What is CT nephrotomography (contrast‑enhanced computed tomography of the kidneys)?

CT Nephrotomography: Definition and Description

CT nephrotomography refers to contrast-enhanced computed tomography of the kidneys that captures the nephrographic phase—the period when intravenous contrast material enhances the renal parenchyma, allowing detailed visualization of kidney structure and function. 1

Technical Components

CT nephrotomography involves several key imaging phases that provide complementary diagnostic information:

Nephrographic Phase

• The nephrographic phase occurs approximately 70-80 seconds after intravenous contrast injection, when contrast material has distributed throughout the renal parenchyma but before significant excretion into the collecting system. 2
• This phase optimally demonstrates renal parenchymal enhancement patterns, allowing detection of focal lesions, inflammatory changes, and vascular abnormalities. 1
• The CT nephrogram serves as an important indicator of underlying functional and structural renal disease, showing both quantitative and qualitative abnormalities. 1

Comprehensive Renal CT Protocols

When performed as part of a complete urographic evaluation (CT urography), the study typically includes: 2

• Unenhanced images (to detect calculi and characterize baseline attenuation)
• Nephrographic phase images (70-80 seconds post-injection for parenchymal evaluation)
• Excretory phase images (acquired at least 5 minutes after contrast injection to visualize the collecting systems, ureters, and bladder)

Some protocols also include an arterial phase (20-30 seconds) for vascular assessment. 2

Clinical Applications

Renal Mass Characterization

• Contrast-enhanced CT in the nephrographic phase is essential for distinguishing enhancing solid masses from non-enhancing cysts. 2
• A homogeneous renal mass measuring 10-20 HU on contrast-enhanced CT is considered a benign cyst; recent evidence suggests masses measuring 21-30 HU on portal venous phase may also represent benign cysts. 2
• Enhancement of >10-20 HU compared to unenhanced images indicates a solid or complex lesion requiring further evaluation. 2

Infection and Inflammation

• Contrast-enhanced CT has superior sensitivity for detecting acute pyelonephritis compared to unenhanced CT, with detection rates of 84.4% versus significantly lower rates with ultrasound (40%). 2
• The nephrographic phase demonstrates parenchymal changes including striated nephrograms, wedge-shaped areas of decreased enhancement, and perinephric inflammatory changes. 2, 1
• Contrast enhancement is critical for identifying renal abscesses, with detection rates of 4.0% by contrast-enhanced CT versus only 1.1% by ultrasound. 2

Vascular Assessment

• CT nephrotomography allows visualization of both the vessel lumen and the renal parenchyma, making it superior to conventional angiography for comprehensive renal vascular evaluation. 3
• The technique can detect renal artery stenosis, renal vein thrombosis, and segmental infarction based on nephrographic patterns. 1
• Multidetector CT with contrast provides 100% sensitivity and 85.7% specificity for detecting the number of renal arteries. 4

Functional Assessment

• The CT nephrogram provides quantitative information about renal function, as the distribution and clearance of iodinated contrast reflects glomerular filtration. 5
• Abnormal nephrographic patterns include global or segmental absence, persistent enhancement, striated patterns, and rim patterns—each indicating specific pathophysiologic processes. 1

Nephrographic Pattern Recognition

Normal Enhancement

• Homogeneous enhancement of renal cortex and medulla in the nephrographic phase indicates normal perfusion and function. 1

Pathologic Patterns

• Global absence (unilateral): Renal pedicle injury, complete arterial occlusion 1
• Segmental absence: Focal renal infarction from arterial emboli 1
• Persistent nephrogram (unilateral): Renal artery stenosis, renal vein thrombosis, urinary obstruction 1
• Striated nephrogram: Acute pyelonephritis, ureteral obstruction, renal contusion, tubular obstruction 2, 1
• Rim nephrogram: Renal infarction, acute tubular necrosis 1

Distinction from Other CT Protocols

CT Nephrotomography vs. Standard CT Abdomen/Pelvis

• Standard contrast-enhanced CT abdomen/pelvis typically captures only portal venous phase (not optimized for renal evaluation) and lacks the unenhanced and delayed excretory phases needed for comprehensive urinary tract assessment. 2
• CT nephrotomography or CT urography is specifically tailored with thin-slice acquisition and multiple phases for optimal kidney and urinary tract visualization. 2

Advantages Over Alternative Imaging

• CT with contrast has 97% sensitivity for detecting urolithiasis and provides superior detection of renal masses, infections, and complications compared to ultrasound or unenhanced CT alone. 2
• Unlike MR urography, CT is more widely available, faster to perform, and better for detecting small calculi. 2
• CT has largely replaced intravenous urography (IVU) at most institutions due to superior diagnostic accuracy and comprehensive anatomic depiction. 2

Important Caveats

Contrast Considerations

• Patients with GFR <60 mL/min require risk-benefit assessment before contrast administration, with adequate hydration and minimized contrast volume for those with GFR 30-60 mL/min. 6
• Contrast-enhanced CT may obscure small renal collecting system stones due to parenchymal enhancement, though stones ≥6 mm are detected with 98% sensitivity even on contrast-enhanced studies. 2

Radiation Exposure

• Multiple-phase CT protocols increase radiation dose; low-dose techniques should be considered, particularly in young patients and those requiring serial imaging. 2
• Single nephrographic phase imaging may be sufficient for many indications, avoiding unnecessary radiation from additional phases. 2