.73 in normal dogs | 0.52 to 0.63 in normal cats | Values above reference range suggest increased vascular resistance but do not identify etiology | | Renal pelvis diameter | Less than 2 to 3 mm | Less than 1.5 to 2 mm | Dilation beyond these values warrants ureteral evaluation | | Ureteral diameter | Less than 2 to 3 mm | Less than 2 mm | Dilation with ipsilateral pelvic dilation indicates obstruction until proven otherwise | | Bladder wall thickness (moderately distended) | 1.4 to 2.3 mm | 1.3 to 1.7 mm | Focal or asymmetric thickening requires characterization and biopsy consideration | ## Physical Principles and Technique Ultrasound image quality in the urinary tract depends on transducer frequency, patient preparation, and systematic scanning. A microconvex or phased array transducer with a frequency of 5 to 10 MHz suits most dogs and cats, with higher frequencies (10 to 18 MHz) reserved for small patients and superficial structures. The kidneys lie retroperitoneally, and the right kidney sits within the renal fossa of the caudate liver lobe, which provides an acoustic window. The left kidney is more caudal and mobile, often requiring a more dorsal approach. Patient positioning varies by operator preference and patient tolerance. Dorsal recumbency with the transducer placed ventrolaterally allows consistent sagittal and transverse imaging of both kidneys. Lateral recumbency may improve access to the dependent kidney. The spleen should not be mistaken for the left kidney, as both are located in the left cranial abdomen, the kidney is distinguished by its reniform shape and distinct capsule. Hair clipping is required for diagnostic-quality images in most patients, and coupling gel should be applied generously to eliminate air interfaces. The ureters are normally difficult to identify unless dilated. Systematic evaluation begins at the renal pelvis and follows the course of each ureter caudally to the bladder trigone. The bladder should be assessed before and after voiding, as wall thickness and luminal content change with distension. Color Doppler is used to identify ureteral jets at the trigone, which confirms patency when visible. ## Normal Sonographic Anatomy The normal kidney has a smooth, well-defined capsule and a distinct corticomedullary junction. The cortex is homogeneous and isoechoic to hypoechoic relative to the liver in both species, and hypoechoic relative to the spleen in dogs. The medulla is hypoechoic relative to the cortex, and the renal pelvis is a hyperechoic linear structure that should not be mistaken for a calculus when nondilated. The feline kidney is more likely to have a mildly irregular contour and a slightly lobulated surface, which is normal. Renal length is the most commonly used size parameter, but it varies with body weight in dogs. The ratio of renal length to aortic diameter, measured in a sagittal plane, normalizes for body size and is useful in both species. A ratio below 5.0 indicates reduced renal size, while a ratio above 9.1 in dogs suggests renomegaly. In cats, renal length of 3.0 to 4.3 cm is typical, with values below 3.0 cm supporting chronic kidney disease when accompanied by other sonographic changes. The medullary rim sign, a hyperechoic line at the corticomedullary junction, is a common incidental finding in both species. It is associated with hypercalcemia, hypercalciuria, and ethylene glycol toxicity in some patients, but in most cases it has no clinical significance. The feline kidney also commonly shows a thin, anechoic subcapsular space that is normal and should not be confused with perinephric fluid accumulation. Griffin (2020) emphasizes that differentiating these incidental findings from pathologic changes is a central challenge in feline renal ultrasonography. ## Doppler Assessment of Renal Blood Flow Duplex Doppler evaluation of intrarenal arterial flow provides a quantitative measure of vascular resistance. The Pourcelot resistive index (RI) is calculated as (peak systolic velocity minus end diastolic velocity) divided by peak systolic velocity. Measurements are typically obtained from arcuate or interlobar arteries at the corticomedullary junction. In sedated normal cats, the 95% confidence interval for the resistive index is 0.52 to 0.60 in the left kidney and 0.55 to 0.63 in the right kidney, as reported by Rivers et al. (1996). Dogs have a slightly higher reference range, generally 0.62 to 0.73. Elevated resistive index indicates increased renovascular resistance but is not specific for any single disease process. It is observed in acute kidney injury, chronic kidney disease, urinary obstruction, and systemic hypotension. A resistive index above 0.75 in dogs or 0.70 in cats warrants investigation, but the measurement must be interpreted alongside B-mode findings and laboratory data. The resistive index is most useful as a serial monitoring tool instead of a one-time diagnostic test. ## Renal Parenchymal Disease Chronic kidney disease produces a spectrum of sonographic changes that vary with severity and chronicity. The kidney may be normal in size or reduced, the cortex becomes hyperechoic, and the corticomedullary junction loses its distinct definition. Irregular cortical contours and small cortical cysts may develop. These changes are progressive but not pathognomonic, and normal ultrasonography does not exclude early chronic kidney disease. Debruyn et al. (2012) note that diffuse parenchymal changes are more challenging to identify than focal lesions, and that B-mode ultrasonography has limited ability to differentiate benign from malignant focal lesions without contrast enhancement. Acute kidney injury may show renomegaly with normal or increased cortical echogenicity, often with a prominent medullary rim sign. The distinction between acute and chronic disease relies on renal size, contour, and clinical history, as sonographic features overlap. Perinephric pseudocysts in cats appear as large, anechoic fluid accumulations surrounding a normal or small kidney, and they are associated with chronic kidney disease. Feline polycystic kidney disease is an inherited autosomal dominant condition characterized by multiple anechoic cysts of varying size distributed throughout the renal parenchyma. Ultrasonography detects cysts in affected cats from as young as 13 weeks of age, and a prospective study of Persian cats found that 45% of examined individuals were positive for the disease based on the presence of anechoic parenchymal cysts (Beck and Lavelle, 2001). Screening programs rely on ultrasound because affected cats may be clinically normal for years. The presence of even a single cyst in a predisposed breed warrants careful evaluation and genetic counseling. Focal renal lesions include cysts, abscesses, hematomas, and neoplasia. Primary renal neoplasia in dogs is most commonly carcinoma, while cats more often develop lymphoma or carcinoma. Sonographic features of malignancy include a mass that disrupts the normal architecture, irregular margins, and mixed echogenicity. Contrast-enhanced ultrasonography improves differentiation between benign and malignant lesions by assessing vascular patterns, but tissue sampling remains necessary for definitive diagnosis. Mast cell infiltration of the kidney, reported in dogs with abdominal mast cell disease, appears as multiple hypoechoic cortical nodules that distort the outer contour (Sato and Solano, 2004). ## Renal Cystic Disease and Mass Lesions Cystic renal disease spans a spectrum from incidental solitary cysts to inherited polycystic kidney disease. In cats, autosomal dominant polycystic kidney disease is an important consideration, particularly in Persian and related breeds. Ultrasonographic screening has demonstrated a high prevalence in Persian cats, with affected individuals identified as young as 13 weeks of age, and the condition is characterized by multiple anechoic cortical and medullary cysts of varying size [feline polycystic kidney disease in Persian and other cats](https://pubmed.ncbi.nlm.nih.gov/11301745/). Screening protocols should therefore include both kidneys in any Persian, Exotic Shorthair, or related breed presented for breeding assessment or geriatric health evaluation. Simple cysts appear as round, anechoic, thin-walled structures with distal acoustic enhancement. They may be solitary or multiple and are generally incidental findings in older animals. Complex cysts with internal septation, mural thickening, or echogenic contents warrant cytologic or histologic evaluation, as B-mode ultrasonography cannot reliably distinguish benign from malignant cystic lesions [ultrasonography of the feline kidney](https://pubmed.ncbi.nlm.nih.gov/23087005/). Renal mass lesions require a systematic approach. Primary renal neoplasia, most commonly lymphoma in cats and renal carcinoma in dogs, typically presents as a focal mass that disrupts normal architecture. Lymphoma often produces a hypoechoic, infiltrative pattern with renomegaly and loss of corticomedullary definition, whereas carcinoma tends to form a discrete, isoechoic or hypoechoic mass that may distort the renal contour. Metastatic disease can produce multiple hypoechoic cortical nodules, as described in dogs with abdominal mast cell disease, where renal infiltration appeared as multiple cortical nodules distorting the outer contour [ultrasonographic findings in abdominal mast cell disease](https://pubmed.ncbi.nlm.nih.gov/15005361/). | Lesion Type | Typical Sonographic Appearance | Recommended Next Step | |---|---|---| | Simple cyst | Anechoic, thin wall, distal enhancement | No further action if solitary and incidental | | Complex cyst | Septations, mural thickening, echogenic contents | Ultrasound-guided aspiration or biopsy | | Solid mass, focal | Discrete hypoechoic or isoechoic nodule | Fine-needle aspiration, consider contrast-enhanced ultrasound | | Solid mass, infiltrative | Renomegaly, loss of layering, diffuse hypoechogenicity | Cytology, consider lymphoma in cats | | Multiple cortical nodules | Hypoechoic nodules distorting contour | Staging for metastatic disease, biopsy | Contrast-enhanced ultrasonography can characterize vascular patterns within focal lesions and offers better differentiation between benign and malignant masses than B-mode alone, though availability limits its routine use [ultrasonography of the feline kidney](https://pubmed.ncbi.nlm.nih.gov/23087005/). Ultrasound-guided fine-needle aspiration is generally safe for solid masses, but care is needed with highly vascular lesions. Coagulation status should be assessed before biopsy, and the patient monitored for hematuria or perirenal hemorrhage after sampling. ## Hydronephrosis and Ureteral Assessment Hydronephrosis is identified by dilation of the renal pelvis with variable loss of medullary tissue. Mild pelvic dilation may be physiologic in a diuretic patient, so the finding must be interpreted alongside bladder distension and hydration status. Progressive dilation on serial examination supports obstruction. The ureters are not visible ultrasonographically in normal dogs and cats, but a dilated ureter can be traced from the renal pelvis to the bladder trigone when obstruction is present [feline abdominal ultrasonography: what's normal? what's abnormal?](https://pubmed.ncbi.nlm.nih.gov/32326858/). Ureteral obstruction in cats is most commonly caused by ureteroliths, which may be difficult to identify because of their small size and the acoustic shadowing from overlying bowel gas. The examiner should search for the ureter at the renal hilus and follow it caudally using a transverse plane, applying gentle pressure to displace gas-filled bowel. A ureteral stone appears as a hyperechoic focus with distal shadowing within the dilated ureter. In dogs, ureteral obstruction is less common but may result from calculi, neoplasia, or retroperitoneal disease. The urinary bladder should be assessed with the patient in dorsal and lateral recumbency, using a high-frequency linear or microconvex transducer. A moderately distended bladder provides the best acoustic window. The bladder wall is measured in the collapsed and distended states, normal wall thickness is approximately 1 to 2 mm in a distended bladder and up to 3 to 4 mm when collapsed. Focal or diffuse thickening may indicate cystitis, neoplasia, or polypoid cystitis. The trigone region requires careful evaluation, as transitional cell carcinoma most commonly arises there. A mass at the trigone with extension into the urethra carries a guarded prognosis, and cytologic confirmation is required before initiating therapy. ## Urolithiasis Bladder uroliths are readily identified as hyperechoic structures with distal acoustic shadowing. The number, size, and mobility of uroliths should be documented, and the bladder should be imaged in multiple planes to avoid missing small calculi that may be obscured by shadowing from larger stones. Uroliths that move with patient repositioning are generally free within the lumen, whereas fixed calculi may be embedded in the wall or associated with a diverticulum. Ultrasound is more sensitive than radiography for detecting small or radiolucent uroliths, but it cannot reliably determine mineral composition. Radiography remains useful for assessing radiopacity and for detecting concurrent nephroliths or ureteroliths. Urate and cystine stones may be radiolucent and are therefore more likely to be identified ultrasonographically. The presence of uroliths should prompt a complete urinalysis with culture, as struvite and infection-associated stones require different management than metabolic stones. ## Chronic Kidney Disease and Diffuse Parenchymal Disease Chronic kidney disease produces a constellation of ultrasonographic changes that include reduced renal size, increased cortical echogenicity, loss of corticomedullary distinction, and an irregular renal contour [feline abdominal ultrasonography: what's normal? what's abnormal?](https://pubmed.ncbi.nlm.nih.gov/32326858/). These changes are progressive and correlate with histologic fibrosis, but ultrasonography cannot quantify functional impairment. A cat with sonographically normal kidneys may still have significant chronic kidney disease, and conversely, an aged cat with mild cortical hyperechogenicity may have clinically normal renal function. The medullary rim sign, a hyperechoic line at the corticomedullary junction, is commonly cited as a marker of renal disease, but it can occur in healthy animals and is not specific for any particular diagnosis [feline abdominal ultrasonography: what's normal? what's abnormal?](https://pubmed.ncbi.nlm.nih.gov/32326858/). Interpretation should therefore be made in the context of the complete sonographic and biochemical picture. Perinephric pseudocysts appear as anechoic fluid collections surrounding the kidney and are associated with chronic kidney disease in cats, though they may also occur as incidental findings. | Sonographic Finding | Differential Considerations | Clinical Significance | |---|---|---| | Reduced renal length | Chronic kidney disease, congenital hypoplasia, end-stage disease | Correlate with creatinine and SDMA | | Increased cortical echogenicity | Fibrosis, inflammation, necrosis, mineralisation | Non-specific, requires biopsy for aetiology | | Medullary rim sign | Normal variant, hypercalcemia, early renal disease | Interpret cautiously, not diagnostic alone | | Loss of corticomedullary distinction | Advanced chronic kidney disease, lymphoma, feline infectious peritonitis | Indicates diffuse parenchymal disease | | Perinephric fluid | Pseudocyst, hemorrhage, abscess, urine leakage | Aspirate for analysis if clinically relevant | ## Documentation and Reporting A structured report should include renal length, width, and height for both kidneys, cortical echogenicity relative to the spleen or liver, corticomedullary distinction, pelvic diameter, and any focal lesions with their location and dimensions. Bladder wall thickness, luminal contents, and the appearance of the trigone and urethra should be recorded. Images should be labelled with patient identification, date, and transducer orientation. Serial measurements are most valuable when obtained by the same operator using consistent technique, as inter-operator variability can be substantial. The resistive index measured by duplex Doppler in the arcuate arteries provides a quantitative assessment of intrarenal vascular resistance. In sedated normal cats, the 95% confidence interval for the resistive index ranges from 0.52 to 0.60 in the left kidney and 0.55 to 0.63 in the right kidney [duplex Doppler estimation of Pourcelot resistive index](https://pubmed.ncbi.nlm.nih.gov/8965265/). Values above these ranges suggest increased vascular resistance, which may accompany obstruction, hypertension, or parenchymal disease, but the index is not specific for any single aetiology and should not be used in isolation. ## Equipment and Protocol Considerations A high-frequency linear transducer (7.5 to 12 MHz) is preferred for the kidneys and bladder in dogs and cats under 20 kg. Larger dogs may require a microconvex or curvilinear transducer (5 to 8 MHz) to achieve adequate penetration. The patient is positioned in dorsal or lateral recumbency, and the hair is clipped from the caudal thorax to the inguinal region. Acoustic coupling gel should be warmed to improve patient tolerance, and sedation is rarely required for a complete urinary tract examination in cooperative patients. For the kidneys, the transducer is placed in the paralumbar fossa and the kidney is identified in a sagittal plane. The renal length is measured from pole to pole, and the cortex is assessed for echogenicity and thickness. The renal pelvis is evaluated for dilation, and the proximal ureter is traced when indicated. The bladder is examined in transverse and sagittal planes with the patient in dorsal and lateral recumbency. Color Doppler is useful for confirming vascular flow in a suspected mass and for identifying ureteral jets at the trigone, which confirms patency of the ipsilateral ureter. Serial monitoring is indicated for patients with known urolithiasis, chronic kidney disease, or cystic disease. The interval between examinations depends on the clinical trajectory and the specific disease process. For polycystic kidney disease in breeding cats, annual screening is appropriate, whereas a patient with a ureteral obstruction may require re-evaluation within 24 to 48 hours after intervention. The ultrasonographic findings should always be integrated with biochemical data, urinalysis, and blood pressure measurement to guide therapeutic decisions. ## Recognized Complications and Early Detection Ultrasound-guided renal biopsy carries recognized risks including hemorrhage, arteriovenous fistula formation, and inadvertent sampling of adjacent viscera. Hemorrhage is the most common complication and is detected early by observing a hyperechoic halo around the kidney immediately after needle withdrawal, which represents perirenal hemorrhage. Serial scans at 5 and 15 minutes post biopsy allow comparison of the halo's extent. A progressive increase in halo thickness, or new anechoic fluid accumulating in the retroperitoneal space, warrants immediate intervention. Doppler interrogation of the biopsy tract can identify arteriovenous fistulae, which appear as a focal color aliasing region with high-velocity, low-resistance flow on spectral analysis. Delayed hemorrhage, presenting 12 to 24 hours later, is detected by re-scanning patients that develop hematuria, hypovolemia, or a falling packed cell volume. Ureteral stent placement carries the recognized failure modes of stent migration, encrustation, and obstruction. Early detection relies on scheduled recheck ultrasonography at 2 to 4 weeks post placement, with particular attention to proximal ureteral diameter and renal pelvic dimensions. A stent that has migrated into the bladder lumen is identified by its echogenic tip within the urine pool, while a persistently dilated renal pelvis despite a patent stent suggests encrustation or luminal obstruction. Color Doppler can confirm ureteral jet flow at the trigone, absence of jets in the presence of a stent indicates obstruction. ## Common Errors and Corrective Actions Less experienced operators frequently mistake the hypoechoic renal pyramids of cats for cystic lesions. The pyramids are uniformly hypoechoic, lack distal acoustic enhancement, and are arranged symmetrically around the medulla. True cysts are anechoic, show distal enhancement, and often distort the cortical contour. Rotating the transducer through orthogonal planes resolves this ambiguity. Another common error is diagnosing hydronephrosis from a single sagittal image. The normal feline renal pelvis can appear prominent, particularly in diuretic patients. The discriminating finding is pelvic dilation that persists across both sagittal and transverse planes and is accompanied by blunting of the papillary tips. A dilated ureter must be traced to its origin at the renal pelvis and followed distally to distinguish pelvic dilation from a tortuous proximal ureter. Misdiagnosis of renal cysts as polycystic kidney disease occurs when age-related cortical cysts are found in breeds not predisposed to the autosomal dominant form. The diagnosis of feline polycystic kidney disease requires multiple bilateral cysts in a predisposed breed, typically Persian or related cats, with affected animals often showing cysts from a young age [Beck and Lavelle, feline polycystic kidney disease prevalence study](https://pubmed.ncbi.nlm.nih.gov/11301745/). A single cyst in an older cat of a non-predisposed breed is incidental. ## Troubleshooting Table | Observation | Likely Cause | Discriminating Check | |---|---|---| | Hypoechoic foci in medulla | Normal feline pyramids | No distal enhancement, symmetric arrangement | | Anechoic renal pelvis | Hydronephrosis vs normal variant | Persists in transverse plane, papillary blunting | | Hyperechoic halo post biopsy | Perirenal hemorrhage | Repeat scan at 5 and 15 minutes, assess progression | | Multiple cortical cysts in Persian cat | Polycystic kidney disease | Bilateral distribution, young age, breed confirmation | | Dilated ureter with stent in place | Stent obstruction vs migration | Trace stent to trigone, assess ureteral jet flow | | Hypoechoic cortical nodules | Mast cell infiltrate vs neoplasia | Cytology or histopathology required [Sato and Solano, abdominal mast cell disease findings](https://pubmed.ncbi.nlm.nih.gov/15005361/) | ## Evidence Limitations and Expert Disagreement The evidence base for feline renal ultrasonography is stronger than for canine counterparts, reflecting the higher prevalence of renal disease in cats. However, much of the published literature consists of descriptive reviews and retrospective case series instead of prospective comparative studies [Griffin, feline renal and perinephric ultrasonography](https://pubmed.ncbi.nlm.nih.gov/32326858/). Diffuse parenchymal disease remains a diagnostic challenge because B-mode ultrasonography cannot reliably differentiate between inflammatory, fibrotic, and neoplastic infiltrates. Contrast-enhanced ultrasonography shows promise for distinguishing benign from malignant focal lesions based on vascularity patterns, but this modality is not widely available in general practice [Debruyn et al, feline renal ultrasonography technique and disease changes](https://pubmed.ncbi.nlm.nih.gov/23087005/). Expert opinion diverges on the clinical significance of the medullary rim sign. Some authors consider it a nonspecific finding associated with hypercalcemia, while others view it as an age-related change of minimal clinical importance. The current consensus is that the rim sign alone does not establish renal disease and must be interpreted alongside biochemical and urinalysis findings. Resistive index values in normal cats have been published with 95% confidence intervals of 0.52 to 0.60 for the left kidney and 0.55 to 0.63 for the right kidney [Rivers et al, duplex Doppler resistive index in normal cats](https://pubmed.ncbi.nlm.nih.gov/8965265/). However, these values were obtained in sedated healthy cats, and the effect of sedation on resistive index in diseased kidneys is not fully characterized. Clinicians should therefore interpret resistive index values cautiously and in conjunction with other diagnostic findings. ## Referral and Escalation Criteria Referral to a veterinary radiologist or internal medicine specialist is warranted when the ultrasonographic findings are equivocal, when diffuse parenchymal disease is suspected but not confirmed, or when ultrasound-guided intervention carries high risk. Suspected renal neoplasia with vascular invasion, complex cystic lesions, and ureteral obstruction that cannot be localized are appropriate reasons for advanced imaging such as CT or MRI. Laboratory involvement is indicated when ultrasonography identifies changes that require cytological or histopathological confirmation. Ultrasound-guided fine-needle aspiration of renal masses can be performed in practice, but core biopsy should be reserved for cases where the risk of hemorrhage is acceptable and where the result will alter management. Coagulation screening before biopsy is mandatory. Regulatory reporting obligations arise when ultrasonographic findings suggest notifiable disease. While renal ultrasonography rarely identifies reportable conditions directly, the discovery of bilateral renomegaly with diffuse parenchymal change in a cat with unexplained fever or weight loss should prompt consideration of infectious causes with public health or trade implications. Veterinary clinicians should consult their regional veterinary authority or the [WOAH terrestrial animal health standards](https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/) for current reporting requirements. Professional guidance on imaging standards and radiation safety is available through the [American College of Veterinary Radiology resources](https://acvr.org/) and the [AVMA practice resources](https://www.avma.org/resources-tools). ## Frequently Asked Questions ### How should I adapt my renal ultrasound protocol when only a low-end machine with a microconvex probe is available? Image quality degrades with lower-frequency transducers, but diagnostic information remains accessible. Use the urinary bladder as an acoustic window for the ureteral papillae and retrograde evaluation of the distal ureters. For the kidneys, patient positioning matters more than machine power. Place the cat in dorsal recumbency and use the liver as a window for the right kidney, as described in [feline renal ultrasonography technique and anatomy](https://pubmed.ncbi.nlm.nih.gov/23087005/). Increase gain slightly to compensate for reduced penetration, but avoid excessive overall gain that masks cortical echotexture. Compare the renal cortex to the spleen or liver on the same image depth. A microconvex probe cannot reliably identify subtle pelvic dilation, so document any suspicion of hydronephrosis and refer for confirmatory imaging. ### What is the minimum database I should record for a renal ultrasound study to support serial monitoring? Record renal length in a sagittal plane, cortical echogenicity relative to liver or spleen, corticomedullary junction distinction, pelvic diameter, and the presence or absence of perinephric fluid. Store at least one longitudinal and one transverse clip per kidney. For cats with chronic kidney disease, measure the same landmarks at each recheck and use identical probe frequency and gain settings. The [feline abdominal ultrasonography review](https://pubmed.ncbi.nlm.nih.gov/32326858/) emphasizes that medullary rim sign and other incidental findings must be documented as such to avoid misinterpretation on later studies. Include a written note on urinary bladder distension at the time of ureteral assessment, because a collapsed bladder can mimic ureteral dilation. ### How do I distinguish a perinephric pseudocyst from severe hydronephrosis in a cat? A perinephric pseudocyst surrounds the kidney as a well-defined anechoic fluid pocket, while the renal parenchyma retains its normal contour and internal architecture. Hydronephrosis dilates the renal pelvis centrally and thins the cortex from within outward. In pseudocyst cases, the kidney may appear small and irregular due to underlying chronic kidney disease, and the fluid is typically anechoic with distal acoustic enhancement. The [feline kidney and perinephric space review](https://pubmed.ncbi.nlm.nih.gov/32326858/) notes that differentiating these conditions is a common clinical challenge. If the renal pelvis is not dilated and the ureter is normal calibre, hydronephrosis is unlikely. Aspiration under ultrasound guidance can confirm the diagnosis, but reserve this for cases where therapeutic drainage is planned. ### When should I recommend contrast-enhanced ultrasound or CT instead of repeat B-mode imaging? Contrast-enhanced ultrasound is indicated when a focal renal lesion is identified and malignancy cannot be excluded on B-mode. The [feline renal ultrasonography review](https://pubmed.ncbi.nlm.nih.gov/23087005/) states that B-mode is limited for differentiating benign from malignant focal lesions, whereas contrast enhancement patterns provide vascularity information that improves distinction. CT is preferred when ureteral obstruction is suspected but not confirmed sonographically, when planning surgery for urolithiasis, or when the entire ureter cannot be visualized. Refer early instead of repeating equivocal B-mode studies. Cost and anesthesia risk should be weighed, but a single definitive study often costs less than repeated inconclusive scans. ### How should I explain an equivocal renal ultrasound finding to a client without causing undue alarm? Use concrete language. State what was seen, for example a slightly irregular kidney contour or a small cortical cyst, and what it does and does not mean. Explain that ultrasound identifies structure, not function, and that bloodwork and urine testing are needed to determine clinical significance. For Persian cats, mention that [feline polycystic kidney disease screening](https://pubmed.ncbi.nlm.nih.gov/11301745/) is available and that early detection allows breeding decisions and monitoring. Avoid terms like lesion or mass without context. Offer a clear next step, such as repeat imaging in three to six months or referral for advanced imaging. Document the conversation in the medical record, including the client's questions and your recommendations. ### What are the practical limits of resistive index measurement in a general practice setting? Resistive index measurement requires duplex Doppler capability, a sedated or cooperative patient, and consistent sampling from arcuate or interlobar arteries. In normal sedated cats, published 95% confidence intervals are 0.52 to 0.60 for the left kidney and 0.55 to 0.63 for the right kidney, as reported in [duplex Doppler resistive index studies in cats](https://pubmed.ncbi.nlm.nih.gov/8965265/). Values above 0.70 are generally considered abnormal, but anesthesia, heart rate, and systemic blood pressure all affect the measurement. Do not use resistive index as a standalone diagnostic test. It is most useful as a serial trend in a patient with suspected obstruction or hypertension. If the machine cannot reliably gate the Doppler waveform, omit the measurement and rely on B-mode findings and laboratory data. ## Related Clinical & Scientific Guides * [MRI Monitoring of Brain Tumor Response to Therapy in Dogs](/knowledge/veterinary-medicine/diagnostic-imaging/mri-monitoring-brain-tumor-response-therapy-dogs) * [Ultrasound-Guided Drainage of Abscesses in Small Animals](/knowledge/veterinary-medicine/diagnostic-imaging/ultrasound-guided-drainage-abscesses-small-animals) * [Radiographic Monitoring of Total Hip Replacement in Dogs](/knowledge/veterinary-medicine/diagnostic-imaging/radiographic-monitoring-total-hip-replacement-dogs) ## References and Further Reading - [Feline abdominal ultrasonography: what's normal? what's abnormal? The kidneys and perinephric space.](https://pubmed.ncbi.nlm.nih.gov/32326858/). 2020. - [Ultrasonography of the feline kidney: Technique, anatomy and changes associated with disease.](https://pubmed.ncbi.nlm.nih.gov/23087005/). 2012. - [Ultrasonographic findings in abdominal mast cell disease: a retrospective study of 19 patients.](https://pubmed.ncbi.nlm.nih.gov/15005361/). 2004. - [Feline polycystic kidney disease in Persian and other cats: a prospective study using ultrasonography.](https://pubmed.ncbi.nlm.nih.gov/11301745/). 2001. - [Duplex Doppler estimation of Pourcelot resistive index in arcuate arteries of sedated normal cats.](https://pubmed.ncbi.nlm.nih.gov/8965265/). 1996. - [Transplantation of amniotic membrane-derived multipotent cells ameliorates and delays the progression of chronic kidney disease in cats.](https://pubmed.ncbi.nlm.nih.gov/27774657/). 2017. - [American College of Veterinary Radiology Resources](https://acvr.org/). American College of Veterinary Radiology. - [MSD Veterinary Manual, Professional Edition](https://www.msdvetmanual.com/). MSD Veterinary Manual. - [American Veterinary Medical Association Practice Resources](https://www.avma.org/resources-tools). American Veterinary Medical Association. ## Related Articles - [Ultrasound of the Canine and Feline Gastrointestinal Tract: Normal and Abnormal Findings](/knowledge/veterinary-medicine/diagnostic-imaging/ultrasound-canine-feline-gastrointestinal-tract-normal-abnormal-findings) - [Ultrasound of the Canine and Feline Liver and Biliary System: Normal and Abnormal Findings](/knowledge/veterinary-medicine/diagnostic-imaging/ultrasound-canine-feline-liver-biliary-system-normal-abnormal-findings) - [Imaging the Canine and Feline Spine: Radiography and Advanced Modalities](/knowledge/veterinary-medicine/diagnostic-imaging/imaging-canine-feline-spine-radiography-advanced-modalities) - [Radiographic Evaluation of the Canine and Feline Thorax: Cardiac and Pulmonary Assessment](/knowledge/veterinary-medicine/diagnostic-imaging/radiographic-evaluation-canine-feline-thorax-cardiac-pulmonary-assessment) - [Ultrasonographic Evaluation of the Canine and Feline Abdomen: A Systematic Approach](/knowledge/veterinary-medicine/diagnostic-imaging/ultrasonographic-evaluation-canine-feline-abdomen-systematic-approach) > This article is educational professional reference material for veterinary audiences. It is not a substitute for veterinary diagnosis, individual clinical judgment, current product labeling, or applicable regulatory requirements.