Assessment of dimensions and parenchymal structure
The primary goal of imaging is to determine whether the pathological process is reversible. This is achieved by assessing kidney size and the thickness of the cortical layer. Chronic kidney disease (CKD) is characterized by small organ size and marked parenchymal thinning. Normally, the average cortical thickness is 13.6 mm, whereas in CKD this value drops to 10.7 mm. It is important to note that parenchymal thinning precedes the reduction in overall kidney length.
Ultrasound is the first-line imaging modality, though it typically yields lower length measurements than classic urography (the lower limit of normal on ultrasound is 9 cm, which is equivalent to 11.5 cm on an excretory urogram). In acute renal failure (ARF), parenchymal echogenicity may be normal or increased, but increased echogenicity alone is not pathognomonic for a specific diagnosis. Currently, ultrasound elastography is being actively investigated as a method for the quantitative assessment of fibrosis.
Diagnosis of obstruction and duplex Doppler sonography
A critical objective is to rule out urinary tract obstruction. On ultrasound, it manifests as dilation of the pelvicalyceal system (PCS) and the ureter above the level of the obstruction. However, visually distinguishing true obstruction from an extrarenal renal pelvis or parapelvic cyst can be challenging.
To differentiate these conditions, the Doppler resistive index (RI) is used. It is calculated as the ratio of the difference between peak systolic and end-diastolic velocity to the peak systolic velocity:
- In acute obstruction, the RI exceeds 0.7.
- An elevated RI is also observed in non-obstructive acute renal failure associated with acute tubular necrosis.
- In prerenal azotemia, the resistive index remains within normal limits.
If renal failure is caused by renovascular disease (e.g., renal artery stenosis), ultrasound will reveal a decrease in the RI with the formation of a parvus-tardus blood flow pattern. An inter-kidney RI difference of more than 0.05 helps identify stenosis, which requires angiography for confirmation.
Cystic renal lesions
Morphological changes in renal failure are frequently associated with cyst formation. Several major pathologies are distinguished:
- Autosomal dominant polycystic kidney disease (ADPKD): accounts for renal failure in 10–15% of dialysis patients.
- Acquired cystic disease of the kidney (ACKD): develops in patients on long-term hemodialysis or peritoneal dialysis. It occurs in 13% of predialysis patients and in more than 90% of patients with a dialysis history exceeding 5 years. Unlike ADPKD, kidney dimensions are normal or reduced. ACKD carries a risk of renal cell carcinoma and persists even after transplantation.
- Tuberous sclerosis: causes renal failure in 5–15% of patients. Cysts may be indistinguishable from ADPKD, so the diagnosis is established based on associated features (angiomyolipomas, central nervous system involvement).
Alternative imaging and biopsy
When ultrasound results are equivocal, non-contrast CT (to detect calculi) or non-contrast MRI (coronal T2-weighted images detail the pelvicalyceal system) is utilized. Iodinated contrast agents are frequently contraindicated in patients with renal failure due to the risk of post-contrast acute kidney injury (PC-AKI).
Dynamic radionuclide renography is optimal for assessing renal perfusion in acute renal failure. It allows for the differentiation of reversible tubular necrosis from conditions with a poor prognosis (such as cortical necrosis).
In cases of acute renal failure of unclear etiology, ultrasound-guided renal biopsy is indicated. This is a high-risk procedure: asymptomatic hematomas occur in 44% of patients on ultrasound and up to 90% on CT. Symptomatic bleeding occurs in less than 10% of cases, and the mortality rate is approximately 1 per 1,000. Important: small kidneys with a parenchymal thickness of 1 cm or less should not be biopsied due to the extreme risk of complications and low diagnostic yield.
Briefly: answers to topical questions
What are the absolute and relative contraindications to percutaneous renal biopsy?
Sources list the following contraindications to percutaneous renal biopsy, but do not classify them into absolute and relative:
- Hemostatic disorders — bleeding diathesis, thrombocytopenia, ongoing anticoagulant therapy.
- Inability to communicate with the patient — comatose states, psychoses.
- Hemodynamic instability — severe, uncontrolled arterial hypertension.
- Anatomical and functional features — presence of a single functioning kidney, shrunken kidneys (primary or secondary).
Decreased RI points to vascular stenosis (parvus-tardus flow). Increased RI suggests acute obstruction or necrosis.
Class and exam questions
Which imaging modality should be used to initiate the workup for renal failure?
Ultrasound (US). It allows for a rapid, safe, and non-contrast assessment of kidney size and cortical thickness, and helps rule out urinary tract obstruction.
Why is the use of contrast agents restricted in patients with renal failure?
The administration of iodinated contrast agents carries a high risk of post-contrast acute kidney injury (referred to as PC-AKI per ACR terminology), which may further worsen the patient's condition.
How can acquired cystic kidney disease be differentiated from polycystic kidney disease on ultrasound?
In acquired cystic kidney disease (ACKD), which commonly develops during long-term dialysis, the kidneys are of normal or reduced size, and the number of cysts is significantly smaller than in congenital polycystic kidney disease.
At what renal dimensions is a biopsy strictly contraindicated?
If the parenchymal thickness has decreased to 1 centimeter or less, performing a biopsy is impractical. The likelihood of obtaining diagnostic tissue is minimal, while the risk of massive hemorrhage increases critically.
What else to study on this topic
- Classification of CKD stages based on the glomerular filtration rate (GFR)
- Differential diagnosis of the causes of renovascular hypertension using duplex Doppler sonography
- Complications of acquired cystic kidney disease (ACKD) and oncogenesis risks
- Dynamic radionuclide renography in tubular and cortical necrosis
- Quantitative assessment of renal fibrosis using ultrasound elastography
Master the entire topic in the "Diagnostic Radiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete WhiteCoat Pro study guide on "Renal Failure" and 2,199 other subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 10 video lessons on the subject
Other topics in the section "Diagnostic Radiology of the Abdominal Organs"
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.