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S B Barnett

Publications and source records attributed to S B Barnett.

At least 19 recordsLinked to original sources
PubMed2001

Intracranial temperature elevation from diagnostic ultrasound.

Tissues of the central nervous system are sensitive to damage by physical agents, such as heat and ultrasound. Exposure to pulsed spectral Doppler ultrasound can significantly heat biologic tissue because of the relatively high intensities used and the need to hold the beam stationary during examinations. This has significant implications for sensitive neural tissue such as that exposed during spectral Doppler flow studies of fetal cerebral vessels. Recent changes in the FDA regulation allow delivery of almost eight times higher intensity into the fetal brain by ultrasound devices that incorporate an approved real-time output display in their design. In this situation, ultrasound users are expected to assess the risk/benefit ratio based on their interpretation of equipment output displays (including the thermal index, TI) and an understanding of the significance of biologic effects. To assist in the assessment of potential thermally mediated bioeffects, a number of conclusions can be drawn from the published scientific literature: the amount of ultrasound-induced intracranial heating increases with gestational age and the development of fetal bone; pulsed spectral Doppler ultrasound can produce biologically significant heating in the fetal brain; the rate of heating near bone is rapid, with approximately 75% of the maximum heating occurring within 30 s; blood flow has minimal cooling effect on ultrasound-induced heating of the brain when insonated with narrow focused clinical beams; the threshold for irreversible damage in the developing embryo and fetal brain is exceeded when a temperature increase of 4 degrees C is maintained for 5 min; an ultrasound exposure that produces a temperature increase of up to 1.5 degrees C in 120 s does not elicit measurable electrophysiologic responses in fetal brain; for some exposure conditions, the thermal index (TI), as used in the FDA-approved output display standard, underestimates the extent of ultrasound-induced intracranial temperature increase.

Animals↗
PubMed2001

Ultrasound and the developing central nervous system.

The potential risk of ultrasonography resulting in adverse biologic effects is particularly important in neurosonographic applications of diagnostic ultrasound in medicine. Key issues relate to the likelihood of producing bioeffects from the level of acoustic output emitted from modern diagnostically powerful ultrasound equipment. Important elements in the assessment of risk include the sensitivity of the tissue structures under examination, the standards of practice in clinical use and the presence of biologic effects identified from laboratory experimentation or from human studies. The World Federation for Ultrasound in Medicine and Biology continues to support activities related to evaluating bioeffects and safety of ultrasound. This paper includes extracts of some of the presentations given during the latest safety meeting, a mini-symposium on "ultrasound and the developing fetal central nervous system" held in conjunction with the WFUMB Congress in May 2000. The speakers covered topics ranging from physics of estimating heating from ultrasound equipment in clinical use to actual measurement of ultrasound-induced intracranial temperature increases in animal fetuses. Finally, some practical scanning strategies were proposed to minimise risk of adverse outcome in various clinical neurosonographic practices.

Animals↗
PubMed2001

Guidelines and recommendations for safe use of Doppler ultrasound in perinatal applications.

Technological development has led to significant improvements in ultrasonographic capabilities in recent years, and this has been accompanied by increases in acoustic output. Meanwhile, there is a developing trend to use ultrasound at early stages of pregnancy when the developing embryo is known to be highly sensitive to damage by physical agents. The advent of pulsed spectral Doppler and color flow imaging has revolutionized perinatal applications. Doppler ultrasound has become widely accepted as a valuable diagnostic tool in obstetric medicine, where it has particular benefits for high-risk pregnancies. The benefits of Doppler screening are less well established. United States Food and Drug Administration (FDA) regulations now provide an option whereby equipment that provides a form of output display can be used to apply substantially higher acoustic output to the embryo or fetus than equipment approved for use under application-specific intensity limits. The Output Display Standard recently adopted by the FDA, in the USA, encourages self-regulation of acoustic exposure by the ultrasound user, on the basis of assumed knowledge of the implications of biophysical interactions. When modern sophisticated equipment is used at maximum operating settings for Doppler examinations, the acoustic outputs are sufficient to produce obvious biological effects, e.g. significant temperature increase in tissue or visible motion of particles due to radiation pressure streaming effects. The risk of inducing thermal effects is greater in the second and third trimesters, when fetal bone is intercepted by the ultrasound beam and significant temperature increase can occur in the fetal brain. Non-thermal bioeffects may be more significant in early gestation, when the relatively loosely tethered embryonic tissues are exposed to an ultrasound beam in a liquid path. The likelihood of producing cavitation-type non-thermal effects is enhanced by the presence in the sound-field of gas-encapsulated echo-contrast media. To ensure the continued safe use of ultrasound in obstetrics, it is important that international ultrasound organizations, such as the International Perinatal Doppler Society, issue advice to members to allow sensible assessment of risk: benefit and the practical implementation of the ALARA (as low as reasonably achievable) principle.

Equipment Safety↗
PubMed2000

International recommendations and guidelines for the safe use of diagnostic ultrasound in medicine.

Modern sophisticated ultrasonographic equipment is capable of delivering substantial levels of acoustic energy into the body when used at maximum outputs. The risk of producing bioeffects has been studied by international expert groups during symposia supported by the World Federation for Ultrasound in Medicine and Biology (WFUMB). These have resulted in the publication of internationally accepted conclusions and recommendations. National ultrasound safety committees have published guidelines as well. These recommendations and safety guidelines offer valuable information to help users apply diagnostic ultrasound in a safe and effective manner. Acoustic output from ultrasound medical devices is directly regulated only in the USA and this is done by the Food and Drug Administration (FDA). However, there is also a modern trend towards self-regulation which has implications for the worldwide use of diagnostic ultrasound. It has resulted in a move away from the relatively simple scheme of FDA-enforced, application-specific limits on acoustic output to a scheme whereby risk of adverse effects of ultrasound exposure is assessed from information provided by the equipment in the form of a real-time display of safety indices. Under this option, the FDA allows a relaxation of some intensity limits, specifically approving the use of medical ultrasound devices that can expose the fetus or embryo to nearly eight times the intensity that was previously allowed. The shift of responsibility for risk assessment from a regulatory authority to the user creates an urgent need for awareness of risk and the development of knowledgeable and responsible attitudes to safety issues. To encourage this approach, it is incumbent on authorities, ultrasound societies and expert groups to provide relevant information on biological effects that might result from ultrasonographic procedures. It is obvious from the continued stream of enquiries received by ultrasound societies that effective dissemination of such knowledge requires sustained strenuous effort on the part of ultrasound safety committees. There is a strong need for continuing education to ensure that appropriate risk/benefit assessments are made by users based on an appropriate knowledge of the probability of biological effects occurring with each type of ultrasound procedure. The primary purpose of this paper is to draw attention to current safety guidelines and show the similarities and areas of general agreement with those issued by the parent ultrasound organisation, the WFUMB. It is equally important to identify gaps in our knowledge, where applicable.

Female↗
PubMed2000

The influence of variations in blood flow on pulsed doppler ultrasonic heating of the cerebral cortex of the neonatal pig.

Pulsed Doppler ultrasound examination of the fetal cerebral circulation may cause potentially harmful temperature elevations in brain tissue immediately beneath the insonated segment of the skull. This study measured the effect of variations in cerebral blood flow on ultrasonic heating of the cerebral cortex of anaesthetised, neonatal pigs. Wide and narrow ultrasound beams were used. Pulsed ultrasound exposures were delivered in 90 s bursts at 5.8 micros pulse length, pulse repetition frequency 8 kHz and centre frequency 3.5 MHz. Studies were performed with the target at the focus of a fixed, stationary beam of 0.3 cm -6 dB beam width (narrow beam) and I(spta) 1.4 W/cm(2) (n = 11), or with the target in the near field of a fixed, stationary beam of 1.6 cm -6 dB beam width (wide beam) and I(spta) 3.6 W/cm(2)(n = 5). The 90 s ultrasound exposures were performed under three different conditions of ambient cerebral blood flow: baseline (during normocarbic, normoxic conditions), increased (during hypercarbic, hypoxic conditions) and absent (postmortem). Cerebral blood flow was measured using the radiolabelled microsphere technique. In the narrow beam studies, cerebral blood flow during baseline was 34 +/- 4 ml/min/100 g, rising to 109 +/- 32 ml/min/100 g during the increased phase (p < 0.001); in the wide beam studies baseline flows were 29 +/- 9 ml/min/100 g, whereas flows in the increased phase were 128 +/- 32 ml/min/100 g (p < 0.001). There was no difference in the heating curves for normal, increased and absent cerebral blood flow for exposure to the narrow beam, when mean temperature increases of 1.5 degrees C at 90 s were recorded in each case (p > 0.21, power > 0.8). However, the heating curves for the wide beam were significantly different for the three rates of blood flow with mean temperature increases of 1.9 degrees C (normal flow), 1.7 degrees C (increased flow) and 2.4 degrees C (no flow) recorded at 90 s (p < 0.05).

Animals↗
PubMed1998

Ultrasound-induced temperature increase in the guinea-pig fetal brain in vitro.

The temperature of the brain of fetal guinea pigs was measured in vitro during exposure to an unscanned beam of pulsed ultrasound at intensity ISPTA 2.8 W/cm2. A mean temperature increase of 5.1 degrees C recorded after 2 min of insonation confirms results of an earlier similar study. The water-bath exposure system provided enhanced cooling of superficial tissue by acoustic streaming. When the scalp was removed, the ultrasound-induced temperature increase was substantially reduced (by 35%) due to cooling through radiation force-induced bulk fluid streaming along the direction of propagation in the water bath. The measured temperature increase in guinea pig fetal brain correlated with a modified cranial thermal index.

Analysis of Variance↗
PubMed1998

Ultrasound-induced temperature increase in guinea-pig fetal brain in utero: third-trimester gestation.

Temperature increase was measured at various depths in the brain of living fetal guinea pigs during in utero exposure to unscanned pulsed ultrasound at ISPTA 2.8 W/cm2. Mean temperature increases of 4.9 degrees C close to parietal bone and 1.2 degrees C in the midbrain were recorded after 2-min exposures. When exposures were repeated on the same sites in each fetus after death, the corresponding mean temperature increases were 4.9 degrees C and 1.3 degrees C, respectively. Cerebral blood perfusion had little cooling effect on ultrasound-induced heating in the guinea pig fetus of 57-61 days gestational age.

Animals↗
PubMed1998

In vivo heating of the guinea-pig fetal brain by pulsed ultrasound and estimates of thermal index.

Temperature was measured in the brain in live near-term fetal guinea pigs (62-66 d gestational age), during in utero exposure to a fixed beam of pulsed ultrasound at intensity ISPTA 2.82 W/cm2. Mean temperature increases of 4.3 degrees C close to parietal bone and 1.1 degrees C in the mid-brain were recorded after 2-min exposures. These values were lower (12%) than those obtained for ultrasound-induced heating near the bone in dead fetuses insonated in utero. A significant cooling effect of vascular perfusion was observed only when guinea pig fetuses reached late gestation, near term, when the cerebral vessels were well developed. The estimated value for the thermal index (TIB), as used in AIUM/NEMA output display standard, underestimated the measured temperature increase at the bone-brain interface. The ratio of measured temperature to the TIB is 1.3. A modification of the cranial thermal index provided a more reasonable, conservative, estimate of the temperature increase at a biologically significant point of interest at the brain-bone interface.

Animals↗
PubMed1998

Effects of pulsed ultrasound on sphenoid bone temperature and the heart rate in guinea-pig foetuses.

Temperature increase induced by exposure to unscanned pulsed ultrasound at an intensity (I(SPTA)) 2.82 W/cm2 was measured in the brain adjacent to the sphenoid bone of foetal guinea-pigs in late gestation under in vitro and in vivo (in utero) conditions. After 120 s exposure a mean temperature increase of 2.6 degrees C was measured in vitro. Removal of the overlying parietal bones increased this value to 5.2 degrees C. Mean temperature increases at the sphenoid bone recorded in utero were 1.5 degrees C live and 2.0 degrees C post mortem. Measurement of foetal ECG showed that ultrasound-induced heating of the hypothalamic region did not significantly alter foetal heart rate.

Animals↗
PubMed1997

The sensitivity of biological tissue to ultrasound.

Mammalian tissues have differing sensitivities to damage by physical agents such as ultrasound. This article evaluates the scientific data in terms of known physical mechanisms of interaction and the impact on pre- and postnatal tissues. Actively dividing cells of the embryonic and fetal central nervous system are most readily disturbed. As a diagnostic ultrasound beam envelopes a small volume of tissue, it is possible that the effects of mild disturbance may not be detected unless major neural pathways are involved. There is evidence that ultrasound can be detected by the central nervous system; however, this does not necessarily imply that the bioeffect is hazardous to the fetus. Biologically significant temperature increases can occur at or near to bone in the fetus from the second trimester, if the beam is held stationary for more than 30 s in some pulsed Doppler applications. In this way, sensory organs that are encased in bone may be susceptible to heating by conduction. Reports in animals and humans of retarded growth and development following frequent exposures to diagnostic ultrasound, in the absence of significant heating, are difficult to explain from the current knowledge of ultrasound mechanisms. There is no evidence of cavitation effects occurring in the soft tissues of the fetus when exposed to diagnostic ultrasound; however, the possibility exists that such effects may be enhanced by the introduction of echo-contrast agents.

Animals↗
PubMed1996

Diagnostic ultrasound in veterinary practice: How safe is it?

This paper provides information on the safety of ultrasonic diagnostic procedures as currently used in veterinary practice. The known mechanisms of action are described and selected literature on biological effects of ultrasound is reviewed. Current international consensus is presented on the safety of medical ultrasound with respect to thermal effects. To date, there is no independently verified clinical evidence that the level of exposure delivered to the tissues during scanned grey-scale ('B-mode') imaging has any adverse effects. Lung haemorrhage has been observed in animal experiments using diagnostic exposures, but the effects have not been reported in the foetus. Equipment that uses pulsed Doppler transmits higher acoustic outputs in a stationary beam, and can produce temperature increases that may have significant biological consequences. When considering sonographic and pulsed Doppler examinations of the prenatal animal, the safety margins are small and the operator should be aware of the acoustic output of the equipment, the exposure time, and the sensitivity of target tissues.

Animals↗
PubMed1995

Ultrasonic heating of the brain of the fetal sheep in utero.

The fetal sheep was used as a model to determine the extent of ultrasound-induced heating of brain tissue in procedures involving pulsed Doppler examination of fetal intracranial arteries. Temperature measurements were recorded in late-gestation fetuses insonated in utero. The centre frequency was 3.5 MHz and a pulse repetition rate of 6 to 10 kHz produced a power output of 0.6 or 2 W. The brain was insonated in the near field of a focussed beam where the -6-dB beam width was 1.7 cm for the 0.6-W transducers and 1.2 cm for the 2-W transducers. Mean (standard error) maximal temperature increases of 3.0 degrees C (0.3) and 12.5 degrees C (1.3), respectively, were recorded in dead fetuses. The mean values obtained in normally perfused living fetuses were lower by 43% and 30%, respectively, showing that vascular perfusion substantially limited ultrasonic heating in sheep fetal brain tissue. There were no changes in blood flow to the heated brain tissue as measured using radiolabelled microspheres.

Animals↗
PubMed1994

Is diagnostic ultrasound safe? Current international consensus on the thermal mechanism.

OBJECTIVE: To describe the potential risk of heating during ultrasound examinations and to report the international consensus on the safety of ultrasound in medicine. DATA SOURCES AND DATA EXTRACTION: Literature on the biological effects of hyperthermia and ultrasound. CONCLUSION: The use of B-mode grey-scale imaging is not contra-indicated on thermal grounds. Some pulsed Doppler equipment has the potential to produce biologically significant temperature increases, specifically at interfaces between bone and soft tissue. Exposures resulting in temperatures less than 38.5 degrees C can be used without reservation.

Female↗
PubMed1994

Current status of research on biophysical effects of ultrasound.

This overview of bioeffects of ultrasound presents some key aspects of selected papers dealing with biophysical end-points. Its purpose is to establish a basis for exposure and dosimetric standards for medical ultrasonic equipment. It is intended to provide essential background resource material for the medical/scientific community, and more specifically for scientific working groups. This document was prepared by members of the Safety Committee of the World Federation for Ultrasound in Medicine and Biology. It was produced as a resource document in response to a request for information by Working Group 12 (Ultrasound exposure parameters) of the International Electrotechnical Commission Technical Committee 87, Ultrasonics. IEC TC 87, WG12 is the working group responsible for generating international standards for the classification of equipment by its acoustic fields based on safety thresholds. Our paper is intended to update and supplement information on the thermal mechanism provided in the publication, "WFUMB Symposium on Safety and Standardisation in Medical Ultrasound: Issues and Recommendations Regarding Thermal Mechanisms for Biological Effects of Ultrasound" (WFUMB 1992). It also provides an overview of trends in research into nonthermal mechanisms as a preliminary to the next WFUMB Symposium on Safety of Medical Ultrasound when this subject will be examined in detail by a select group of international experts. The WFUMB-sponsored workshop will take place in Utsunomiya, Japan during 11-15th July, 1994. The purpose of the meeting is to evaluate the scientific literature and to formulate internationally accepted recommendations on the safe use of diagnostic ultrasound that may be endorsed as official policy of the WFUMB. It should be noted that the current publication is not intended for review or endorsement as an official WFUMB document. It is produced as a scientific paper by individuals who are members of the WFUMB Safety Committee, and it therefore represents the opinions of the authors. Nevertheless, during the preparation of this document, contributions were received from members of the International Electrotechnical Commission Technical Committee 87 as well as many other individual experts, and the authors sincerely acknowledge their support.

Air↗
PubMed1993

Pulsed ultrasound and electrocortical activity in fetal sheep.

The aim of this study was to determine if low intensity ultrasound affects electrocortical activity in chronically instrumented fetal sheep. A 3.5-MHz transducer was attached to each fetal skull (n = 9) and activated periodically to emit pulses of 5.8 microseconds duration at a repetition rate of 2 kHz generating a power output of 60 mW. The power supply was activated in 30-s bursts every 180 s but random electronic switching energized the transducer with 50% of bursts. On average, 175 bursts of ultrasound were delivered to the fetus over 19.5 h. Blinded analysis of continuous recordings of electrocortical activity showed it to be unaffected by the ultrasound exposures. Therefore, it is unlikely that ultrasonic examination of human fetuses will affect normal cyclical electrocortical activity.

Animals↗