Angle of Insonation & B-flow Imaging

Angle of Insonation

Angle dependency of the Doppler shifted frequencies is also a critical factor in blood flow analysis. In sector scanning, multiple scan lines spread out from the transducer in a fanlike manner. When the sector scanner is used to interrogate circulator system in which the direction a flow is across these scan lines in a color window the angle of insonation between the flow axis and the ultrasound beam changes. The angle is smallest when flow stream enters in the sector filed and progressively rises to 90 0 as flow approaches the center of the field. Concurrently the Doppler shifted frequencies progressively decline and may become undetectable at the center of the color filed. A sector scanner may also create apparently. Contradictory directional information in a vessel transferring across the color encoding for flow toward to the transducer which usually is red. as the flow moves away, it will show bi directional flow. This paradox actually highlights the basic concept of representation of flow directionally by any Doppler system.

B-FLOW IMAGING
B-mode blood flow (B-flow) imaging is a new method that improves the resolution,frame rate, and dynamic range of B-mode to image blood flow and tissue simultaneously (Chiao et al., 2000).  The gray scale of an echo is adjusted by correlating the echo waveforms temporally. The correlation function measures the similarity of two echo waveforms and is determined by blood echogenicity, blood flow velocity, and beam width. A filter is designed to suppress large and slow or nonmoving echoes. The result is that the image of blood in an image is enhanced so as to allow the better visualization of blood flow, especially close to the vessel wall. A comparison of the B-mode and B-flow images of a carotid artery. It is evident that blood flow is better visualized by B-flow imaging. The vessel lumen where blood flows in the B-mode image is basically anechoic.