A new technique for imaging the full space-time combut these may not be truly quantitative, and almost invariably plexity of transient ultrasound fields is presented. The theoretical baimage field projections, rather than the field itself. These two sis of the method is developed in terms of the directivity spectrum, classes of technique are not considered here.
which is a generalization of the angular spectrum. The input field Another approach relies on measurements made in a limited measurement set, acquired at a single distance from the transducer, region of the field itself, and using those data to accurately predict consists of spatial projections of the field, which are directly measured the field throughout a region of interest. In this case, reliance must with a large aperture hydrophone, as described in the text. The be made on computer calculation (as for the impulse response method compares very favorably with the angular spectrum approach technique), but no specific knowledge of the transmitting surface, and is devoid of evanescent waves. It allows computed pulse propawhich can be of arbitrary shape, is required. Point hydrophone gation over extended ranges, even in the backward direction from approaches to the problem usually involve measurements perthe measurement location.