Comparative study of temperature measurements in ex vivo swine muscle and a tissue-mimicking material during high intensity focused ultrasound exposures.

Tissue-mimicking materials (TMMs) can provide a convenient, stable, and reproducible means for testing high intensity focused ultrasound (HIFU) devices. When TMMs containing thermal sensors are used to measure ultrasound-induced temperature rise, it is important that measurement results reasonably represent those that occur in biological tissue. Therefore the aim of this paper is to compare the thermal behavior of the TMM under HIFU exposure to that of ex vivo tissue. This was accomplished using both a previously developed TMM and fresh ex vivo swine muscle that were instrumented with bare 50 µm thin wire thermocouples. HIFU at 825 kHz was focused at the thermocouple junction. 30 s exposures of increasing peak negative pressure (1 to 5 MPa) were applied and the temperature profile during and after sonication was recorded. B-mode imaging was used to monitor bubble activity during sonication. If bubble formation was noted during the sonication, the sonication was repeated at the same pressure levels two more times at 20 min intervals. Temperature traces obtained at various pressure levels demonstrated similar types of heating profiles in both the tissue and TMM, the exact nature of which depended on whether bubbles formed during the HIFU exposure. The onset of bubble activity occurred at lower ultrasonic pressures in the TMM, but the basic temperature rise features due to HIFU exposure were essentially the same for both materials.

A methodology for probabilistic predictions of regional climate change from perturbed physics ensembles.

A methodology is described for probabilistic predictions of future climate. This is based on a set of ensemble simulations of equilibrium and time-dependent changes, carried out by perturbing poorly constrained parameters controlling key physical and biogeochemical processes in the HadCM3 coupled ocean-atmosphere global climate model. These (ongoing) experiments allow quantification of the effects of earth system modelling uncertainties and internal climate variability on feedbacks likely to exert a significant influence on twenty-first century climate at large regional scales. A further ensemble of regional climate simulations at 25km resolution is being produced for Europe, allowing the specification of probabilistic predictions at spatial scales required for studies of climate impacts. The ensemble simulations are processed using a set of statistical procedures, the centrepiece of which is a Bayesian statistical framework designed for use with complex but imperfect models. This supports the generation of probabilities constrained by a wide range of observational metrics, and also by expert-specified prior distributions defining the model parameter space. The Bayesian framework also accounts for additional uncertainty introduced by structural modelling errors, which are estimated using our ensembles to predict the results of alternative climate models containing different structural assumptions. This facilitates the generation of probabilistic predictions combining information from perturbed physics and multi-model ensemble simulations. The methodology makes extensive use of emulation and scaling techniques trained on climate model results. These are used to sample the equilibrium response to doubled carbon dioxide at any required point in the parameter space of surface and atmospheric processes, to sample time-dependent changes by combining this information with ensembles sampling uncertainties in the transient response of a wider set of earth system processes, and to sample changes at local scales. The methodology is necessarily dependent on a number of expert choices, which are highlighted throughout the paper.

The impact of piezoelectric PVDF on medical ultrasound exposure measurements, standards, and regulations.

This paper describes the development of PVDF hydrophones for characterizing medical ultrasound fields. The polymer hydrophone approaches that have resulted from this work are discussed, with emphasis given to the spot-poled membrane design that has become the de facto reference device for these measurements. The various national and international standards and regulations that have followed from the successful use of PVDF hydrophones also are summarized. The works discussed encompass polymer-based hydrophones designed primarily for diagnostic and lithotripsy exposure measurements, both in water and in vivo. The advent of these hydrophones has made possible accurate and reliable measurements of exposure levels encountered in medical ultrasound and, thus, has allowed questions of ultrasound bioeffects and device safety to be addressed in a consistent and scientifically sound manner.

Time delay spectrometry for hydrophone calibrations below 1 MHz.

Knowing the response of miniature ultrasonic hydrophones at frequencies below 1 MHz is important for assessing the accuracy of acoustic pressure pulse measurements in medical ultrasound applications. Therefore, a time delay spectrometry (TDS) system was developed as an efficient means to measure hydrophone sensitivity in this frequency range. In TDS a swept-frequency signal is transmitted. A tracking receiver distinguishes arrivals with different propagation delays by their frequency offset relative to the signal being transmitted, thus eliminating spurious signals such as those reflected from the water surface or tank walls. Two piezoelectric ceramic source transducers were used: a standard planar disk and a disk with varying thickness to broaden the thickness-resonance. This latter design was preferred for its more uniform response without significant sensitivity loss. TDS is not an absolute method, but it was demonstrated to provide efficient, accurate calibrations via comparison with a reference hydrophone using a substitution technique.

Desmoplastic (sclerotic) nevus: an underrecognized entity that resembles dermatofibroma and desmoplastic melanoma.

Desmoplastic (sclerotic) nevus, a benign melanocytic neoplasm characterized by predominantly spindle-shaped nevus cells within a fibrotic stroma, can be confused with fibrous lesions and other melanocytic proliferations, including desmoplastic melanoma. We compared the histologic and immunohistochemical features of 16 desmoplastic nevi, nine desmoplastic melanomas, four hypopigmented blue nevi, and six dermatofibromas. The similarities between desmoplastic nevi and dermatofibromas included epidermal hyperplasia (12 of 16), presence of keloidal collagen (15 of 16), hypercellularity (16 of 16), and increased numbers of factor XIIIa-positive dendritic cells (12 of 12). The absence of adnexal induction (0 of 16), the rarity of lesions with multinucleated cells (3 of 16) or epidermal hyperpigmentation (2 of 16), and the presence of S-100 immunoreactivity (16 of 16) and melanocytic proliferation (9 of 16) helped differentiate desmoplastic nevi from dermatofibromas. The similarities between desmoplastic nevi and desmoplastic melanomas included the presence of atypical cells (16 of 16) and HMB-45 expression in the superficial portion of the lesions (11 of 16). The infrequent location on the head or neck (1 of 16), the absence of mitotic figures (0 of 16), a significantly lower number of Ki-67-reactive cells, and a decrease in HMB-45 expression in the deep area of the lesions (8 of 11) helped distinguish desmoplastic nevi from desmoplastic melanoma. Desmoplastic nevi had overlapping features with hypopigmented blue nevi, but features tending to favor the latter included a predominance of ovoid nuclei, higher numbers of atypical cells, and homogeneous staining with HMB-45. We conclude that a combination of histologic and immunohistochemical criteria facilitates the reliable diagnosis of desmoplastic nevus from its simulators.

Theoretical study of steady-state temperature rise within the eye due to ultrasound insonation.

The soft tissue thermal index (TIS), as defined in the AIUM/NEMA Output Display Standard, may not be relevant with respect to eye exposure, primarily because of differences in actual vs. assumed acoustic and thermal properties. Therefore, a theoretical study of temperature rise within the eye due to ultrasound insonation was undertaken to compare the TIS with more exact calculations. At each plane in the direction of propagation, the focused ultrasound beam was modeled as a disc of uniform intensity. Each disc becomes a heat source, and integration over all discs provides the total temperature rise at any axial position. Calculations were done assuming the ultrasound beam intersects the lens of the eye as well as for the case in which the beam does not intersect the lens. Results were found for frequencies of 7.0 MHZ to 40 MHZ, transducer diameters of 0.2 cm to 1.0 cm, and focal lengths ranging from 0.2 cm to 3.0 cm. Perfusion was assumed negligible and thermal and acoustic parameters were taken from reported studies. For every case, the ratio of maximum temperature rise to the TIS (assuming constant output power) was calculated. For the lens case, the ratio varied from 7.35 to 0.8. For the no-lens case, the ratio varied from 4.1 to 0.4. These results indicate that the TIS is not adequate to represent the temperature rise occurring within the eye upon insonation.

Are current hydrophone low frequency response standards acceptable for measuring mechanical/cavitation indices?

The purpose of this paper is to determine the error introduced by ultrasonic hydrophones used to measure current or proposed Mechanical (MI) and Cavitation (CI) Indices, assuming that the hydrophones meet bandwidth specifications contained in US and IEC measurement standards. These indices are based on the peak rarefactional pressure, pr. Since the portion of the pressure waveform where pr occurs is dominated by low frequency components, attention was placed on the low frequency hydrophone response specifications. Both simulated and actual diagnostic pressure pulses (with center frequency fc) were subjected to single-pole high-pass filtering for a range of -3 dB cut-off frequencies (fa). The error in the indices introduced by this filtration was evaluated. At both fa = 0.5fc (the US requirement) and fa = 0.86fc (calculated from the IEC -6dB bandwidth specification at 0.5fc), results showed that errors exceeding -30% could be expected. Furthermore, to reduce errors to less than 5%, the low frequency hydrophone response should extend at least an order of magnitude below the center frequency of the pressure wave. For example, for a 3.5 MHz transducer, the hydrophone should have a lower cut-off frequency of less than 350 kHz, which at present constitutes a challenge because of the lack of commercial hydrophones calibrated below 1 MHz.

Lithotripsy pulse measurement errors due to nonideal hydrophone and amplifier frequency responses.

When using miniature ultrasonic hydrophones to probe the focal region of extracorporeal shock wave lithotripsy devices, the frequency response of the measurement hydrophone and any associated amplifier must be broad enough to minimize pulse distortion. To study the potential effects of the bandwidth-limited behavior, a mathematical model was used. Several parameters of a simulated lithotripsy pulse were compared before and after being filtered by hydrophone and amplifier response functions. Errors were computed for the peak positive and negative pressures, risetime, pulse duration, and pulse intensity integral as functions of hydrophone and amplifier bandwidths. Although most of the energy in a shock wave pulse lies at frequencies below a few megahertz, it is found that significant errors can occur unless measurement bandwidths are much wider.

A model of the effects of hydrophone and amplifier frequency response on ultrasound exposure measurements.

To examine how less-than-ideal hydrophones and amplifiers could affect the measurement of pulse parameters, a simulated distorted pressure pulse is analyzed before and after being filtered by hydrophone and amplifier response models. The hydrophone model used is similar to the response of piezopolymer membrane hydrophones. The amplifier model is taken from typical responses of integrated circuit operational amplifier-based designs. The pressure pulse, hydrophone, and amplifier are characterized respectively by the pulse center frequency, f (c), the hydrophone resonance frequency, f(h), and the amplifier low-pass corner frequency, f(a).

Relationship between image quality and ultrasound exposure level in diagnostic US devices.

Ultrasonographic B-mode images were obtained at various exposure levels with three real-time diagnostic scanners. Adult human and tissue-equivalent phantom images were compared in terms of diagnostic content and depth of penetration. For the exposure level settings used, spatial-peak pulse-average intensities ranged from approximately 10 to 500 W/cm2. At the 3.50-3.75-MHz nominal frequencies used in the study, images of the human abdomen showed little discernible change in quality with varying exposure levels. However, phantom tests confirmed that depth of penetration is a function of exposure level. The results suggest that a judicious use of exposure level and receiver gain controls can be a practical means for minimizing patient exposure to ultrasound without sacrifice of diagnostic effectiveness.

Exposure to and precautions for blood and body fluids among workers in the funeral home franchises of Fort Worth, Texas.

In 1982 the Centers for Disease Control published a set of recommendations and measures to protect persons working in health care settings or performing mortician services from possible exposure to the human immunodeficiency virus. This study of a number of funeral homes in the Fort Worth area was designed to determine the level of exposure of funeral home workers to blood and other body fluids and also to assess existing protective measures and practices in the industry. Workers in 22 funeral home franchises were surveyed with a predesigned questionnaire. Eighty-five responses from 20 of the 22 establishments were received. All 85 respondents admitted exposure of varying degrees to blood and body fluids. Sixty persons (70%) admitted heavy exposure, that is, frequent splashes. Analysis of the responses showed that 81 of 85 (95.3%) persons consistently wore gloves while performing tasks that might expose them to blood or other body fluids. Of the 60 persons who were heavily exposed, 43 wore long-sleeved gowns, 27 wore waterproof aprons, 17 surgical masks, and 15 goggles. The study further revealed that 52.9% (45/85) of the respondents had sustained accidental cuts or puncture wounds on the job. In light of these findings it is important to target educational efforts to persons in this industry to help them minimize their risks of infection with blood and body fluid borne infections.

Hydrophone measurements in diagnostic ultrasound fields.

Some of the methods, calculations, and problems associated with making hydrophone measurements in diagnostic ultrasound fields are considered. Several organizations have proposed definitions of various peak and average intensities that need to be specified when characterizing medical ultrasound fields. All of these can be determined from hydrophone measurements, but the bandwidths encountered (>50 MHz), along with the small focal diameters achievable ( approximately 1 mm), can place great demands on hydrophone performance. Two general types of hydrophones are available-the spot-poled membrane and needle types. Both employ the piezopolymer polyvinylidine fluoride (PVDF), and for the most part they have effective dimensions in the 0.5-1.0-mm range. Hydrophones can be made with useful bandwidths extending beyond 50 MHz. However, above approximately 15 MHz the nature of the response becomes highly dependent on the method of construction and PVDF film thickness used, as well as the characteristics of any associated preamplifier circuitry. Other factors that can affect measurement accuracy are discussed.

Interlaboratory comparison of hydrophone calibrations.

The results of an interlaboratory comparison of hydrophone calibration techniques in the frequency range 1-10 MHz are reported. Two membrane hydrophones were calculated to six laboratories, and each laboratory determined the end-of-cable loaded sensitivities using their normal calibration methods; these included optical interferometry, planar scanning, reciprocity combined with time-delay spectrometry, and suspended-sphere radiometry. After converting the results to end-of-cable open-circuit sensitivities, in most cases agreement between the various values was within +/-10% at all frequencies.

Assessing cancer incidence in a small community.

Many small communities are concerned about health effects from environmental pollutants. One such community, Orwell, in Oswego County, New York, was investigated to determine if the level of cancer was significantly high in comparison to the county and the state. A pilot health study was administered among residents living in a defined area. Results of the survey demonstrated that cancer incidence was higher than both Oswego County and New York State, while cancer mortality was higher for men and lower for woman. A closer examination of the data showed that the way in which health surveys are usually analyzed may not be appropriate for small communities. In particular, some parametric statistics, such as chi-square analysis, might show significant differences between populations when standardized incidence and mortality rates are derived from small samples, yet standard deviations calculated from these data are so large as to cast doubt on the analysis. Researchers have discussed this problem theoretically, but empirical studies illustrating the difficulties have not heretofore appeared in the published literature. For health studies in small communities, greater consideration should be given to risk ratios and the implications of standard deviations.

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.

Instrument for measuring the temporal characteristics of therapeutic ultrasound fields.

An instrument is described that measures the ultrasonic frequency; pulse repetition rate; pulse duration; timer accuracy; peak-to-rms pressure amplitude ratio; and, peak-to-average intensity ratio for ultrasonic therapy devices designed for use in physical medicine. These temporal beam characteristics are included in a Food and Drug Administration performance standard for ultrasonic therapy products. They can be measured conveniently and efficiently when the instrument is used with an ultrasound transducer (hydrophone) to detect the radiated ultrasound beam. Instrument design is straightforward, employing standard analog and digital circuit components and modules. Frequency and time measurement errors are typically 1% or better. Pressure and intensity ratio errors are less than 2% and 4%, respectively.

Angular response of miniature ultrasonic hydrophones.

The voltage response of ceramic and polyvinylidene fluoride (PVDF) hydrophones was measured in the receive mode for angles of incidence ranging from 0 degrees to 90 degrees. The measurements were performed at 2.5, 3.5, 5.0, and 8.0 MHz; these frequencies are typical of those used in medical diagnosis. The results are compared to three theoretical models based on diffraction theory; correlation between the measured response and theoretical models is evident for some PVDF hydrophones but not for others, and not for any ceramic hydrophone. The effective radius, as defined in the AIUM-NEMA standard for diagnosis ultrasound, is calculated and compared to the test criteria established in that standard. All of the ceramic hydrophones and two of the five PVDF hydrophones failed to meet the criteria.

A comparison of two calibration methods for ultrasonic hydrophones.

Although miniature ultrasonic hydrophones are frequently used to measure the acoustic pressure distributions from diagnostic ultrasound sources, relatively little attention has been devoted to the methods for absolute calibration of these hydrophones. In this study a polyvinylidene (PVDF) hydrophone was used to compare two calibration methods currently in use. One is based on a reciprocity technique and the second involves the planar scanning of a source transducer having a known radiated ultrasonic power. The reciprocity method revealed that the hydrophone response did not vary by more than +/- 1.6 dB from -262.8 dB re IV/microPa over the frequency range of 1-10 MHz. For the planar scanning technique seven ultrasound sources between 1-10 MHz were used, and all calibration points were within +/- 0.5 dB of the corresponding points found by the method of reciprocity.