High frame rate imaging with a small number of array elements

Recently, a high frame rate imaging method has been developed to construct either 2-D or 3-D images (about 3750 frames or volumes/s at a depth of abou t 200 mm in biological soft tissues because only one transmission is needed ). The signal-to-noise ratio (SNR) is high using this method because all ar ray elements are used in transmission and the transmit beams do not diverge . In addition, imaging hardware with the new method can be greatly simplifi ed. Theoretically, the element spacing (distance between the centers of two nei ghboring elements) of an array should be lambda/2, where lambda is the wave length, to avoid grating lobes in imaging. This requires an array of a larg e number of elements, especially, for 3-D imaging in which a 2-D array is n eeded. In this paper, we study quantitatively the relationship between the quality of images constructed with the new method and the element spacing o f array transducers. In the study, two linear arrays were used. One has an aperture of 18.288 mm, elevation dimension of 12.192 mm, a center frequency of 2.25 MHz, and 48 elements (element spacing is 0.381 mm or 0.591 lambda) . The other has a dimension of 38.4 mm x 10 mm, a center frequency of 2.5 M Hz, and 64 elements (0.6 mm or 1.034 lambda element spacing). Effective lar ger element spacings were obtained by combining signals from adjacent eleme nts. Experiments were performed with both the new and the conventional dela y-and-sum methods. Results show that resolution of constructed images is no t affected by the reduction of a number of elements, but the contrast of im ages is decreased dramatically when the element spacing is larger than abou t 2.365 lambda for objects that are not too close to the transducers. This suggests that an array of about 2.365 lambda spacing can be used with the n ew method. This may reduce the total number of elements of a fully sampled 128 X 128 array (0.5 lambda spacing) from 16384 to about 732 considering th at the two perpendicular directions of a 2-D array are independent (ignorin g the larger element spacing in diagonal directions of 2-D arrays).