Non-contrast ultrasound assessment of blood flow in clinical practice

Since the first clinical use of ultrasound in the 1940s, significant advancements have been made in its applications. Color Doppler imaging and power Doppler imaging are considered the first and second generations of flow ultrasound assessment tools, respectively. Subsequently, the introduction of contrastenhanced ultrasound has significantly improved the assessment of arterial and venous vascular patterns in lesions and vessels. 'Blood flow brightness-mode imaging' or 'B-flow', a non-Doppler ultrasound flow assessment mode introduced more recently, provides even more information for ultrasound users in flow assessment. Microvascular imaging, introduced about a decade ago, is the third generation of Doppler non-contrast ultrasound flow modes, and is growing in popularity. Using a special wall filter, microvascular imaging overcomes the limitations of color Doppler imaging and power Doppler imaging in the detection of slow flowing signals. Advanced dynamic flow is a third-generation non-contrast Doppler flow technology that has so far gained popularity in obstetric ultrasound, commonly used to evaluate fetal umbilical vessels and heart chambers. This review article presents some recent updates on the various non-contrast ultrasound flow modalities available in clinical practice. It focuses on the design principles of individual flow modalities, discussing their strengths, limitations, and clinical applications, along with a review of the relevant literature.