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

498 17. Commercial and Medical Ultrasound ApplicationsX-rays, it is used in fetal monitoring, detection of aneurysms, and echocardiography.The production of heat in the body through ultrasound is applied for its therapeuticvalue. The selectively greater absorption of ultrasound in cancerous tissueshas proven its usefulness in hypothermic treatment of cancer. Emerging developmentsinclude the use of ultrasonic waves to perform noninvasive or “bloodless”surgery, stop internal bleeding in trauma patients, and control delivery of drugs orother compounds. Enormous advances in electronic miniaturization are resultingin fairly compact handheld diagnostic units.Diagnostic Uses of UltrasoundDiagnostic medical applications are based on the imaging procedures described inSection 16.7. One diagnostic technique is based on the pulse method and seconddiagnostic technique is based on the Doppler effect where the reflected wave isshifted in frequency from that of the incident wave impinging on a moving target. Inthe reflection or pulse-type equipment, A-scan, B-scan, or a combination of thesetwo methods are utilized to present data on an oscilloscope display or processthe data for permanent record. The A-scan presents echo amplitude and distance,and it is used principally in echoencephalography for the detection of midlineshifts traceable to tumors or concussions. It has also been applied in obstetrics,gynecology, and ophthalmology in conjunction with B-scanning techniques. InB-scanning, the radar/sonar techniques of data processing are applied to synthesizethe reflected signals into a pattern on the oscilloscope display that corresponds toa cross section of the region scanned lying in a plane parallel to the direction ofbeam propagation.The position of the probe is synchronized with the sweep of one of the axes of theoscilloscope, and the echo amplitude appears as a spot with a specific intensity at aposition on the screen corresponding to the position of the plane causing the echo.The TM-mode (or M-mode) is a diagnostic ultrasound representation of temporalchanges in echoes in which the depth of echo-producing interfaces is displayedalong one axis, and time (T) is displayed along the second axis, thus recordingmotion of the interfaces toward and away from the transducer.In order to resolve small structural details, the transmitted pulse should be asshort as possible, which means that the transducer must be highly damped. In orderto promote good coupling between the transducer and the body, a film of oil orgrease is applied at the selected spot, and care must be taken that the contactingfilm is free of air bubbles. In immersion procedures (such as that for kidney stonepulverization), coupling is effected through a bath of liquid, usually water. Theimpedance match between water and soft tissue is good, and little energy is lost inirradiating soft tissue. But the match between water and bone is poor and also theattenuation in bone is high. The result in echoencephalography is that considerableenergy is lost, even where almost immediate contact is made with the skull bone.Ultrasound densitometry is used to measure bone density in the heel, shinbone,or kneecap. It is used as a screening tool and while currently not as precise asabsorptiometry techniques, it is still effective, inexpensive, portable and uses no