THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

500 17. Commercial and Medical Ultrasound Applicationsmeasuring the length of the axis of the eye, and detect foreign bodies close to theposterior eye wall. One instrument combines a diagnostic transducer for locatingforeign bodies with a surgical instrument for removing an object, which enablesrapid removal of foreign bodies from the eye by directing the surgical tool to theobject with least damage to the eye.In the field of neurology, ultrasonic echoencephalography provides an immediatemeans of detecting lateral shifts in the midline septum caused by tumorsor concussion. It is notable that every emergency ambulance in Japan is outfittedwith echoencephalographic equipment in order to identify victims with possiblesubdural hemorrhage so that they may be transported directly to special neurologicalunits for treatment. Posttraumatic intercrannial hemorrhage and skull andbrain trauma can be quickly diagnosed and lesions can be located rapidly withoutdiscomfort to the patient. Ultrasonic pulses are transmitted through the temples. Inthe A-scan mode, echo indications from the midline and the opposite temple areasof the skull are presented on an oscilloscope screen. A shift in the midline is readilydiscernible. B-scans, which could provide more information, are more effectivewith small children and infants because their skulls are soft and have attenuationcoefficients lower than those in adults.The ultrasonic B-scan technique for examining the abdomen is useful for detectingpelvic tumors, hydatiform moles, cysts, and fibroids. It is also used to diagnosepregnancy at 6 weeks (counted from the first day of the last menstrual period) andafterwards. The obstetrician can follow the development of the fetus throughoutthe pregnancy, including size and maturity and positioning of the placenta. Thepresence of twins or multiple pregnancy is also revealed. This avoids the need forX-rays and the attendant danger of irradiation to mother and child. Fetal death canbe confirmed much earlier by ultrasound than it can by radiography.With improvements in ultrasonic and computer technology, 3D visualizationbegan to appear in the early 1980s. Some work came from the domain of cardiologistswhere initial efforts were directed to determining the volume of cardiacchambers. Real-time scanner probes mounted on articulated arms were often employedwhere positions of the probe can be accurately established. The principleof 3D imaging has always been to stack parallel image sections together with theirpositional information into a computer.Ultrasonic diagnosis by echo methods extends to all parts of the body. Air andother gases have much lower impedance than either liquid or solids, so air cavitiesproduce distinct echoes. Stationary air embolisms can be so identified becausethey are located in areas usually filled with fluids. Moving embolisms can beidentified by Doppler methods. Gallstones have an acoustic impedance equivalentto that of bone and so can provide good ultrasonic echoes. The ultrasonic B-scantechnique has been proven effective in diagnosing thyroid disorders. Disordersdetected ultrasonically include neoplastic lesions including cystic nodules, solidadenoma and carcinoma, nonneoplastic lesions, subacute thyroidiis, and chronicthyroiditis.The Doppler method takes advantage of the fact that a shift in frequency occurswhen an ultrasonic wave is reflected from a moving target. Also any variation in