**1.4.2.1 Doppler ultrasound**

It is an established method for the assessment of blood flow based on the frequency variation of a US beam hitting a target (in our case, groups of red blood cells) in motion. The difference in frequency and its positive or negative character are elements that allow vectorial representation of blood speed and direction of movement against the transducer. Doppler ultrasound has several technical options: spectral and color coded, each of them with their advantages and disadvantages. Thus: spectral ultrasound describes the flow type (arterial or venous) and allows measurements. Color flow mode (CFM) ultrasound encodes speed vectors related to red blood cells groups thus detecting blood flows. Evaluation of tumor vasculature will be made first using the CFM module and then spectral Doppler module by positioning the Doppler sample on reference regions. On CFM investigation tumor vessels have tortuous paths, aberrant spatial ramifications, they intercommunicate and can abruptly end in "glove finger" (Cherrington et al, 2000). Spatial positioning of intratumoral vessels can also be illustrated by 3D/4D reconstruction procedures and quantitative assessment can be achieved with techniques using color pixel count per unit area. Spectral investigation measurements into the tumor vessels lumen show either low values (RI<0.65; S/D <2.5) suggesting the absence of arterial precapillary sphincter and the presence of arterio-venous shunts or high values secondary to increased interstitial pressure. Accelerated flow speed can be identified in the input vessels from the tumor hilum suggesting a circulatory bed avid for blood (Cosgrove & Eckersley, 1997).
