Ultrasonics in medical diagnosis

Abstract

Beginning with the wave equation, the characteristics of ultrasound are described in terms of propagation, reflexion, beam formation, scattering and attenuation. In medical diagnosis, ultrasound in the low megahertz frequency range is both generated and detected by piezoelectric transducers. At these frequencies the wavelength is in the order of 1 m m ; this is one of the fundamental limitations of attainable resolution. Because attenuation increases with frequency, it is necessary to compromise between resolution and required penetration. The time delays between pulse transmission and echo reception are proportional to the distances between the transducer and the reflectors along the ultrasonic beam. The echo amplitudes are controlled both by the characteristics of the reflectors, and by attenuation in the intervening media. The pulse-echo method is used to produce A-scan, B-scan and C-scan displays. At present the B-scan is the most important. It is used for the study of structure motion by time-position recording, and for tomography by two-dimensional scanning. Grey-scale display gives clues about tissue characteristics. Manually operated, automatic, and very rapid (real-time) two-dimensional scanners are in clinical use, especially in obstetrics and gynaecology, internal medicine and cardiology. The Doppler frequency-shift method is used to detect the movements of structures (especially the foetal heart) and to measure blood flow velocities. Analysis of Doppler blood-flow signals gives data on vessel characteristics. Pulsed Doppler systems can be used to estimate both velocity and position. Newer methods include phase compensation for the distortion otherwise introduced by the skull in brain scanning, tissue characterization, computerized tomography, Doppler blood flow transfer function analysis and tumour detection.