The goal of this observational study is to quantitatively assess the renal microcirculation changes by contrast-enhanced ultrasound (CEUS) and to obtain systemic hemodynamic information by ultrasound Doppler at the same time, to analyze the relationship between renal microcirculation changes and systemic hemodynamic changes, and to explore the diagnostic value of CEUS in critically ill acute kidney injury.

The main questions it aims to answer are:

1. To explore the quantitative parameters of contrast-enhanced ultrasound which can reflect the changes of renal microcirculation。
2. To explore the relationship between renal microcirculation and systemic hemodynamics。
3. To explore the value of quantitative evaluation of renal microcirculation changes by contrast-enhanced ultrasound in the diagnosis of acute kidney injury

Inclusion Criteria:

• Age>18 years old
• First admission to ICU < 24h after admission and expected ICU stay ≥48 hours
• Acute kidney injury risk factors (KDIGO 2012)
• Contrast-enhanced ultrasound

Exclusion Criteria:

• Known severe chronic kidney disease (CKD≥ stage 4) or undergoing hemodialysis treatment
• Kidney transplantation or renal malignancy
• Terminal stage of malignant tumor
• Pregnancy
• Renal artery stenosis or renal vein thrombosis
• The quality of CEUS images is poor, and quantitative analysis cannot be performed

Through literature review, the latest meta-analysis results showed that the area under the curve of CEUS parameters for diagnosing AKI was 0.84 and the width of the confidence interval was 0.22. When α=0.05, the sample size N=54 was obtained by PASS software. Considering the 10% shedding rate, 60 patients were expected to be enrolled in this study.