The maximal peak systolic velocity is expressed in centimeters per second (cm/s) and corresponds to the maximum flow rate during systole. For a proper evaluation, the accurate positioning of the angle of correction has a fundamental importance; it must be less than 60 ° and corrected for the direction of flow, also using a narrow sampling port, aligned along a straight portion of the cavernous artery. The evaluation of arterial flow must be performed in the artery origin segment. The end-diastolic velocity is expressed in centimeters per second (cm/s) and defines the residual flow in a vessel at the end of the diastolic phase. In physiological conditions, the blood flow increases during the phase of tumescence, determining filling of the sinusoids that press against the tunica albuginea, which is a relatively inextensible fibroelastic membrane. This determines compression of the sub-albugineous venules until interruption of the venous outflow occurs, and the achievement of a higher intracavernous pressure than diastolic pressure. The resistance index is the equivalent of the semiquantitative end-diastole speed, the resulting formula being:

RI = PSV-EDV/PSV. RI is expressing peripheral resistance to blood flow.

Objective evaluation of the erection is always associated with hemodynamic parameters and is expressed in fifths (x/5), based on the five basic stages of erection: