1. A method of obtaining an ultrasonic image of a patient comprising administering to the patient microparticles comprising a cavity containing a gas or a volatile fluid, which particles upon exposure to an effective color Doppler sonography modality produce a Doppler signal when the particles are essentially motionless in the patient, and imaging said patient with an effective ultrasonic color Doppler modality in a region where said microparticles are essentially motionless.