Acoustic signal transmission couplants for use in tomographic, large aperture ultrasound imaging and therapeutic ultrasound are described that couple acoustic signal transducers to body structures for transmitting and receiving acoustic signals. The acoustic signal transmission couplants can conform to the receiving medium (e.g., skin) of the subject such that there is an acoustic impedance matching between the receiving medium and the transducer. In one aspect, an acoustic coupling medium includes a hydrogel including polymerizable material that form a network structured to entrap an aqueous fluid inside the hydrogel. The hydrogel is structured to conform to the receiving body, and the acoustic coupling medium is operable to conduct acoustic signals between acoustic signal transducer elements and a receiving medium when the hydrogel is in contact with the receiving body such that there is an acoustic impedance matching between the receiving medium and the acoustic signal transducer elements.