Spatial Average Intensity Research Articles

Measurement of diagnostic electronic linear arrays by miniature hydrophone scanning

Ultrasonic pressures, intensities and power were computed using a calibrated miniature hydrophone scanning system. To accomplish these measurements, the electronically scanned beam was stopped, and raster scans were made with the hydrophone over a number of planes perpendicular to the beam axis. Ultrasonic power also was measured using a radiation force balance, and the results compared well with those obtained using the hydrophone. Spatial average-temporal average intensities for the stopped beam ranged from 0.13 to 3.71 mW/cm(2 ) (for a -40-dB beam area), and spatial peak-temporal peak intensities varied from 8.31 to 45.2 W/cm(2). Such measurements are important in setting equipment safety standards, in encouraging minimum use of ultrasonic exposures, and in quantifying exposure levels that may result in harmful biological effects.

Ultrasound and spermatogenesis in the rat.

Using exposure conditions comparable to those which have been associated by others with positive results, we tested for effects of ultrasound on sperm production over a period of 12 weeks following treatment. Continuous wave exposure with spatial average intensities of 1, 2 and 4 W/cm2 and exposure times up to 10 minutes were used. In some experiments, the exposures were repeated after an elapsed time of 48 h. No significant changes in spermatogenesis were related to any of the exposure conditions in spite of the fact that some of the treatments caused thermal tissue damage near bone. No effects of exposure were found in weights of the testis, prostate, seminal vesicle, or whole body.

The effect of pulsed ultrasound on the survival of drosophila

There are reports in the literature that the temporal peak intensity of pulsed ultrasound may be a better predictor of biological damage than is average intensity. The effects of temporal peak intensity on the survival of Drosophila melanogaster pupae exposed to pulsed ultrasound were investigated. Pulse average intensity and pulse repetition frequency were varied, while temporal average intensity and exposure duration were held constant. Thus, the total energy delivered was held constant. Survival was found to be directly dependent on pulse average intensity. We observed decreased survival at spatial average pulse average intensities as low as 0.5 W/cm 2.

Thermal and heart rate response to ultrasonic exposure in the second and third trimester dog fetus.

A total of 39 fetuses from five pregnant dogs were used in this study. Following pentobarbital anesthesia of the dams the gravid uterus was exposed and partially exteriorized. Fetal and maternal electrocardiograms were recorded, along with fetal abdominal temperatures, during ultrasound exposures of 3.4 MHz and 2, 4, 6, 8 or 12 W/cm2 (continuous wave), spatial average intensities, for 6 minutes. Baseline fetal abdominal temperatures were 35.2 +/- 1.5 (SD) degrees C. Baseline fetal heart rates were 176.8 +/- 4.3 (SD) beats per minute. These studies indicate that the product of intensity and time of exposure, under the conditions of these experiments, is directly related to increases in fetal abdominal temperature and fetal heart rate.

A discussion of procedures for ultrasonic intensity and power calculations from miniature hydrophone measurements.

In this paper, some of the fundamental procedures for evaluating medical ultrasound fields using miniature hydrophones are discussed. Examples are considered for both static (fixed beam) and real-time (moving beam) ultrasound systems. Intensity and power quantities are calculated from hydrophone measurements using definitions contained in current standards, and where reasonable some simplified computational procedures are introduced. Assumptions implicit in common expressions for spatial average-temporal average intensity and ultrasonic power are identified, and in some cases illustrations of calculations using idealized pressure waveforms and beam shapes are given and compared with more exact results. Also, the adequacy of present guidelines for choosing the hydrophone size is evaluated using a simple time domain approach. This material is presented to augment and help clarify descriptions of procedures contained in existing standards, and to provide a basis for pursuing some of the more difficult problems associated with hydrophone measurements.

Stable cavitation at low ultrasonic intensities induces cell death and inhibits 3H-TdR incorporation by Con-A-stimulated murine lymphocytes in vitro.

Murine spleen cell suspensions stimulated by Concanavalin A (Con-A) were exposed to 1.6-MHz continuous-wave ultrasound at low intensities (spatial-peak values ranging from 16 to 300 mW/cm2) in the presence of a Nuclepore membrane that contained stabilized gas bodies. The ultrasonically activated gas bodies induced cell lysis and reduced the fraction of intact cells that excluded trypan blue. At a spatial-peak intensity of 75 mW/cm2 (spatial-average intensity 15 mW/cm2), Con-A-induced Methyl[3H]thymidine (3H-TdR) incorporation was reduced in cultures exposed at 24 or 48 hr after addition of Con-A but not at 7 or 13 hr. There was no observable effect on cell survival or 3H-TdR incorporation at spatial-peak intensities below 75 mW/cm2.

A comparison of two methods for determining ultrasonic intensity for medical transducers

The spatial and temporal average intensity (/ SATA) is determined for four diagnostic ultrasound transducers by two methods: (a) direct measurements (beam profiling) with a miniature hydrophone, as described in the AIUM-NEMA standard, and (b) estimating from measurements of total power and assumptions about beam size. The latter method is frequently used by diagnostic ultrasound manufacturers when reporting output levels to users and regulatory agencies. However, due to the conventions for defining beam area, this method actually overestimates the spatial average intensity. For the four transducers tested, the estimated / SATA exceeds the measured / SATA by several hundred percent. The spatial peak, temporal average intensity (/ SPTA) was also measured for the four transducers and is less than the estimated / SATA in every case.

The induction of hyperthermia by ultrasound: its value and associated problems I. Single, static, plane transducer

The temperature distributions that may be achieved in mammalian tissues by ultrasonic heating have been studied. Thigh muscle of Large White pigs has been irradiated with 0.75 and 3.0 MHz ultrasound at spatial average intensities in the range 1.5-3.0 W cm-2 in both pulsed and continuous modes of delivery. Resultant temperature profiles are described and some of the problems arising are discussed.

Observations of ultrasound-induced effects in the fish xiphophorous maculatus

Tails of the fish Xiphophorous maculatus have been studied by transmission light microscopy during irradiation with continuous wave ultrasound (frequencies 0.78–3 MHz, spatial average intensities 0.01–3 Wcm −2). Blood flow started to increase a few minutes after the start of an irradiation, reaching a maximum after 5–10 min. Periodic variations in blood flow rate were often seen, and the response varied considerably among individual specimens. Acoustic microstreaming, which resulted in rapid rotation of clusters of cells, was observed in blood vessels adjacent to cartilaginous rods in the tail. The threshold average spatial intensities to initiate this were a few hundred mW cm −2 at 0.78, 1.5 and 3 MHz. The microstreaming resembled that occurring around ultrasonically stimulated microbubbles, but no evidence of any association was found. Microbubbles, possibly originating in water, were sometimes seen in the tissue of the fish following treatment with ultrasound.

The action of low-intensity pulsed ultrasound on amphibian embryonic tissues.

Amphibian embryos at different stages of development, and ectomesodermal explants (small pieces of embryonic tissue extirpated from lateral parts of the embryos just after neurulation), have been irradiated with continuous and pulsed ultrasound at a frequency of 0.88 MHz, spatial and temporal average intensities being within the range 0.025-0.1 W cm2. Ultrasound pulse repetition frequencies were varied within 10-1000 Hz, duty factor being constant and equal to 0.5. Destruction of embryonic tissue as an immediate effect, and interruption or distortion of development as delayed (24 hr) effects, were registered. Significant damaging effects of non-cavitational and non-thermal origin and dependent on pulse repetition frequencies, were observed. Highly frequency-dependent resonance-type effects were obtained for irradiated explants. The maximum effect on Rana temporaria explants was obtained at pulse repetition frequencies of 10-20 Hz and on Xenopus laevis explants at 110-130 Hz.

Ultrasonic irradiation of mouse tissues in vivo and human amniotic cells in vitro

Recently published results have been substantiated by enlarging the group sizes of mice subjected to whole-body exposure for 200s and of human amniotic cells in vitro for 2000s at 34°C. Spatial average intensity was 1 W/cm 2. Cyclic AMP, cyclic GMP and histamine in mouse skin, lung and peritoneal cells, and cAMP and cGMP in human amniotic cells were assayed as sensitive indicators of membrane damage. No significant effect of ultrasound was observed. Statistical analysis indicates an 80% probability that we would have detected a difference in the means for exposed and sham exposed groups if it had been greater than one standard deviation in 9 out of 18 comparisons for mouse tissues and 7 out of 8 comparisons for human amniotic cells.

Ultrasonic effects on alveolar macrophages in suspension.

Rabbit alveolar macrophages in suspension were exposed to 5 or 10 min of continuous 2-MHz ultrasound with 5, 10, and 15 W/cm2 spatial average intensities. Viability as determined by dye exclusion decreased with increasing intensity. Pressure experiments indicated that this was a result of acoustic cavitation. Ultrasound induced clumping of cells and often reduced membrane ruffling. Some cells were disintegrated. Cells that appeared to be otherwise intact had swollen mitochondria with ruptured cristae.

Ultrasonic irradiation of mammalian cells in vitro at hyperthermic temperatures.

Suspensions of V79 cells and HeLa cells have been irradiated with continuous 3 MHz ultrasound at a spatial average intensity of 3W cm-2. This irradiation condition did not give rise to cell lysis. When the cells were irradiated with ultrasound for up to six hours at 37 degrees C no cell killing was observed. However, at temperatures in the hyperthermia range 42--45 degrees C the increase in cell killing that resulted from the irradiation was greater than that which could be attributed to its heating effect alone.

Toxicity of ultrasound in mice: neonatal studies.

Pregnant mice were exposed to ultrasound (continuous wave, 2 MHz) on Day 8 of gestation to determine effects on the progeny. The most significant finding was a decrease in mean uterine weight of the female progeny. The thresholds for this effect were 140 s at 0.5 W/cm2 and 60 s at 1 W/cm2, which were below the thresholds previously reported for other effects in mice. We suggest that this indicates a delay or impairment of maturation of the mice exposed in utero. Exposure of the dams to spatial average intensity of 1 W/cm2 for 40 and 60 s had no effect on body weight of the progeny, compared with sham-treated controls. In this experiment the body weights of progeny from sham-treated controls were significantly lower than those from untreated controls on Days 10, 17, and 25 of age. After exposure in utero to 0.5 W/cm2 for 180 s, statistically significant decreases in mean body weights of the neonates were observed, but only on Day 25 of age, in both sexes compared with sham-treated controls. At necropsy at Day 25 of age, neonatal organ weights relative to body weights were not significantly affected for the thymus in either sex or for the seminal vesicles and tests ion comparison with sham-treated controls.

Post-partum survival of mice exposed in utero to ultrasound

On day 8 of pregnancy we exposed pregnant Swiss Webster CFW mice to continuous ultrasound at 2 MHz, 0.5(± 10%) W/cm2 spatial average intensity [in the vacant water bath (37°C)] for 60, 100, 140, and 180 s. Estimated intensity at the uterus was 0.45(±20%) W/ cm2. By comparison with sham treated controls we found the ultrasound had no effect on the survival of neonatal mice through day 25 of age (84 to 108 neonates per group). This result may be compared with Curto's (1976) report [K. A. Curto, Ultrasound in Medicine 2, 535-6 (1976)] of 13% to 26.7% reduction in neonatal survival after exposure of CFI mice on day 13 of pregnancy to 1 MHz, 0.125 to 0.5(±20%) W/cm2 spatial average intensity [in the vacant saline bath (30°C)] for 180 s. Curto sought enhancement of exposure and her conditions could have resulted in intensities at the uterus as great as 1.0(±20%) W/cm2. We conclude that the expression of this end-point at (0.5 W/cm2, 180 s) as cited in summaries is at least questionable and is possibly dependent on other conditions of exposure. [Research supported by NIH grant ♯GM 22684.]

Postpartum survival of mice exposed in utero to ultrasound.

We found no credible enhancement of neonatal mortality in mice exposed in utero on Day 8 of gestation to ultrasound at 0.44 W/cm2 (+/- 20%) spatial average intensity, 2 MHz, continuous wave, for 60--180 s in a 37 degrees C water bath. This result is compared with the adverse effect on neonatal survival reported by Curto [in Ultrasound in Medicine (Plenum, New York, 1976), Vol. 2, pp. 535--536] for exposure on Day 14 of gestation at about 0.125 to 0.5 W/cm2, 1MHz, continuous wave, for 180 s in a 30 degrees C bath. It is concluded that a temporary reduction in fetal viability, lasting from 4 days to less than 10 days after exposure, could account for both sets of data. It is also evident from these data that improved methods of determining intensities within mammalian tissues are highly desirable.

Ultrasonic power and intensities produced by diagnostic ultrasound equipment.

Abstract The ultrasonic radiation force on an absorber was measured with a feedback microbalance to determine the ultrasonic power emitted into water by 26 diagnostic ultrasound instruments. Included in the measurements were pulse echo scanners, echocardiographic units, ophthalmic units, Doppler fetal heart detectors, fetal monitors, and continuous and pulsed Doppler units for peripheral vascular studies. The temporal and spatial average intensity at the transducer was calculated for those instruments by dividing the total power by the nominal area of the radiating surface. Estimates of the spatial maximum intensity in water, both temporal average and temporal peak, were made by measurements with miniature hydrophones normalized to the power measurements. Hydrophone measurements only were performed on linearly-scanned real time arrays. For transducers with single element transmitters, the maximum obtainable power ranged from 60μW to 37 mW, and maximum obtainable intensity at the transducer face ranged from 1 to 1700 W/m2. A much smaller range of power and intensity was encountered among systems designed for a given set of applications and providing similar performance in those applications. The temporal average and temporal peak intensities at the focal point of focused transducers employed frequently in the last few years for general purpose scanning were somewhat higher than previously quoted values because of the increased focusing.