Instrumentation in ultrasound cardiographyVMK Swamy
Environmental and Medical Products Division, Scientific Instrument Company Ltd., Bombay-400001, India
Cardiac ultrasound/Echocardiography is a. simple, safe and non-invasive method for detecting heart diseases. When combined with ECG, X-ray and isotopic studies, ultrasound provides significant diagnostic information not previously available.
Ultrasound cardiography, introduced by Edler and Hertz  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.
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.
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.
I wish to thank Dr. C." K. Deshpande, Dean, Seth G.S. Medical College and K.E.M. Hospital for giving us facilities.