1. A method for adaptive volume rendering in medical diagnostic ultrasound, the method comprising: acquiring B-mode and flow ultrasound data representing a volume of a patient, wherein the flow ultrasound data includes energy and velocity estimates; applying energy and velocity thresholds at different levels in first and second thresholding passes to the same flow ultrasound data, the first thresholding pass associated with lower thresholds than the second thresholding pass; computing a spatial variance of the velocity estimates subjected to the first thresholding pass; determining an adaptive threshold as a function of three-dimensional gradients calculated from the energy estimates subjected to the second thresholding pass, wherein the adaptive threshold is lower for larger gradients and higher for lower gradients; and for each location associated with flow ultrasound data subjected to the first thresholding pass, identifying locations with the spatial variance below a spatial variance threshold and the energy estimate below the adaptive threshold, identifying that the flow data subjected to the second thresholding pass indicates flow within a neighborhood of locations around the location, and setting the flow data to show flow, otherwise showing no flow; receiving a viewing direction relative to the volume; determining values for at least one rendering parameter as a function of depth along the viewing direction, the B-mode ultrasound data, and the flow ultrasound data; and rendering an image from the B-mode and flow ultrasound data, the rendering being as a function of the viewing direction and the values for the at least one rendering parameter.