Compliance with ethical standards

Guarantor
The scientific guarantor of this publication is Jian Yang, Department of Radiology, the First Affiliated Hospital of Xi’an Jiaotong University.

Conflict of interest
The authors declare that they have no conflict of interest.

Statistics and biometry
No complex statistical methods were necessary for this paper.

Informed consent
Written informed consent was obtained from all subjects (patients) in this study.

Ethical approval
This study was approved by the Ethics Committee of the First Affiliated Hospital of Xi’an Jiaotong University. Institutional review board approval was obtained.

Methodology
• retrospective
• diagnostic or prognostic study
• performed at six institutions