Ultrasonic visualization of a subject, particularly of perfusion in the myocardium and other tissues, is performed using novel gas-containing contrast agent preparations which promote controllable and temporary growth of the gas phase in vivo following administration and can therefore act as deposited perfusion tracers. The preparations include a coadministerable composition comprising a diffusible component capable of inward diffusion into the dispersed gas phase to promote temporary growth thereof. In cardiac perfusion imaging the preparations may advantageously be coadministered with vasodilator drugs such as adenosine in order to enhance the differences in return signal intensity from normal and hypoperfused myocardial tissue respectively.