Introduction
Imaging plays a crucial role in the diagnosis and management of kidney diseases. Various modalities offer unique insights into renal anatomy, function, and pathology. This section provides an overview of the most common imaging techniques used in nephrology.
Ultrasonography (US)
Key Uses of Renal Ultrasound
- Initial evaluation of AKI and CKD: The first-line imaging modality.
- Kidney Size and Echogenicity: Small, echogenic (bright) kidneys suggest chronic, irreversible disease. Normal-sized kidneys in AKI suggest an acute process.
- Hydronephrosis: Excellent for detecting urinary tract obstruction. The presence of a dilated collecting system is a key finding.
- Cysts and Masses: Can differentiate simple cysts from complex cysts or solid masses.
- Doppler US: Assesses renal blood flow, useful for evaluating renal artery stenosis, renal vein thrombosis, and transplant rejection.
Ultrasound is non-invasive, widely available, and does not use ionizing radiation or require contrast, making it the ideal initial imaging test for most renal conditions.
Computed Tomography (CT)
Applications of CT in Nephrology
- Kidney Stones: Non-contrast CT is the gold standard for detecting nephrolithiasis, able to identify stones of all compositions.
- Complex Cysts and Tumors: Contrast-enhanced CT is superior for characterizing complex renal masses (e.g., using the Bosniak classification for cysts) and staging renal cell carcinoma.
- Renal Trauma: The primary imaging modality for evaluating kidney injury after trauma.
- Infections: Useful for diagnosing complications like pyelonephritis, renal abscesses, and emphysematous pyelonephritis.
- CT Angiography (CTA): Provides detailed imaging of the renal arteries and veins, essential for diagnosing renal artery stenosis and other vascular abnormalities.
Contrast Considerations
Iodinated contrast used in CT scans carries a risk of contrast-induced nephropathy (CIN), particularly in patients with pre-existing CKD, diabetes, or volume depletion. Risk-benefit assessment is crucial before ordering a contrast-enhanced CT in these patients.
Magnetic Resonance Imaging (MRI)
MRI offers excellent soft-tissue contrast and is a valuable problem-solving tool in renal imaging, often used when CT is contraindicated or inconclusive.
Key Uses of MRI
- Characterization of Renal Masses: Especially useful for indeterminate masses on CT or in patients who cannot receive iodinated contrast.
- Magnetic Resonance Angiography (MRA): An alternative to CTA for evaluating renal vasculature, though it can sometimes overestimate stenosis.
- Functional MRI: Emerging techniques like Blood Oxygen Level-Dependent (BOLD) MRI can assess tissue oxygenation, offering insights into renal pathophysiology.
Gadolinium Contrast Warning
Gadolinium-based contrast agents used in MRI are associated with Nephrogenic Systemic Fibrosis (NSF) in patients with severe renal dysfunction (eGFR < 30 mL/min/1.73m²). The use of gadolinium is relatively contraindicated in this population.