What is the recommended protocol for a computed tomography (CT) scan of the kidneys (renal protocol)?

CT Renal Protocol

For most clinical indications requiring renal CT imaging, perform IV contrast-enhanced CT of the abdomen and pelvis with immediate (nephrographic phase at 90-100 seconds) and delayed (excretory phase) images to comprehensively evaluate both renal parenchyma and the collecting system. 1

Standard Protocol Components

Contrast Administration

• Use IV iodinated contrast material for optimal visualization of renal parenchyma, vasculature, and collecting system 1
• Administer contrast at injection rates approximately equal to the flow rate in the vessel being evaluated 2
• Maximum recommended total dose of iodine for adults is 80 grams 2
• Hydrate patients before and after contrast administration 2

Imaging Phases

Nephrographic Phase (90-100 seconds post-contrast):

• This is the most critical phase for detecting parenchymal abnormalities, including pyelonephritis, abscesses, and renal masses 3, 4
• Accuracy of 90-92% for diagnosing acute pyelonephritis and complications 4
• Provides optimal assessment of renal perfusion and parenchymal enhancement 3

Delayed/Excretory Phase (5-15 minutes post-contrast):

• Essential for evaluating the collecting system, ureters, and bladder 1
• Critical for detecting contrast extravasation from collecting system injuries in trauma 1
• Necessary for identifying urothelial abnormalities and obstruction 1

Precontrast Phase:

• Useful for detecting calculi (most sensitive modality for urinary stones) 1
• Provides baseline attenuation values 1
• Can be omitted if stone disease is not suspected and radiation dose reduction is prioritized 3

Indication-Specific Protocols

Renal Trauma

• Perform triphasic CT (precontrast, nephrographic, and delayed phases) in stable patients with gross hematuria, microscopic hematuria with hypotension, or concerning mechanism of injury 1
• Delayed images are mandatory to detect collecting system injuries and contrast extravasation 1
• Include pelvis to evaluate for bladder injury, especially with pelvic fractures 1

Suspected Pyelonephritis or Renal Abscess

• CT with IV contrast (nephrographic phase) is superior to all other modalities for detecting parenchymal changes and complications 3, 4
• CT detects renal abscesses at 4.0% rate compared to 1.1% with ultrasound 4
• Include pelvis to identify distal ureteral stones, congenital anomalies, or other sources 4
• Imaging is NOT indicated for uncomplicated pyelonephritis unless patient fails to respond within 72 hours 3

Acute Kidney Injury (AKI)

• Unenhanced CT is preferred to avoid contrast nephrotoxicity 1
• Use CT primarily to characterize ultrasound-detected hydronephrosis and determine level/cause of obstruction 1
• CT with IV contrast is NOT appropriate for diagnosing AKI 1
• Most sensitive modality for detecting urinary calculi 1

Renovascular Disease

• CTA with contrast-enhanced imaging when eGFR >30 mL/min/1.73 m² 1
• For eGFR <30 mL/min/1.73 m², consider unenhanced MRA techniques or duplex Doppler ultrasound to avoid contrast nephrotoxicity 1
• Recent evidence suggests low risk of contrast-induced nephropathy with stable eGFR >45 mL/min/1.73 m² 1

Critical Contraindications and Modifications

Renal Insufficiency

• eGFR <30 mL/min/1.73 m²: Avoid iodinated contrast; use unenhanced CT or MRI alternatives 1
• eGFR 30-45 mL/min/1.73 m²: Consider reduced iodine dose; risk-benefit assessment required 1
• eGFR >45 mL/min/1.73 m²: Intravenous iodinated contrast is not an independent nephrotoxic risk factor 1
• Iso-osmolar contrast (iodixanol) causes less SCr elevation than low-osmolar agents in high-risk patients 5

Pregnancy

• Avoid CT whenever possible; use ultrasound or MRI as first-line 1
• When CT is absolutely necessary, use low-dose protocol 1
• Reserve for situations where diagnosis cannot be made on ultrasound or MRI 1

Pediatric Patients

• Dose: 1-2 mL/kg for both intra-arterial and intravenous administration 2
• Maximum dose should not exceed 2 mL/kg for CT or excretory urography 2
• Maximum dose should not exceed 4 mL/kg for angiography 2

Common Pitfalls to Avoid

• Never perform intrathecal administration of iodixanol—it is contraindicated and can cause death, seizures, and cerebral hemorrhage 2
• Do not omit delayed phase imaging in trauma cases—up to 34% of renal injuries may be missed without excretory phase 1
• Avoid extravasation, especially in patients with severe arterial or venous disease 2
• Do not mix iodixanol with other drugs or inject in IV lines containing other medications 2
• Do not rely solely on hematuria to determine need for imaging in trauma—mechanism of injury and physical findings are equally important 1