|
The main requirement of pulsed-echo ultrasound applications such as cardiac imaging, 3-D imaging, or blood flow imaging can be identified as frame rate. Currently, frame rate is limited by the imaging depth range and the number of ultrasonic fires. For example, a 15 cm imaging range gives rise to 200 microsecond(s) lines or to a 2 s acquisition time for 100 planes of 100 lines in 3-D medical applications. The only way to increase frame rate is parallel beam formation in the receive mode. Simultaneous parallel beam forming allows us to divide the acquisition time by a factor proportional to the number of beam formed lines. In the technique developed by S. W. Smith, an increase frame rate of 16 is achieved. However, this technique is limited by a loss in lateral resolution due to the requirement for a wide illumination beam of the explored medium in the transmit mode. We propose an alternate illumination scheme that minimizes the loss in resolution in the transmit mode. In this technique, ultrasonic energy is transmitted simultaneously in several narrow beams. This technique works in pulsed mode and we have built the hardware needed for the simultaneous production of several beams. Each transducer is connected to a transmitter able to generate a sequence of excitation pulses. If n beams are to be transmitted, the excitation signal is the sum of n cylindrical wave fronts. For those, among elements where the n wave fronts are disjoint, the excitation signal is thus the succession of n pulses with specific time positions with respect to the system synchronization. For the others, the excitation signal is more complex since it consists in a (n - 1) level signal (0, 1, 2, ... n - 1 times the basic excitation signal). We show the performances of such a parallel transmit scheme based on beam plots as well as on tissue phantom images. This leads to an evaluation of the maximal number of beams compatible with current medical imaging quality standards. It is shown that a gain of 16 in the acquisition time can be achieved without any loss in lateral resolution.
|
