Instrumentation in ultrasound cardiography

VMK Swamy 
 Environmental and Medical Products Division, Scientific Instrument Company Ltd., Bombay-400001, India

Correspondence Address:
VMK Swamy
Environmental and Medical Products Division, Scientific Instrument Company Ltd., Bombay-400001
India

Full Text

Ultrasound cardiography, introduced by Edler and Hertz [1] more than 20 years ago is a well established, non-invasive diagnostic procedure in cardiovascular diseases. An impressive and growing list of cardiac disorders can be identified by their characteristic echocardiographic features. Because of their relative simpli­city, safety and non-invasive nature, these echocardiographic techniques are readily adaptable as screening procedu­res and may aid in developing a rational approach to diagnosis prior to the perfor­mance of more invasive studies or the initiation of therapy. The accelerated interest in echocardiography in recent years is the result of improved instrumen­tation. The echogram when recorded simultaneously with other non-invasive physiological indicators such as electro­cardiogram, phonocardiogram and carotid pulse tracing, provides excellent means of correlating electrical and mechanical events of the heart. This article descri­bes the various instrumentation techniques available for echocardiography.

Instrumentation Techniques

The techniques in use at present are: (1) A-Mode; (2) M-Mode (Time Motion Mode); (3) B-scan; (4) Gray Scale Imaging and (5) Real Time Imaging.

(1) A-Mode Technique

The earliest type of display was the A-Mode (Amplitude Mode). The A-Mode display presents echo amplitudes in the vertical axis (Y-axis), and time (distance) between echoes in the horizontal axis (X-axis). A-Mode presentation is still used in encephalography and echocardio­graphy.

(2) B-Mode/Time Motion Mode

When the amplitude intensity is dis­played by the corresponding brightness (called Z-axis), the resulting image is called B-Mode (Brightness) presentation. In M-Mode, dot intensity is related to echo amplitude and width is related to the base of the echo spike. Distance from the transducer surface is still displayed in . the X-axis. For moving structures, such as the heart, the Time-Motion Mode (M-Mode) has been developed. M-Mode can be recorded by electronically sweep­ing the oscilloscope screen at a given rate and photographing the results.

(3) B-Scan

B-mode display in two dimensions is popularly called "B-Scan". However, for B-scanning another component is neces­sary. That component must have capab­ility to provide information on the exact location of the transducer and the direc­tion of the beam. In contact scanning, the scanning arm and its associated elec­tronics -provide this information.

Until recently two dimensional displays have been of the bistable mode. (For simplicity it can be said as black and white picture). This modality is best ap­preciated in terms of the storage oscil­loscope. In the bistable mode only that information above a given threshhold is displayed. The oscilloscope does not display amplitude information in terms of brightness but rather that the ampli­tude is above the given threshhold. That is, if the echo is displayed, then the am­plitude of the echo is greater than that of the threshhold. If the echo is not dis­played, the amplitude is such that it falls below the threshhold setting. This is often referred to as "outline" or "leading edge" scanning.

(4) Gray Scale

A more useful display of the two dimensional ultrasound information is Greatone (gray scale). This modality not only displays the echo in relation to its X-Y co-ordinates but also displays the echo intensity in terms of the Z or brightness information. Early images were recorded from non-storage oscillo­scopes into Polaroid film. This has been found to be less than optimum. Present­ly, most of the commercial manufacturers have begun to utilize the scan converter for gray scale ultrasound. This device re­cords the ultrasound signal similarly to the oscilloscope, but has the ability to store 32 levels of Z-axis information. The output format from a scan converter is in a television format and is viewed on a television monitor.

Greatone ultrasound combines this in­formation into one picture by relating the intensity of the echo to the intensity of the image displayed. It is a point by point representation of the echo intensi­ties. Latest extended digital processing sonograph system receives better to "see more" because of a tuned receiver and more sensitive system response, high re­solution through the implementation of higher frequency transducers and multi­ple modes of continuous gray scale pro­cessing.

(5) Real Time Imaging (Sonofluoroscopy)

This is a different procedure from B-­scan imaging. In B-scanning, one constructs an entire cross section image through compound scanning techniques. This image is the result of all the dots of information forming a display on the screen. Real time imaging, on the other hand, visualizes only the information directly under the transducer array and for only as long as the array is over that information. For example, the linear and the phased array real tame systems have a large field of view, continuous line den­sity and are preferred for abdominal and cardiological applications.

Recording o f ultrasound Images

Several different approaches have re­cently been developed for photographing ultrasound images. Each technique has its own advantages and limitations. In places where it is difficult to procure pola­roid films, it is better to use either Tek­tronix Recording Camera with graflex back to take 120 roll films or Hasselblad 70 mm camera apart from Dunn Multi­format Camera wherein conventional X­ray film has been used in ultrasound im­aging. A Viedeo Hard Copy Recorder can also be used.

A strip chart recorder such as the Ho­neywell Model 1856 Fiberoptic system is highly desirable for echocardiography where demonstration of the continuity or discontinuity of various structures and complex re-arrangements of the great vessels requires sweeping movements of the sound beam. This recorder minimi­ses performer bias and also facilitates the recording of amplitude signals such as those that arise from the aortic or pul­monic cusps or endocardium and the mo­nitoring of changes produced by pharma­cological interventions such as the use of amyl nitrite.

Thus echocardiographic examination can give valuable information on the mi­tral valve, aortic root and valve, pulmon­ary valve, septum and posterior wall of the left ventricle etc. Stroke volume and ejection-fraction can be calculated from these. One can diagnose pericardial effu­sion, left ventricular wall thickness, con­gestive and hypertrophic cardiomyopathy, infective endocarditis, coronary artery disease and congenital heart disease.

 Acknowledgements

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