Evaluating renal microcirculation in patients with acute kidney injury by contrast-enhanced ultrasonography: a protocol for an observational cohort study.

This study is retrospectively registered on the Chinese Medical Research Registration information System( https://61.49.19.26/login ) on December 31, 2021: MR-11-22-003,503. This study has been approved by the Ethics and Scientific Research Department of Peking University First Hospital.