A spatial compounding system has been designed to improve the quality of B-mode echographic images. It consists of constructing an improved image from the combination of several different images of the same cross-sectional plane. The "final" image is constructed by the registration and the superposition of the "original" images. For this, the relative position in the space of the original images has to be known. The use of a localization articulated arm, on which the ultrasonic probe is fixed, makes this possible. The main advantages of the technique are, on one hand, the elimination of the acoustic shadows following a strong reflector structure and, on the other hand, the reduction of the speckle generated in echographic images. The method of reconstruction has been validated on agar gel phantoms and provides good accuracy. In vivo experiments on human beings have also been performed. Acoustic shadows caused by bones in cross-sectional images of the thigh and the arm are eliminated. All the contours of the femur and humerus can be observed in the final images. The reduction of speckle is shown in kidney images and the signal-to-noise ratio improvement is quantified as a function of the number of images involved in the reconstruction.