The present invention relates to medical instrumentation and, more particularly, to a digital ultrasound system for medical imaging. A primary objective of the present invention is to provide a high-resolution digital ultrasound system with modest memory and processing requirements.
A typical ultrasonic imaging system comprises an electronics module and a probe. The electronics module generates an electrical pulse which is converted to an ultrasonic pulse by a transducer in the probe. When the probe is pressed against a body, the ultrasonic pulse is transmitted into the body and is reflected to different degrees at tissue boundaries within the body. The reflections from the various tissue boundaries reach the transducer at different times, depending on their distances from the probe. The transducer converts the reflections to a time-varying electrical signal. This electrical signal is processed to form a video representation of the body being imaged.
Relatively simple ultrasound systems are known which employ spherical or parabolic transducers to transmit and receive ultrasound signals. Generally, these transducers are fixed focus in that their focal point is a fixed distance from the transducer. Provisions are typically made to steer the transducer to obtain image information over a range of angles. The resolution of the ultrasound image is limited by the aperture of the transducer, with larger apertures allowing greater resolution. However, larger apertures produce shallower depths of field so that a smaller range of depths to either side of the focal point can be imaged within a given tolerance of maximum resolution.
It is theoretically possible to provide both greater range and high resolution by deforming a transducer to vary its focal length so that a high resolution image is obtain for each of many focal depths. Apparently, it has not been practical to achieve the desired focal length control by mechanically deforming a transducer. On the other hand, "electronic deformation" of phased array transducers, a technology derived from radar, has permitted high-resolution imaging without significant depth-of-field limitations.
Phased array transducers comprise multiple transducer elements arranged in annular, linear or planar arrays. By varying time delays between elements of any array one can vary the depth of focus dynamically. Thus, a large aperture array transducer can be used to obtain high-resolution imaging and its depth of focus can be varied to overcome the limitation of a shallow depth of field.
Annular arrays come closest to simulating a mechanically deformable single-element spherical transducer. An annular array comprises multiple annular transducer elements arranged coaxially. As reflections are received by each of the annular elements, each annular element generates a corresponding electrical signal. By controlling the relative delays introduced in these electrical signals, the focal depth of the annular array transducer can be controlled. As with a spherical single-element transducer, an annular array must be mechanically steered to obtain a two-dimensional ultrasound image. Thus, an annular array involves a hybrid of mechanical and electronic control of the transducer focal point.
A linear phased array comprises a series of narrow elements arranged side-by-side. Such an array can be electronicallyu steered and focussed. A disadvantage of a linear phased array is poor azimuthal resolution in the direction perpendicular to the plane of the image. Planar array transducers comprise a multitude of small-aperture elements arranged in a two-dimensional array. As with linear array transducers, both focal depth and steering can be effected electronically. In fact, steering can be in two-dimensions. A major advantage of planar array transducers, when contrasted with linear array transducers is that they resolve in the azimuthal dimension. However, planar array transducers are not widely implemented due to the large number of separate signal channels, one for each transducer element, which must be processed.
Most ultrasound systems use analog processing to obtain video representations of the subject being imaged. However, as in other technological fields, several advantages are obtainable using digital processing. Once the signals are converted to digital format, they are less vulnerable to distortion, noise etc. In addition, digital systems are more amenable to automated testing and require few adjustments. Therefore, it is generally easier to manufacture reliable digital systems. Costs can be lowered through integration and flexibility can be provided through programming.
Digital ultrasonic imaging systems are disclosed in U.S. Pat. No. 4,290,310 to Anderson. These systems are all based on linear phased arrays, although the suggestion is made that the principles can be applied to other transducer configurations, including other phased array transducers. Each transmit/receive transducer or each transmit/receive transducer pair corresponds to a channel, with 32 or more channels being preferred to obtain practical resolution. Steering and focus are controlled as a function of relative delays between the channels. These relative delays are implemented by controlling the time between reading in and reading out reflection data from a analog memory device such as a serial analog memory (SAM) or a charge-coupled device (CCD), or a digital memory such as a first-in-first-out memory (FIFO). Analog-to-digital converters (ADCs) are used to convert the analog reflection signals to digital form before they are read into the FIFO memories. The output of the memory devices are combined by a summing circuit, the output of which is directed to a video detector, and thence to a display. A master controller synchronizes the ADCs, memories, summing circuit and video sweep to provide a coherent image of the subject being scanned.
A major disadvantage of the system disclosed by Anderson is that focus depth cannot be swept dynamically. Anderson controls focal depth by filling memory devices to achieve a desired delay and then creates an image at the corresponding focal depth. Each depth requires a refilling of the memories, introducing delays in the imaging process. In effect, Anderson uses zone focussing--relying on a relatively small aperture, rather than dynamic focus, to provide the required resolution throughout the range of depths represented by the zones.
Other disadvantages of these digital systems are basically cost and complexity, the latter impinging on reliability. Anderson required 32 channels of ultrasonic reflection information to be synchronized and processed in parallel. A high performance system using the same design principles would require more than 100 linear transducer elements and a corresponding number of signal processing channels. Each channel requires its own ADC and memory as well as ancillary components. The activity of each component in each channel must be synchronized with the transmitter and the video output section.
In addition, each of the components must be capable of handling data at high speeds. Ultrasound imaging frequencies typically range from 2-10 MHz. Sampling by the ADCs generally is over two and, preferably, about four times the maximum ultrasound frequency so that components should be rated from about 15 MHz to 40 MHz. Anderson discloses a 60 MHz master clock to produce component clocks of 15 MHz and 20 MHz. Component costs rise dramatically with maximum clock rate, so a premium must be paid for these high speed components. In addition, operation of the memories at high speeds lowers the range of delays that can be implemented using the components. Anderson combines memory devices serially to obtain longer delays, but this multiplies cost and complexity as well.
Furthermore, Anderson's implementation using linear arrays results in poor azimuthal resolution. This problem can be addressed by opting for two-dimensional arrays. However, this would greatly exacerbate the problems of component count, component, cost and system complexity. Several hundred channels would be required for practical resolution in a digital two-dimensional phased array system.
What is needed is an ultrasonic imaging system which provides the advantages of dynamic focus and digital control without the cost and complexity of previously disclosed systems. Preferably, high resolution in all three dimensions would be provided. Furthermore, the component clock ratings should be modest so that less costly components can be used and so that longer delays can be implemented without serially combining memory devices or using larger, more expensive memories.