Ultrasound imaging artifacts of acoustic origin relating to resolution, propagation path, and attenuation are reviewed. Lateral and axial resolution limitations are artifactual in nature since a failure to resolve means a loss of detail and two adjacent structures may be visualized as one. Apparent resolution close to the transducer (speckle) is not directly related to tissue texture but is a result of interference effects from the distribution of scatterers in the tissue. Reverberation produces a set of equally spaced artifactual echoes distal to the real reflectors. The mirror image artifact is the presentation of objects that are present on one side of a strong reflector, appearing on the other side as well. Shadowing and enhancement are useful artifacts for determining the nature of masses. Enhancement results from low attenuation objects in the sound path while shadowing results from strongly reflecting or strongly attenuating objects. Additional artifacts include section thickness, refraction, multipath, side lobe, grating lobe, focal enhancement, comet tail, ring down, speed error, and range ambiguity.

Attenuation and propagation speed in the 1- to 5-MHz frequency range were measured in vitro in 22 tissue samples from six anatomic sites in five normal human brains obtained unfixed at autopsy. Two methods were used to measure attenuation: (1) received amplitude reduction resulting from insertion of tissue sample between a pair of transducers, and (2) radiation force reduction resulting from insertion of tissue sample between a transducer and a reflector. The piezoelectric transducer method gave attenuation values −10% to +90% higher than the radiation force method. Attenuation (by transducer method) was a function of frequency to the 1.08 power with a value of 0.87 dB ⋅ cm−1 at 1 MHz. Propagation speed was measured by observing signal arrival time change resulting from insertion of the tissue sample between the transducer pair. Propagation speed was 1562 m ⋅ s−1 at 1 MHz with a dispersion of 1.2 m ⋅ s−1  ⋅  MHz−1 over the 1- to 5-MHz frequency range. Ages at death were 3 days, 20, 21, 42, and 72 years. Results were similar in the 20- to 72-year range but attenuation for the 3-day case was about one-third that for adult. Formalin fixing decreased propagation speed by 10 m ⋅ s−1. One-day aging of unfixed tissue resulted in up to a 21% decrease in attenuation and up to a 10 m ⋅ s−1 decrease in propagation speed. White matter (corna radiata, medulla, pons) had attenuation 1.5 times that for gray (Island of Riel, cerebellum) with mixed (basal ganglia) having an intermediate value. Attenuation decreased with increasing temperature. Propagation speed as a function of temperature exhibited a minimum at about 15 °C.

The 45-year history of research in cancer therapy involving ultrasound may be divided into the four periods of initiation, enthusiasm, pessimism and revival. There have been three approaches to the subject: studies have sought to measure the effects on tumors of a) ultrasound alone; b) ultrasound in combination with radiotherapy, and c) ultrasound in combination with chemotherapy. With the first approach the results have varied. In some cases, decreased growth rates or regressions of tumors have been reported; in other cases, either no effect has been observed or growth has been increased. With the second approach, for some tumors, combined treatment has produced greater effects on tumors than has x-ray alone, whereas in other tumors the addition of ultrasound has produced no change. With the third approach, enhancement of the effects of drugs has been observed in melanoma and mouse tumor cells treated with ultrasound and several anticancer drugs. The mechanism of action in most (but not all) cases has appeared to be absorption heating. The potential of ultrasound to provide local tumor control and to enhance other therapy modes has motivated the current efforts by several groups to further study and understand it actions on malignancies.