• Digital systems digitize the acoustic signal immediately, and then perform the focusing using digital electronics.
• Analog systems, including those using analog electronics under digital control (hybrid systems), perform their focusing using analog electronics.
• Digital systems will provide better image quality just as digital electronics (CD players) provide better fidelity than analog electronics (tape players).
• All of GE's LOGIQ products are digital ultrasound systems.

The world in which we live is full of degrees of difference between two extremes. The car is neither blue nor green; it is bluish-green. The day is neither extremely warm nor cold; it is unseasonably cold this morning with a warming trend toward the evening rush hour. While we recognize that we live in a world of grays, we have found it convenient to devise precise ways of talking about them. The paint companies tell us that the car color is Pantone 307, a precise measure of color. The temperature at 5 o'clock was 37 degrees.

This process of determining discretely different levels of continuously variable (analog) physical phenomena is known as quantification and is the fundamental process behind digital technologies.

The principal disadvantage to the digital technologies is that they tend to be more expensive than their analog counterparts when they are first introduced to consumers. Whether they eventually become less expensive depends largely on whether the demand for the product allows suppliers to reduce the per-unit cost by increasing their manufacturing volume.

Occasionally, high volumes in one industry will have an impact in another industry. For instance, the high volume of portable computers has driven down the costs of high-speed memory devices. This has benefited other industries that use the same electronics in their products, even if their product sales volumes are low.

In all cases, however, the digital devices have one important attribute: Once the "value" of the data is determined, it can be copied, stored, reproduced, or modified without degradation. Consider the difference in accuracy of the reproduction between making a copy of a copy of a copy of a computer disk and making a copy of a copy of a copy of an audio cassette.

Digital ultrasound is like any of the digital products described above. The electronics in a digital ultrasound instrument make discrete measurements of continuously variable phenomena.

Nearly every ultrasound system in production today includes an analog-to-digital converter. This includes the $5,000 portable scanner and the $300,000 premium ultrasound workstation. This, of course, opens the doors for many manufacturers to exploit the ambiguity in their promotional programs. A common working definition that the ultrasound community can use is in order. Figure 1 shows two simple diagrams that illustrate the difference between the two choices.

As with the compact disk player, the principal advantage of a digital ultrasound machine over its analog counterpart is its image quality (audio fidelity in the case of a compact disk player). Perhaps the most important parameter in ultrasound image processing is the beam formation. The beamformer is the part of the ultrasound system that provides the focusing for the ultrasound beam.

As a point of review, in a transducer with multi-element solid state crystals, the beam is focused and steered by exciting each of the elements at a different time so that the resulting sound wave coming from each crystal will arrive at the intended focal point simultaneously.

Figure 2 demonstrates this principal. In case A, the beam is being focused and steered to the left. Note that the distance from the focal point to element 1 of the transducer is shorter than the distance from the focal point to element 4. This means that in order for the waves generated by each element to arrive at the focal point simultaneously, element 4 must be excited before elements 1, 2, and 3.

Likewise, in case B, the focal point is to the right, so the elements of the transducer must be excited in the reverse order. This process of phasing the firing of elements is "beam formation."

We have illustrated the case in which the beam is formed on transmission. A similar beam formation must be done on reception. Consider Figure 3. In this case, it is easy to see that the echo returning from point A encounters element 1 before it encounters element 4. Thus, the signals coming into the ultrasound scanner from the various elements must be delayed so that they all "arrive" at the same moment. The signals from each element are summed together to form the ultrasound signal that is subsequently processed by the rest of the ultrasound instrument.

The precision with which all of these delays are made, whether on transmission or reception, determines the precision with which the focusing can be done. As one can imagine, slight miscalibrations of any of the delays can cause artifacts in the image. Though the rest of tie imaging chain cannot be ignored, this particular step is crucial.

The definition of a digital beamformer is one in which the echo returning from the tissue is digitized before the time delays, so that the time delay and the summation that follow it can both be done with digital electronics.

• The CD-like digital image quality is locked in at the beginning of the imaging chain. The earlier the signal is locked in, the more likely the signal integrity will be maintained.
• Digital time delays permit much greater precision in the shaping of the ultrasound beam. An example follows below.
• Since the time delays and summation are all done digitally, they do not require the frequent calibration associated with analog beamformers.
• The digital beamformer permits the system considerable flexibility in reprogramming the size, shape, direction, and intensity of the beam. This permits considerable flexibility to implement future image formation enhancements.

With the definition well understood, one might now ask "Does it matter?" Though the beamforming performance has several implications, let's consider the effect of beamforming precision on spatial resolution. Recently designed ultrasound instruments can now routinely resolve distances well below a millimeter (see Micron Imaging). Lateral resolution, for instance, is the product of wavelength and F-number:

where D is the depth of scan and A is aperture of the transducer. A 10 MHz transducer, with a 1 cm aperture focused 2 cm deep, has a theoretical lateral resolution of 0.3 mm.

The distance from the center of the aperture to the focal point is 2 cm, as described above. The distance from the outside of the aperture to the focal point is 2.061 cm. In terms of beam formation, with the speed of sound at 1540 m/s, the total travel time of the sound to the focal point is between 12.98 µs and 13.38 µs. The difference (0.4 µs) is the coarsest that the time delay resolution can ever be and still retain the theoretical 0.3 mm lateral resolution target. That 0.4 µs would be further divided between the number of elements in the aperture, leaving an astonishingly small margin of error.

Remember from the earlier discussion that among the advantages of digital technology is its ability to maintain fidelity. In the context of the current discussion, the additional benefit to digital is its ability to hold the precision once it is established.

To understand what will happen when the tolerances are not held, consider the summation of the signals that is done after the time delays are done. Figure 4a shows the summation of signals from each of the transducer elements when the time delays are properly done. Figure 4b shows the summation when the time delays are slightly miscalibrated. Note that in the well-aligned case (4a), all of the signals sum coherently, providing not only good focusing, but significant signal strength as a result. In the poorly-aligned case (4b), two artifacts become apparent. First, the length of the resultant pulse is longer. This reduces the system's axial resolution. The second effect is that the additional signal strength gained from coherent summation of the signals is significantly less than in the well-aligned case.

Image quality is certainly the primary reason to select a digital beamformer. Color flow imaging illustrates an example of these additional benefits:

The greatest physical constraint on color flow frame rate is the speed of sound. Sound travels at roughly 1540 m/s in human soft tissues. Color flow imaging involves sending out a burst of sound to a specific position in the tissue and waiting for the echo to return. Most of the time spent in this process is consumed waiting for the sound to travel out and back. To make matters worse, the process is repeated for every pixel in the region of interest. This means that the system spends a considerable amount of time waiting. This is time that cannot be reduced with a faster computer or better designs.

It is possible, however, to take advantage of a sufficiently flexible digital beamformer to reduce the amount of time required. Here is how it works: When the echo is received, it is digitized and stored immediately. (Something not possible in an analog beamformer.) Then two different sets of time delays can be applied to the same received acoustic data.

In this case, the acoustic data is processed twice (or more times). The first processing applies one set of digital time delays to create a "virtual focus" in the left side of the beam. The second processing creates a "virtual focus" on the right side of the beam. Thus, from one set of acoustic data, the system derives two sets of color flow information. These two processes are done simultaneously through separate paths through the ultrasound system. This two-for-one parallel processing allows the system to double the information processed which can then be used to improve the color flow frame rate.

To be clear, this parallel processing is not a spatial interpolation or averaging that estimates data that is not really in the ultrasound signal. Parallel processing of this form extracts additional acoustic information that would have been ignored in systems with a different design.

Even with precise definitions, points of confusion are likely to arise. Consider the following possibilities:

Digital Beamformer vs. Digital Scan Converter: Remember that ultrasound information generally comes from the body to the transducer in an up and down direction on the screen. TV monitors, however, scan out data right to left on the screen. Figure 5 shows a block diagram of a typical ultrasound system.The role of the scan converter is to convert the ultrasound information from the up-down format used in the image processing to the left-right format required by the TV displays.

Nearly every ultrasound system on the market today has a digital scan converter. This alone, however, does not meet the definition outlined above which requires digital data in the beamformer.

Though the application of digital electronics to ultrasound beam formation has been pioneered by a few ultrasound manufacturers, the fundamental electronic technologies that have made digital ultrasound possible were not developed by ultrasound companies. These technologies, such as high-speed digital-to-analog converters, were developed in other industries that have much larger consumer bases to fund the research required for this kind of advance. To illustrate the point, consider the difference in the number of sales of cellular telephones as compared to the number of sales of high-performance ultrasound instruments. The availability of these technologies to ultrasound has been made possible by other areas.

As of the writing of this paper there were 7 digital ultrasound systems available from 4 suppliers. GE offers three different digital platforms; the LOGIQ 700, LOGIQ 500 and LOGIQ 400, each with slightly different applications but with the same fundamental structure: A digital beamformer, with digital time delays, and a digital sum followed by digital processing.