[Ultrasound and the bone: a difficult relationship].

The principles of US physics and technology hampering both the production of US images of the bone and the assessment of the soft tissue structures underlying it are discussed. In theory, two US parameters play a role in this field: the different transmission velocity of the US beam through soft tissues and bone, and the marked attenuation of the US beam when crossing the bone. The former parameter, due to higher density and lower compressibility of the bone with respect to soft tissues, causes both intense reflection (and thus beam weakening) and refraction (and thus lateral resolution and US image distortions) at the soft tissue/bone interfaces. Moreover, US transmission velocity in the bone differs significantly from the reference velocity of the US scanners on the market based on that of soft tissues. As a consequence, during the reconstruction of a bone US image, artifacts resulting in the axial compression of bone structures should, at least in theory, occur. The latter parameter, due to both conversion into heat (absorption) and local dispersion (scattering), is likely to be main factor causing the loss of US energy when the US beam passes through the bone. Although the amount of matter (bone mass or density) undoubtedly accounts for some attenuation, local bone architecture (bulk and shear moduli of bone and marrow, bone and marrow density, marrow viscosity and porosity, permeability and tortuosity of cancellous bone structure) seems nevertheless also responsible for some attenuation through both absorption and scattering. Other consequences of attenuation reflecting on US imaging of the bone are: marked lowering of central transducer frequency and US beam widening preventing the correct identification of the interfaces originating echoes by relating them to the structures on the transducer axis. In conclusion, based on the above parameters, echoencephalography and transcranial Doppler US can be expected to improve, in the near future, their bone-crossing capabilities, even though no true gray-scale bone sonogram will ever be feasible.