Ultrasonic diagnostic imaging systems are in widespread use for performing ultrasonic imaging and measurements. For example, cardiologists, radiologists, and obstetricians use ultrasonic imaging systems to examine the heart, various abdominal organs, or a developing fetus, respectively. Diagnostic images are obtained from these systems by placing a scanhead against the skin of a patient, and actuating ultrasonic transducer elements located within the scanhead to transmit ultrasonic energy into the body of the patient. In response, ultrasonic echoes are reflected from the interior structure of the body, and the returning acoustic echoes are converted into electrical signals by the transducer elements in the scanhead.
FIG. 1 shows an ultrasonic imaging system 10 according to the prior art. A scanhead 12 includes a handle portion 14 that supports a transducer assembly 16. The transducer assembly 16 is generally formed from a crystalline material, such as barium titanate or lead zirconate titanate (PZT), that is constructed to form an array of piezoelectric transducer elements 18 that are each capable of transmitting and receiving signals at ultrasonic frequencies. The transducer elements 18 thus formed may be arranged in a linear array, or alternatively, they may be arranged in a variety of two-dimensional configurations. A scanhead cable 20 is coupled to the scanhead 12 at one end, and to an ultrasonic processor 22 at the opposing end to permit the processor 22 and the scanhead 12 to communicate. The ultrasonic processor 22 contains a beamformer 24 capable of exchanging signals with the scanhead 12 to focus the ultrasonic signals emitted by the transducer assembly 16. Focus is achieved by controlling the relative time delays of the applied voltages on each element so that they are combined to produce a net ultrasonic signal focused at a selected point within the body being scanned. The focal point thus achieved can be moved on each successive transmitter excitation, so that the transmitted signals can be scanned across the body at various depths within the body without moving the transducer. Similar principles apply when the transducer receives a return echo from an interior region of the body. The voltages produced at the transducer elements 18 are individually delayed in time and then summed so that the net signal is dominated by the acoustic echoes reflected from a single receive focal point in the body. The focused signals may then be transferred to an image processor 26 located within the ultrasonic processor 22 for subsequent additional processing prior to displaying a visual image of the scanned region of the body on a visual display 28. A system controller 30 cooperatively interacts with the beamformer 24 and the image processor 26 to control the processing of the beamformed signals and the data flow from the beamformer 24.
The need for diagnostic images having a finer resolution and three-dimensional diagnostic images requiring a 2-dimensional array of transducer elements has progressively led to the development of systems with transducer assemblies that contain ever larger numbers of individual transducer elements 18. As a result, the transducer assembly 16 may contain individual transducer elements 18 in numbers that range from a few hundred elements to as many as three thousand. Generally, each transducer element 18 in the transducer assembly 16 must be coupled to the processor 22 by an individual coaxial line. Since all of the coaxial lines extend through the scanhead cable 20, the diameter of the scanhead cable 20 increases as the number of transducer elements 18 increases. Consequently, as transducer assemblies 16 increase in size, the scanhead cable 20 becomes increasingly more difficult to manipulate during ultrasonic procedures due to decreased cable flexibility and increased cable size and weight. As the size and complexity of transducer arrays steadily increases, the diameter and weight of the scanhead cable 20 can become prohibitively large.
One technique for allowing a scanhead 12 to be more easily manipulated is to use a communications link between the scanhead 12 and the ultrasonic processor 22 other than coaxial cables. For example, a radio or optical link could be used instead of coaxial cables. However, the need to couple a signal from each of a large number of transducer elements 18 to the processor 22 can be problematic for other reasons. For example, it can be difficult to avoid cross coupling between radio links, and it can be difficult to maintain a free “line-of-sight” between a scanhead 12 and an ultrasonic processor 22 needed for an optical link. Thus, although a radio or optical link can solve the problem of scanhead cable weight and flexibility, it creates other problems that are difficult to solve.
Some prior art ultrasonic imaging systems have employed circuitry in the scanhead 12 that have resulted in a reduction in the number of coaxial lines in the cable 20 extending between the scanhead 12 and the ultrasonic processor 22. Some of these prior art ultrasonic imaging systems were initially designed for scanheads having a relatively small number of transducer elements. When scanheads 12 having a larger number of transducer elements 18 were developed, they were adapted for use with the existing ultrasonic processors 22 by placing a multiplexer (not shown) in the scanhead 12. A beam could then be synthesized using signals from an aperture consisting of less than all of the transducer elements 18 in the scanhead 12 by using the multiplexer to selectively couple different groups of transducer elements 18 to the ultrasonic processor 22. An ultrasonic image was then obtained by a process of multiple transmission and receiving cycles in which the transmit aperture, receive aperture, or both, were repositioned using the multiplexer for each cycle.
While this approach was successful in adapting scanheads having a relatively large number of transducer elements 18 to ultrasonic processors having a lesser number of beamformer 24 input channels, this success did not come without a price. In particular, the need to perform multiple transmission and receiving cycles to obtain each ultrasonic image can greatly reduce the frame rate of the ultrasonic imaging system, thus making it relatively time consuming to obtain an ultrasonic image. In addition, the system is not capable of using the entire available aperture in the scanhead at any one time.
Another technique used in prior art ultrasonic imaging system that has resulted in a reduction in the number of coaxial cables in the scanhead cable 20 is to place pre-processing circuitry in the scanhead 12 that performs at least some of the processing functions performed by the beamformer 24. Pre-processing signals from the transducer elements 18 results in the signals from the transducer elements 18 being combined to produce a lesser number of signals that must be coupled through coaxial cables. As a result, using pre-processing circuitry in the scanhead 12 reduces the number of coaxial cables in the scanhead cable 20. Although this approach allows an overall reduction in the number of coaxial lines in the scanhead cable 20, significant shortcomings still exist. For example, the flexibility with which the ultrasonic processor 22 can interface with the scanhead 12 depending on the type of image being obtained and other factors is reduced because at least some of the functionality is fixed by the design of the scanhead 12. Further, in the event the transducer elements 18 assembly either wholly or partially fail, the scanhead 12, including relatively costly preprocessing circuitry, may have to be discarded along with the failed transducer elements 18.
Therefore, there is a critical need for a scanhead that can be coupled to an ultrasonic processor through a relatively thin cable or communications link having relatively few channels despite having a large number of elements without sacrificing the flexibility and functionality that can be obtained by simultaneously coupling all of the transducer elements to the ultrasonic processor through a relatively thick cable.