Medical ultrasound: from inner space to outer space

During the last decade, medical ultrasound has rapidly become a widely accepted imaging modality used in many medical specialties. It has the advantages that it is noninvasive, does not use ionizing radiation, is relatively inexpensive and is easy to use. Future trends in ultrasound include expanded areas of use, advanced signal processing and digital image analysis including tissue characterization and three dimensional reconstructions. One important advantage of ultrasound is that it can easily and safely be used for longitudinal serial studies on a healthy patient population. In order to reduce the errors associated with such studies, we have been developing an advanced spatial location and image processing system. The system permits the operator to take a series of ultrasonic scans with known relationship to each other and reconstruct a three dimensional represen­ tation of the targeted organ or vessel. The present spatial locator system uses a spark that is discharged in air across a small gap between two electrodes which generates a shock wave traveling radially at the speed of sound. It is possible to detect this wave with an appropriate microphone. The distance to the spark-gap can then be computed by measuring the transit time of the shock wave and knowing the local speed of sound. It has the advantage that it places no signifi­ cant constraints on the operator, since only small, flexible cables are used to attach the acoustic sources to the transducer assembly. Also, it provides him or her with accurate, nearly-real-time, transducer position information with respect to a fixed coordinate system. Three spark-gaps are fixed to the probe in a known relationship. The microprocessor controlled system sequentially fires the acoustic sources to produce three separate shock waves. These are detected by an array of microphones, filtered and processed by the micro­ processor. The coordinates of the three sources are used to calculate the position of the surface of the transducer assembly. The normal to this surface is found by taking the cross product of the vectors between the acoustic sources and the orientation of the scan plane. The system has as its output, nine quantities which are the X, Y and Z coordinates of the apex of the scan plane and passing through its apex and the direction cosines of the vector normal to the scan plane. The performance of both the JPL spatial locator system and the locator concept are undergoing evaluation as part of a continuing effort. Experience with the locator system indicates that positions within a cubic region 100 cm on a side can be determined to with­ in 0.5 mm and 1° error In the direction cosines with increased accuracy if averaging of determination Is incorporated. Positional accuracies of 0.3 mm have already been achieved. A microprocessor computer system has been modified for use with any modern ultrasound Instrument. This is a real-time data acquisition system which encodes human-readable and machine-readable information into each video frame and each video field. The system coor­ dinates from the spatial locator as well as video frame numbers. Measurements are pos­ sible with joystick controlled cursors on playback. The images may be digitized subse­ quently and the binary Information may be decoded for use in image processing.