The Digital Scan Converter

Excerpt from patent claim "Ultrasound imaging system with improved scan conversion", United States Patent 405800, Waxman, Albert S.; Application Number: 710942, Filing Date: 1976-08-02, Publication Date: 1977-11-15.

Many commercial ultrasonic imaging units employ some form of analog scan converter. Such scan converters typically use a storage tube which employs a cathode to generate a beam of electrons focused on an anode to produce a distribution of electrostatic charge thereon in a pattern representative of the tissue interfaces detected within the body of the patient. The pattern of charges is "read" from the cathode of the storage tube and concurrently displayed in the form of an image on a conventional television receiver.

The video images displayed on the television receiver may be in the form commonly referred to as a "B" scan. B scans depict cross-sectional areas of the patients under study in the plane of the ultrasound probe (typically a vertical plane). Echoes received from particular coordinate locations in this plane are depicted as spots of light on the television receiver. Coordinate locations from which no echoes are received, result in an absence of illumination on the display unit. Because of background noise, an amplitude demarcation is necessary to determine which signals are to be selected for display as echoes, and which signals are background and should be suppressed.

B scan displays may be derived from one or two different forms of pulse processing techniques in conventional ultrasonic imaging systems. The echo pulses attributed to any particular coordinate location within the body of the patient may be integrated, so that the display at the corresponding point on the display unit represents the amplitude sum of all of the pulses deem to have originated from that point. The information obtained from this form of display, however, is subject to a high degree of variation from operator techniques in moving the ultrasonic probe across the body of the patient. If the probe dwells too long in a single position, the echoes detected from the locations scanned at that probe position will be weighted out of proportion to echoes from points which are scanned more rapidly. By the same token, if particular locations are not scanned at all or scanned for a shorter period of time, the detected echoes will reflect an inadequate sampling from that position.

To obviate this problem, a different technique was developed. This technique is known as pulse amplitude peak detection and is characterized in that only the largest echo signal is registered from each coordinate location. In this way, the ultrasound diagnostic unit becomes more nearly operator-independent. Excessive sampling of some locations and inadequate sampling of other locations does not result in distortions, since only the largest echo amplitude peak from each coordinate location will be ultimately registered on the display unit. The echo amplitude peak detection technique has been used to wide advantage in ultrasound medical imaging.

It has been found, however, that when ultrasound diagnostic units having conventional analog scan converters are adapted for echo amplitude peak detection, the full advantages which might be expected from the peak detection technique are not realized. Generally reduced resolution is found in peak, detection mode, especially when an image portion is over-scanned, and because edge resolution is less than center resolution. The image will tend to be less clear toward the edges due to the growth of spot size in the storage tube, and the tubes inherently exhibit non-uniformities of 10% typically. Image alignment is difficult due to poor gray scale matching and geometry distortions.

The write speed of ultrasonic units equipped with analog scan converters is limited, so that in order to avoid loss of echo information due to this lack of speed, larger fields of view than optimum for particular applications must be used. Even so, all of the available echo information may not be registered on the storage tube, and in any event resolution is degraded. Further, writing speed limitations, along with the characteristic long erase time of the analog converter, are significant enough to prevent practical real time imaging.

Another facet of the foregoing problem is the distinct time interval between read and write, which functions cannot be done simultaneously with the foregoing systems. This need to multiplex the read and write functions result in image artifacts, particularly a "venetian blind" effect, on the CRT monitor. That is, the monitor goes blank and blinks during the erase interval just prior to a new information recording operation. This effect is, of course, distracting to those viewing the monitor. Finally, post-image processing and analysis is difficult, since the entire system and associated circuitry is in analog form.

Moreover, this improvement which is possible with peak detection, while significant, does not by any means exhaust the possibilities of improving image quality. The tissue sought to be studied may include both strong, specular scatterers of ultrasonic energy, and diffuse spherical scatterers; on the macroscopic level, the former may be thought of as a high acoustic impedance interface, and the latter as a low acoustic impedance interface. Peak detection helps mostly with the former, by discriminating in favor of the strongest echo, that normal to the addressing acoustic pulse.

In the case of the diffuse scatterers, (or low impedance interfaces), however, good image information may continue to be lost, since a location associated with such a scatterer or interface does not provide an echo which is significantly larger or more representative than the others from such location. Such diffuse scatterers or low impedance interfaces are precisely those which characterize those tissue formations which are the most difficult to analyze, such as the internal portions of organs having fairly similar or uniform structures, or tumors which are similar to surrounding healthy tissues. Accordingly, any lacks in this regard can have serious diagnostic consequences.

Image improvement comprising a digital scan converter for receiving inputs from the transducer and having location determination means for assigning two-dimensional coordinates in the scanning plane and having addressable memory storage locations in communication with the transducer for storing representations of the amplitudes of the echo signals at corresponding memory storage locations having the addresses of the aforesaid two-dimensional coordinates, and circuit means for communicating the stored representations to the display unit.

With the digital scan converter associated in the foregoing manner with the system, a greatly improved degree of sensitivity and resolution is attained, the non-uniformities and distortions, both of linearity and of gray scale, found in the analog converter are avoided, multiple image storage capability is improved, and write and erase speed are improved to permit real time imaging.

The digital scan converter improvement permits and facilitates the incorporation of this improvement of peak detection operation for larger pulse echo amplitudes, and the integration-normalization analysis for smaller amplitudes. These techniques provide great improvement in the quality of the ultimate diagnostic image produced. Small echo amplitudes emanating from interfaces between similar tissues bearing useful image information would normally be lost in the peak detection mode. In this invention, however, they are saved, cumulated and normalized to obtain values for particular locations which significantly exceed background noise and thus become useful, particularly in visualizing slight discontinuities within the tissue of a patient which would otherwise be obscured. Gray scale possibilities and resolution, especially for those details which are most difficult to detect, are greatly improved. At the same time, inaccuracies in the contribution of large pulses are effectively controlled by the peak detection technique. The inherent speed of the digital scan converter allows echo information processing in either static mode or real time, along with all of the advantages set forth above.