The expanding role of ultrasound in acute kidney injury: from B-mode to microcirculation

Background: Acute kidney injury (AKI) is increasingly recognized as a dynamic systemic disorder reflecting hemodynamic instability rather than a purely nephrological complication. Conventional diagnostic criteria based on serum creatinine and urine output are delayed and provide limited insight into the underlying pathophysiological mechanisms. Methods: This narrative review summarizes current evidence on the application of multiparametric renal ultrasound in AKI. We examined the role of B-mode imaging, arterial and venous Doppler techniques, microvascular Doppler, and contrast-enhanced ultrasound (CEUS) in the assessment of renal perfusion, microcirculation, and venous congestion. Results: Doppler-derived parameters, including the renal resistive index and intrarenal venous Doppler patterns, provide dynamic information on renal hemodynamics and congestion. Microvascular Doppler and CEUS enable sensitive visualization of cortical and medullary perfusion, supporting AKI phenotyping and identification of potentially reversible mechanisms. Each modality presents specific strengths, limitations, and confounding factors. Conclusions: Multiparametric renal ultrasound complements laboratory and clinical assessment by offering real-time functional evaluation of renal perfusion and venous outflow. Within a physiology-driven framework, ultrasound may improve risk stratification, guide fluid and decongestive therapy, and support monitoring of renal recovery, shifting AKI assessment from delayed biochemical diagnosis to real-time bedside functional interpretation.