Intracranial temperature elevation from diagnostic ultrasound

Australian sonographers' perceptions of patient safety in ultrasound imaging: Part two - translation into practice

Introduction

A lack of patient safety research hampers capacity to improve safety in healthcare.Ultrasound is often considered 'safe' as it does not use ionising radiation, a simplistic view of patient safety. Understanding sonographers' actions towards patient safety is crucial; however, self-reported measures cannot always predict behaviour. This study is part of a PhD exploring patient safety in medical diagnostic ultrasound. The aim of this paper is to explore sonographers' responses to the patient safety concerns identified in Part one of this study. The ultimate aim of the study is to inform the final phase of the doctoral study which will consider the next steps in improving the quality and safety of healthcare experienced by patients.

Methods

A qualitative study using semi-structured, one-on-one interviews. The Theory of Planned Behaviour (TPB) explained how sonographers respond to perceived patient safety risks in practice.

Results

Thirty-one sonographers were interviewed. Based on the seven themes identified in Part one of the study, results showed that incongruences exist between identifying patient safety risks and the actions taken in practice to manage these risks.

Conclusion

The TPB showed that behavioural, normative and control beliefs impact sonographers' responses to perceived patient safety risks in practice and can lead to risk avoidance. Lack of regulation in ultrasound creates a challenge in dealing with Fitness to Practice issues. Collective actions are required to support sonographers in taking appropriate actions to enhance patient safety from multiple stakeholders including accreditation bodies, regulatory authorities, educational institutions and employers.

A Review on Biological Effects of Ultrasounds: Key Messages for Clinicians

Ultrasound (US) is acoustic energy that interacts with human tissues, thus, producing bioeffects that may be hazardous, especially in sensitive organs (i.e., brain, eye, heart, lung, and digestive tract) and embryos/fetuses. Two basic mechanisms of US interaction with biological systems have been identified: thermal and non-thermal. As a result, thermal and mechanical indexes have been developed to provide a means of assessing the potential for biological effects from exposure to diagnostic US. The main aims of this paper were to describe the models and assumptions used to estimate the "safety" of acoustic outputs and indices and to summarize the current state of knowledge about US-induced effects on living systems deriving from in vitro models and in vivo experiments on animals. This review work has made it possible to highlight the limits associated with the use of the estimated safety values of thermal and mechanical indices relating above all to the use of new US technologies, such as contrast-enhanced ultrasound (CEUS) and acoustic radiation force impulse (ARFI) shear wave elastography (SWE). US for diagnostic and research purposes has been officially declared safe, and no harmful biological effects in humans have yet been demonstrated with new imaging modalities; however, physicians should be adequately informed on the potential risks of biological effects. US exposure, according to the ALARA (As Low As Reasonably Achievable) principle, should be as low as reasonably possible.

Transcranial low-intensity ultrasound stimulation for treating central nervous system disorders: A promising therapeutic application

Transcranial ultrasound stimulation is a neurostimulation technique that has gradually attracted the attention of researchers, especially as a potential therapy for neurological disorders, because of its high spatial resolution, its good penetration depth, and its non-invasiveness. Ultrasound can be categorized as high-intensity and low-intensity based on the intensity of its acoustic wave. High-intensity ultrasound can be used for thermal ablation by taking advantage of its high-energy characteristics. Low-intensity ultrasound, which produces low energy, can be used as a means to regulate the nervous system. The present review describes the current status of research on low-intensity transcranial ultrasound stimulation (LITUS) in the treatment of neurological disorders, such as epilepsy, essential tremor, depression, Parkinson's disease (PD), and Alzheimer's disease (AD). This review summarizes preclinical and clinical studies using LITUS to treat the aforementioned neurological disorders and discusses their underlying mechanisms.

Contemporary directions of application of low power ultrasounds in anticancer therapy

In the recent years, research has been conducted on the role of ultrasounds (US) in anticancer therapy. Although the mechanisms of impact on cancer cells have not yet been fully understood, it is known that the best results are obtained using low power ultrasound. Currently applying ultrasounds to organisms is considered in three areas of influence: thermal (thermic effect), cavitation (cavitation effect), other than thermal and cavitation ones (non-thermal, non-cavitation effect). Under the influence of ultrasonic wave with low power, the absorption of drugs is increased as well as of anti-angiogenic activity. Sonodynamic therapy is aimed at destroying dividing cancer cells through the formation of free radicals in the cavitation mechanism and in the presence of sonosensitizers. At the same time under the influence of US, local hyperthermia is generated. In vivo studies showed a synergistic increase in cytotoxicity due to the effects of ultrasonic hyperthermia and adriamycin. The thermal effect and inertial cavitation are described as two factors induced by US, which may lead to damage to the vascular network within the neoplastic lesion. A proportional increase in tumor echogenicity to the frequency range of the applied ultrasound wave has been demonstrated. The strategy of combining US with photosensitizers, chemotherapeutics or contrast agents is gaining more and more recognition. Obtained results from inter developed studies on antineoplastic sonodynamic therapy indicate that it may become a new additional cancer treatment strategy.

Blood Pressure Profiles in Infants With Hypoxic Ischemic Encephalopathy (HIE), Response to Dopamine, and Association With Brain Injury

Objective: To describe mean arterial blood pressure (MABP), responsiveness to dopamine, and relationship to brain injury in infants with moderate/severe hypoxic-ischemic encephalopathy (HIE) undergoing therapeutic hypothermia (TH). We hypothesized that, when utilized, dopamine would rapidly and effectively increase MABP in treated patients. Methods: Continuous arterial blood pressure measurements were prospectively recorded from infants with moderate/severe HIE undergoing TH in a multi-institutional cohort from 2010 to 2018. Treatment with dopamine was at the discretion of the medical team for hypotension/hypoperfusion. MABP values of treated infants were compared to those obtained at an equivalent time period in control infants receiving TH but not dopamine (24 h after birth). MRI was obtained per unit protocols and included T1/T2/DWI sequences. Injury was classified as no injury/mild injury or moderate/severe injury using a standardized scoring system. Seizures were confirmed with conventional EEG. Results: Eighteen infants were treated with dopamine and were similar to untreated controls (n = 36) with the exception of lower cord gas pH (6.92 ± 0.2 vs. 7.07 ± 0.2, p < 0.05). Dopamine was initiated at a mean of 24 h after birth. MABP was significantly lower in the dopamine group at the start of therapy (39.9 ± 2.0 vs. 49.1 ± 1.3, p < 0.01) and 1 h later (44.3 ± 2.0 vs. 49.8 ± 1.1, p < 0.05). However, after 9 h of treatment, dopamine increased the MABP by an average of 9 mmHg and MABP values were similar to untreated controls for the remainder of the observation period. There were no significant differences in rates of seizures, brain injury, or death. Conclusion: Neonates with moderate/severe HIE treated with dopamine during TH had MABP significantly lower than controls. The majority of infants responded to dopamine monotherapy following adequate volume resuscitation. An association between requirement for dopamine and severity of brain injury was not detected.

Iterative Curve Fitting of the Bioheat Transfer Equation for Thermocouple-Based Temperature Estimation In Vitro and In Vivo

Temperature measurements with thin thermocouples embedded in ultrasound fields are strongly subjected to a viscous heating artifact (VHA). The artifact contribution decays over time; therefore, it can be minimized at late temperature readings. However, previous studies have failed to demonstrate a rigorous method for determining the optimal time point at which the artifact contribution is negligible. In this study, we present an iterative processing method based on successive curve fittings using an artifact-independent model. The fitting starting point moves at each iteration until the maximum R² indicates where the viscous heating is minimum. A solution of the bioheat transfer equation is used to account for blood perfusion, thus enabling in vivo measurements. Three T-type thermocouples with different diameters and sensitivities were assessed in an excised canine liver and in the mouse brain in vivo. We found that the artifact constitutes up to 81% ± 5% of wire thermocouple readings. The best-fit time varied in the liver samples ( n = 3 ) from 0 to 3.335 ± 0.979 s and in the mouse brain ( n = 5 ) from 0 to 0.498 ± 0.457 s at variable experimental conditions, which clearly demonstrates the need of the method for finding the appropriate starting time point of the fit. This study introduces a statistical method to determine the best time to fit a curve that can back-estimate temperature in tissues under ultrasound exposure using thermocouples. This method allows temperature evaluation in vivo and in vitro during a validation and safety assessment of a wide range of therapeutic and diagnostic ultrasound modalities.

Association of Prenatal Ultrasonography and Autism Spectrum Disorder

Importance

The prevalence of autism spectrum disorder (ASD) has been increasing rapidly, with current estimates of 1 in 68 children affected. Simultaneously, use of prenatal ultrasonography has increased substantially, with limited investigation into its safety and effects on brain development. Animal studies have demonstrated that prenatal ultrasonography can adversely affect neuronal migration.

Objective

To quantify prenatal ultrasound exposure by the frequency, timing, duration, and strength of ultrasonographic scans in children with later ASD, developmental delay, and typical development.

Design, Setting, and Participants

This case-control study included 107 patients with ASD, 104 control individuals with developmental delay, and 209 controls with typical development. Participants were identified from medical records based on prenatal care and delivery at Boston Medical Center, a diverse, academic, safety-net medical center, from July 1, 2006, through December 31, 2014, with a gestational age at birth of at least 37 weeks. Data were analyzed from May 1, 2015, through November 30, 2017.

Exposures

Ultrasonographic exposure was quantified by the number and timing of scans, duration of exposure, mean strength (depth, frame rate, mechanical index, and thermal index), and time of Doppler and 3- and 4-dimensional imaging.

Main Outcomes and Measures

Among participants with ASD and controls with developmental delay and typical development, ultrasound exposure was quantified and compared per trimester and for the entire pregnancy, with adjustment for infant sex, gestational age at birth, and maternal age.

Results

A total of 420 participants were included in the study (328 boys [78.1%] and 92 girls [21.9%]; mean age as of January 1, 2016, 6.6 years; 95% CI, 6.5-6.8 years). The ASD group received a mean of 5.9 scans (95% CI, 5.2-6.6), which was not significantly different from the 6.1 scans (95% CI, 5.4-6.8) in the developmental delay group or the 6.3 scans (95% CI, 5.8-6.8) in the typical development group. Compared with the typical development group, the ASD group had shorter duration of ultrasound exposure during the first (290.4 seconds [95% CI, 212.8-368.0 seconds] vs 406.4 seconds [95% CI, 349.5-463.3 seconds]) and second (1687.6 seconds [95% CI, 1493.8-1881.4 seconds] vs 2011.0 seconds [95% CI, 1868.9-2153.1 seconds]) trimesters but no difference in the number of scans. The ASD group had greater mean depth of ultrasonographic penetration than the developmental delay group in the first trimester (12.5 cm [95% CI, 12.0-13.0 cm] vs 11.6 cm [95% CI, 11.1-12.1 cm]). The ASD group had greater mean depth than the typical development group during the first (12.5 cm [95% CI, 12.0-13.0 cm] vs 11.6 cm [95% CI, 11.3-12.0 cm]) and the second (12.9 cm [95% CI, 12.6-13.3 cm] vs 12.5 cm [95% CI, 12.2-12.7 cm]) trimesters.

Conclusions and Relevance

This study found significantly greater mean depth of ultrasonographic penetration in the ASD group compared with the developmental delay group in the first trimester and compared with the typical development group in the first and second trimesters. Further research is needed to determine whether other variables of ultrasound exposure also have adverse effects on the developing fetus.

Cerebral Autoregulation, Brain Injury, and the Transitioning Premature Infant

Improvements in clinical management of the preterm infant have reduced the rates of the two most common forms of brain injury, such as severe intraventricular hemorrhage and white matter injury, both of which are contributory factors in the development of cerebral palsy. Nonetheless, they remain a persistent challenge and are associated with a significant increase in the risk of adverse neurodevelopment outcomes. Repeated episodes of ischemia-reperfusion represent a common pathway for both forms of injury, arising from discordance between systemic blood flow and the innate regulation of cerebral blood flow in the germinal matrix and periventricular white matter. Nevertheless, establishing firm hemodynamic boundaries, as a part of neuroprotective strategy, has challenged researchers. Existing measures either demonstrate inconsistent relationships with injury, as in the case of mean arterial blood pressure, or are not feasible for long-term monitoring, such as cardiac output estimated by echocardiography. These challenges have led some researchers to focus on the mechanisms that control blood flow to the brain, known as cerebrovascular autoregulation. Historically, the function of the cerebrovascular autoregulatory system has been difficult to quantify; however, the evolution of bedside monitoring devices, particularly near-infrared spectroscopy, has enabled new insights into these mechanisms and how impairment of blood flow regulation may contribute to catastrophic injury. In this review, we first seek to examine how technological advancement has changed the assessment of cerebrovascular autoregulation in premature infants. Next, we explore how clinical factors, including hypotension, vasoactive medications, acute and chronic hypoxia, and ventilation, alter the hemodynamic state of the preterm infant. Additionally, we examine how developmentally linked or acquired dysfunction in cerebral autoregulation contributes to preterm brain injury. In conclusion, we address exciting new approaches to the measurement of autoregulation and discuss the feasibility of translation to the bedside.

Neonatal Cranial Ultrasound: Are Current Safety Guidelines Appropriate?

Ultrasound can lead to thermal and mechanical effects in interrogated tissues. We reviewed the literature to explore the evidence on ultrasound heating on fetal and neonatal neural tissue. The results of animal studies have suggested that ultrasound exposure of the fetal or neonatal brain may lead to a significant temperature elevation at the bone-brain interface above current recommended safety thresholds. Temperature increases between 4.3 and 5.6°C have been recorded. Such temperature elevations can potentially affect neuronal structure and function and may also affect behavioral and cognitive function, such as memory and learning. However, the majority of these studies were carried out more than 25 y ago using non-diagnostic equipment with power outputs much lower than those of modern machines. New studies to address the safety issues of cranial ultrasound are imperative to provide current clinical guidelines and safety recommendations.

Brain Surface Heating After Exposure to Ultrasound: An Analysis Using Thermography

Ultrasound is the imaging modality of choice to monitor brain pathologies in neonates after complicated deliveries. Animal studies have indicated that ultrasound may cause heating of brain tissues. To date, no study has explored brain surface heating by ultrasound during clinically relevant exposure. Hence, we investigated heating effects of B-mode and pulsed Doppler (PD) mode on ex vivo lamb brains using thermography. Five brains were scanned for 5 min in B-mode or for 3 min, 1 min, 30 s or 15 s in PD mode. Brain surface temperature was measured pre- and post-exposure using thermography. The highest mean temperature increase was recorded by B-mode (3.82 ± 0.43°C). All five PD exposure protocols were associated with surface temperature increases of 2.1-2.7°C. These outcomes highlight for the first time that B-mode ultrasound can contribute to brain surface heating during a routine cranial scan. Scan duration should be minimised whenever possible.

Benefits and risks of ultrasound in pregnancy

Ultrasound is, arguably, the most commonly used diagnostic procedure in obstetrics. It is convenient, painless, yields immediate, extensive results, and is widely considered to be safe. Some (but not all) benefits described in the literature have been validated by evidence-based analysis, such as pregnancy dating. Others are considered clinically useful, although objective evidence may be less strong. As is the case with almost any medical procedure, however, its performance carries some risks: misdiagnosis on the one hand and possible undesired effects on the other. The general belief exists that diagnostic ultrasound (DUS) does not pose any risk to the pregnant patient nor to her fetus. Nonetheless, ultrasound is a form of energy and, as such, demonstrates effects in biological tissues it traverses (bioeffects). The physical mechanisms responsible for these effects are thermal or non-thermal (mechanical). It is the role of science to show whether any of these bioeffects may be harmful. A risk-benefit analysis may also be important, as well as education of the end users to assure patients' safety.

Sonodynamic therapy (SDT): a novel treatment of cancer based on sonosensitizer liposome as a new drug carrier

Cancer is a major cause of morbidity and mortality. Chemotherapy is one of the main means of cancer treatment. Conventional chemotherapy agents kill rapidly proliferating cells, resulting in some of the most common side effects of chemotherapy. Liposome-encapsulated drugs offer the possibility to increase target efficacy as well as reducing toxic side effects of chemotherapeutic drugs. However, the target specificity of liposome is dissatisfied. We urgently need to develop new approaches to improving drug target efficacy. Recently, sonodynamic therapy (SDT) of cancer, which is based on preferential uptake and retention of a sonosensitizer in tumor tissues and subsequent activation of drug by ultrasound radiation, is a developing field. In this article, we propose the use of sonosensitizers in combination with liposome to target chemotherapy drugs directly to tumor cells. SDT with low-intensity ultrasound combined with a sonosensitizer may be a promising approach to cancer therapy.

Taboos and opportunities in sonothrombolysis for stroke

Systemic thrombolysis with tissue plasminogen activator (tPA) is the only approved treatment for acute ischaemic stroke that improves functional outcome if given up to 4.5 h from symptom onset. At least half of treated patients have unfavourable outcomes long-term though, emphasising the need to amplify the only approved acute stroke therapy. Ultrasound targeting of an intra-arterial occlusive clot and delivering mechanical pressure to its surrounding fluids (referred to as sonothrombolysis) accelerates the thrombolytic effect of tPA. Higher recanalisation rates produce a trend towards better functional outcomes that could be safely achieved with the combination of 2 MHz frequency ultrasound and systemic tPA. To further accelerate the clot-dissolving effect of ultrasound, a variety of frequencies and intensities as well as other adjuvant treatment elements are being studied. However, literature reports argue efficacy and safety of these novel approaches doubting promptly translation into the clinical practice. This review will summarise our current knowledge about potentially harmful (taboos) directions and what we think are promising avenues for these future stroke therapies. We also give a prospect for novel technologies such as operator-independent devices that aim to further spread the use of sonothrombolysis for stroke.

Minimization of treatment time for in vitro 1.1 MHz destruction of Pseudomonas aeruginosa biofilms by high-intensity focused ultrasound

Medical implants are prone to colonization by bacterial biofilms. Normally, surgery is required to replace the infected implant. One promising noninvasive modality is to destroy biofilms with high-intensity focused ultrasound. In our study, Pseudomonas aeruginosa biofilms were grown on implant-mimicking graphite disks in a flow chamber for 3 days prior to exposing them to ultrasound pulses. Exposure time at each treatment location was varied between 5, 15 and 30s. Burst period was varied between 1, 3, 6 and 12 milliseconds (ms). The pulses were 20 cycles in duration at 1.1 MHz from a spherically focused transducer (f/1, 63 mm focal length), creating peak compressional and rarefactional pressures at the graphite disk surface of 30 and 13 MPa, respectively. P. aeruginosa were tagged with green fluorescent protein, and killed cells were visualized using propidium iodide before determining the extent of biofilm destruction. The exposure-induced temperature rise was measured to be less than 0.2°C at the focus, namely the interface between graphite disk and water. Then, the temperature rise was measured at the focus between the graphite disk and a tissue-mimicking phantom to evaluate therapy safety. Two thresholds, of bacteria destruction increase and of complete bacteria removal, respectively, were identified to divide our eight exposure conditions. Results indicated that 30-s exposure and 6-ms pulse period were sufficient to destroy the biofilms. However, the 15-s exposure and 3-ms pulse period were viewed as optimum when considering exposure time, efficacy, and safety.

Transcranial ultrasound (TUS) effects on mental states: a pilot study

BACKGROUND/OBJECTIVE

Transcranial ultrasound (TUS) can modulate brain function. To assess possible TUS modulation of mental states, we investigated effects on subjective reports of pain and mood of sub-thermal TUS versus placebo applied to frontal scalp and brain of chronic pain patient volunteers.

METHODS

With IRB approval and informed consent, subjects with chronic pain completed two visual analog scales for pain (NRS) and mood (VAMS/Global Affect), and their vital signs were recorded 10 min prior to, and 10 min and 40 min following exposure to either subthermal TUS (8 MHz) or placebo (in a double blind crossover study) using the 12L-RS probe of a LOGIQe ultrasound imaging machine (General Electric, USA). A physician, also blinded for TUS versus placebo, applied the probe (with gel) to scalp over posterior frontal cortex, contralateral to maximal pain, for 15 seconds. A second investigator operated the ultrasound machine, randomizing TUS versus placebo. The process was then repeated, applying the opposite modality (TUS or placebo).

RESULTS

Subjective reports of Mood/Global Affect were improved 10 min (P = 0.03) and 40 min (P = 0.04) following TUS compared with placebo. NRS pain reports slightly improved following TUS (P = 0.07) at 40 min.

CONCLUSION

We found improvement in subjective mood 10 min and 40 min after TUS compared to placebo. TUS can have safe neurophysiological effects on brain function, and is a promising noninvasive therapy for modulating conscious and unconscious mental states and disorders. We suggest TUS acts via intra-neuronal microtubules, which apparently resonate in TUS megahertz range.

Prenatal ultrasound exposure and children's school performance at age 15-16: follow-up of a randomized controlled trial

OBJECTIVE

To evaluate the association between prenatal ultrasound exposure and school performance at 15-16 years of age.

METHODS

The study population consisted of children born to women who participated in a randomized controlled trial on the second-trimester ultrasound examination in Sweden from 1985 to 1987. Information about the children's grades when graduating from primary school and information on socioeconomic factors was obtained from Swedish nationwide registers. Comparisons were made using linear and logistic regression analyses according to randomization to ultrasound, ultrasound exposure in the second trimester and ultrasound exposure at any time during pregnancy. Boys and girls were analyzed separately.

RESULTS

Of the 4756 singleton children from the randomized trial, we identified 4458 (94%) in the National School Register. There were no statistically significant differences in school performance for boys or girls according to randomization or exposure to ultrasound in the second trimester. Compared to those who were unexposed, boys exposed to ultrasound at least once at any time during fetal life had a tendency towards lower mean school grades in general (-4.39 points; 95% CI, -9.59 to 0.81 (max possible, 320) points) and in physical education (-0.45 points; 95% CI, -0.91 to 0.01 (max possible, 20) points), but the differences did not reach significance.

CONCLUSION

In general, routine ultrasound examination in the second trimester had no effect on overall school performance in teenagers.

A new safety parameter for diagnostic ultrasound thermal bioeffects: safe use time

It is widely accepted that diagnostic ultrasound has the potential to elevate the temperature of tissue being scanned. Because both the maximum value of the temperature rise and the temporal profile of that rise are necessary to estimate the risk correctly, the temperature rise [DeltaT(t)] at an observation point for an exposure condition is presumed to have two components, that is, DeltaT(t)=DeltaT(max)X(t). The amplitude component DeltaT(max) is the maximum value of DeltaT(t), and the exposure time component X(t) represents the time dependency of that DeltaT(t). Ninety-six cases were investigated to obtain the proposed DeltaT(t) model at six frequencies, four source diameters, and four f-numbers. Then, using the relative change in the rate of induction of a thermal effect due to ultrasound exposure that produces DeltaT(t) different from a threshold exposure, the safe use time (SUT) model was constructed. SUT informs the user of the maximum duration of exposure in a region at a particular output level that would be no more hazardous than scanning at the threshold exposure. Using the SUT model, high power ultrasound can be applied for a short time so that the user can improve imaging performance while staying within safe limits.

Knowledge of the bio-effects of ultrasound among physicians performing clinical ultrasonography: Results of a survey conducted by the Italian Society for Ultrasound in Medicine and Biology (SIUMB)

INTRODUCTION

Safety issues should always be kept in mind when using diagnostic ultrasound, which is associated with potentially hazardous bio-effects, especially with the introduction of new technologies.

AIM

To assess the level of awareness and knowledge of safety issues related to the clinical use of ultrasound among physician-members of the Italian National Society for Ultrasound.

MATERIALS AND METHODS

A questionnaire with 11 multiple-choice questions was sent to members of the Italian Society for Ultrasound in Medicine and Biology. The answers were forwarded from the Society's Secretariat to the investigators, who statistically analyzed the data.

RESULTS

The mean age of the 105 respondents was 44 years. The most frequent kind of ultrasound examinations (in addition to conventional B-mode) were: Doppler (74%), contrast-enhanced US (43%), and pediatric studies (43%). Only 50-60% of the responders knew the correct definitions of the terms thermal index and mechanical index. Almost all respondents understood the bio-effects reflected by the thermal index, but only a minority knew the most likely organ target of bio-effects related to the mechanical index and what do indicate the units in which the thermal index is expressed. The majority knew that fetuses are at higher risk of damage. Few respondents were able to identify the correct safety statements included in the recommendations of the International Ultrasound Societies.

CONCLUSION

In conclusion, the present findings indicate that greater efforts of National Ultrasound Societies are warranted in disseminating knowledge of the bio-effects of diagnostic ultrasound modalities among operators.

Prenatal ultrasound and the risk of childhood brain tumour and its subtypes

We carried out a nationwide case-control study of childhood brain tumours in Sweden (n=512) by histological subtype in relation to prenatal ultrasound, extracting data from antenatal records and the Medical Birth Register. We found no increased risk for brain tumour after ultrasound exposure, either for all tumours or for any subgroup.

An increased thermal index can be achieved when performing Doppler studies in obstetric sonography

OBJECTIVE

The present study was aimed at evaluating acoustic outputs during clinical ultrasound examinations, as expressed by the thermal index (TI) and the mechanical index (MI), during the second half of pregnancy and comparing acoustic outputs between B-mode and Doppler examinations.

METHODS

Patients with suspected fetal growth problems undergoing Doppler studies of the fetal circulation in addition to B-mode sonography were selected. Examinations took place between 21 and 40 weeks' gestation. An obstetrician collected data prospectively. Sonographers were unaware of the data being sought. The analysis of variance test was applied for differences in continuous variables.

RESULTS

A total of 63 examinations were evaluated. The mean gestational age +/- SD was 31.6 +/- 5.1 weeks. The mean duration of the total examinations was 17.6 +/- 8.6 minutes, whereas the Doppler studies lasted 0.9 +/- 0.8 minutes. The TI was significantly higher in the pulsed wave Doppler studies (mean, 1.5 +/- 0.5; range, 0.9-2.8) and color flow imaging studies (mean, 0.8 +/- 0.1; range, 0.6-1.2) compared with B-mode sonography (mean, 0.3 +/- 0.1; range, 0.1-0.7; P < .01). During the examination, 190 B-mode MI variations were recorded (mean, 1.1 +/- 0.1), which were comparable with those of the 31 color flow Doppler studies (mean, 1.0 +/- 0.1; P = .09) but higher than the 190 pulsed wave Doppler MI variations (mean 0.9 +/- 0.2; P < .001).

CONCLUSIONS

Increased acoustic output levels, as expressed by TI levels, are reached during obstetric Doppler studies. In particular, TI levels may reach 1.5 and higher. Doppler procedures should be performed with caution and be as brief as possible during obstetric sonography.

Evaluation of biological effects induced by diagnostic ultrasound in the rat foetal tissues

In recent years, there has been growing interest in estimating the degree of heating caused by the diagnostic ultrasound in clinical practice. Both theoretical and experimental methods have been suggested for estimating the heating potential, or thermal hazard, of diagnostic ultrasound. Aim of this study was to evaluate in vivo effects of ultrasound exposure of variable duration (from 10 up to 20 min) with commercially available imaging systems commonly used for diagnostic imaging. Numerical results related to the thermal effect are obtained by simulation program based on B-mode (scanning) and Doppler (non-scanning). To investigate the biological effects of the ultrasound exposure to the brain and liver tissues, the antioxidant enzyme activity and thiobarbituric acid reactive substances (TBARS) of the tissues were evaluated. In liver tissue, as a lipid peroxidation index, TBARS levels very significantly increase in Doppler group compared to control. However, in B-mode, TBARS levels are the same with the control group. Use of B-mode in foetal tissue is more reliable than Doppler mode because temperature rise is very small compared to the Doppler mode. On the other hand, the antioxidant enzyme activities tend to increase in B-mode and Doppler groups compared to the control group as a defensive mechanism. In the brain tissue, lipid peroxidation is increased slightly in B-mode compared to the control group. This situation is related to the molecular structure of the brain tissue because of its high lipid concentration. In brain tissue, the antioxidant enzyme activities and lipid peroxidation were significantly increased, such as liver tissue in Doppler groups. Doppler ultrasound may produce harmful effects in rat foetus liver and brain tissues as a result of the high temperature rises.

Brain temperature during 340-kHz pulsed ultrasound insonation: a safety study for sonothrombolysis

BACKGROUND AND PURPOSE

Because ultrasound is used for improving thrombolysis of cerebral infarction but continuous ultrasound insonation also has significant thermal effects, we evaluated brain temperature increase and tissue destruction during pulsed ultrasound emission.

METHODS

We examined 340-kHz pulsed ultrasound effects in male Wistar rats. Ultrasound was applied transcranially for 30 minutes on different power levels (1 to 7 W/cm2). Temperature was measured at different locations (brain, in the focus of ultrasound beam, inner ear, temporalis muscle, and rectum). The cooling time after 30-minute insonation for every power level was recorded, and animals were examined by postmortem brain histology (TUNEL and hematoxylin/eosin).

RESULTS

Brain temperature increased within 2 to 5 minutes of insonation. Brain temperature increase and cooling time were in proportion to power level, and even with the highest intensity of 7 W/cm2 for 30 minutes, the maximum elevation of mean brain temperature was 0.9 degrees C, with the highest cooling time of 40 minutes. No deleterious side effects of this treatment could be found in histological examination.

CONCLUSIONS

Using a pulsed ultrasound design, only a moderate temperature increase could be observed with no histopathological abnormalities. Deleterious side effects of mid-kilohertz ultrasound (eg, intracerebral hemorrhage) are therefore not a consequence of local brain temperature increase.

Broadband measurements of the frequency dependence of attenuation coefficient and velocity in amniotic fluid, urine and human serum albumin solutions

The frequency dependence of attenuation coefficient in amniotic fluid, urine and 4.5% and 20% human serum albumin solutions over the frequency range 5 MHz to 25 MHz was measured at both room temperature and physiological temperature using a variable path length technique. A 15 MHz (13 mm diameter) transducer was used to produce a broadband single-cycle pulse and a 4 mm diameter bilaminar polyvinylidene difluoride membrane hydrophone was used to detect the attenuated pulse. Standard time-of-flight measurement techniques were used to measure the acoustic velocity in the same fluid samples. At physiological temperature, the attenuation coefficients in amniotic fluid, urine and 4.5% and 20% human albumin solution were found to be 0.0053 f(1.65), 0.0047 f(1.67), 0.019 f(1.57) and 0.167 f(1.27) dB cm(-1), respectively, where f is in MHz. The velocities in amniotic fluid, urine and 4.5% human albumin solution at physiological temperature were found to be 1541.1 m s(-1) +/- 1.3 m s(-1), 1551.3 m s(-1) +/- 1.3 ms(-1) and 1547.3 m s(-1) +/- 1.0 m s(-1), respectively. The results provide unique data over the diagnostic and therapeutic ultrasonic frequency range that can be used as input data for theoretical models that attempt to simulate nonlinear pressure fields and temperature rises from medical ultrasonic transducers.

A model for estimating ultrasound attenuation along the propagation path to the fetus from backscattered waveforms

Accurate estimates of the ultrasound pressure and/or intensity incident on the developing fetus on a patient-specific basis could improve the diagnostic potential of medical ultrasound by allowing the clinician to increase the transmit power while still avoiding the potential for harmful bioeffects. Neglecting nonlinear effects, the pressure/intensity can be estimated if an accurate estimate of the attenuation along the propagation path (i.e., total attenuation) can be obtained. Herein, a method for determining the total attenuation from the backscattered power spectrum from the developing fetus is proposed. The boundaries between amnion and either the fetus' skull or soft tissue are each modeled as planar impedance boundaries at an unknown orientation with respect to the sound beam. A mathematical analysis demonstrates that the normalized returned voltage spectrum from this model is independent of the planes orientation. Hence, the total attenuation can be estimated by comparing the location of the spectral peak in the reflection from the fetus to the location of the spectral peak in a reflection obtained from a rigid plane in a water bath. The independence of the attenuation estimate and plane orientation is then demonstrated experimentally using a Plexiglas plate, a rat's skull, and a tissue-mimicking phantom.

Insonation facilitates plasmid DNA transfection into the central nervous system and microbubbles enhance the effect

Many of the diseases which affect the central nervous system are intractable to conventional therapies and therefore require alternative treatments such as gene therapy. Therapy requires safety, since the central nervous system is a critical organ. Choice of nonviral vectors such as naked plasmid DNA may have merit. However, transfection efficiencies of these vectors are low. We have investigated the use of 210.4 kHz ultrasound and found that 5.0 W/cm(2) of insonation for 5 s most effectively transfected a plasmid DNA into culture slices of mouse brain (147.68-fold increase compared with 0 W/cm(2) of insonation for 5 s). The effect was reinforced by combination with echo contrast agent, Levovist. One hundred fifty mg/mL of Levovist significantly increased gene transfection by ultrasound (5.23-fold when insonated at 5.0 W/cm(2) for 5 s). When DNA was intracranially injected, Levovist also enhanced gene transfection in newborn mice (4.49-fold increase when insonated at 5.0 W/cm(2) for 5 s). Since ultrasound successfully transfected naked plasmid DNA into the neural tissue and Levovist enhanced the effect, this approach may have a significant role in gene transfer to the central nervous system.

Biological effects of high-frequency ultrasound exposure during mouse organogenesis

Little has been reported on bioeffects of high-frequency ultrasound (US) and guidelines for US use do not necessarily apply to high frequencies. Pregnant CD-1 mice were exposed to Doppler or B-mode US biomicroscopy (UBM) on embryonic day (E) 8.5 or E10.5, during organogenesis. Operating frequency was 40 MHz with a free field I(SPTA) of 11.9 W/cm(2) (Doppler) and 2.6 mW/cm(2) (B-mode), peak negative pressures of 6.61 MPa and MI of 1.05 (B-mode). Offspring were assessed weekly from 1 day postnatally to euthanasia at 6 weeks, with no significant difference in pup weight, body length or crown-rump length observed. E8.5 Doppler-exposed mice showed a small reduction in weight and length at 3 weeks and in weight at 6 weeks. E10.5 Doppler-exposed animals exhibited slight growth reduction in weeks 2 to 4, but were not significantly different at 6 weeks. Our results indicate similar exposures of mice should not cause significant adverse bioeffects.

Does low-energy ultrasound, known to enhance thrombolysis, affect the size of ischemic brain damage?

OBJECTIVE

In view of the potentially beneficial effects of ultrasound on fibrinolysis, we aimed to study possible drawbacks of low-energy ultrasound on nonperfused and perfused brain tissue.

METHODS

A model of transient focal ischemia in anesthetized rats was used. Rats were randomly assigned to 1 of 3 groups: the first exposed to ultrasound, the second a control of the ultrasound group, and the third a method control group. In each group, the right middle cerebral artery was occluded for 1.5 hours, during which time the rats in the ultrasound group were exposed to 1 hour of pulsed ultrasound (1 MHz; spatial-average temporal-average intensity, 0.1 W/cm2 at a duty cycle of 10%). The occlusion period was followed by a 24-hour recirculation period, after which the brains were excised and evaluated.

RESULTS

Ultrasound did not affect the volume of ischemic damage in nonperfused brain tissue or add ischemic damage to perfused brain tissue.

CONCLUSIONS

Under these experimental conditions, ultrasound does not cause additional ischemic damage to the rat brain during middle cerebral artery occlusion.

The cooling effect of liquid flow on the focussed ultrasound-induced heating in a simulated foetal brain

There is a need to investigate the thermal effects of diagnostic ultrasound (US) to assist the development of appropriate safety guidelines for obstetric use. The cooling effect of a single liquid flow channel was measured in a model of human foetal brain and skull bone heated by a focussed beam of simulated pulsed spectral Doppler US. Insonation conditions were 5.7 micros pulses, repeated at 8 kHz from a focussed transducer operating with a centre frequency of 3.5 MHz, producing a beam of -6 dB diameter of 3.1 mm at the focus and power outputs of up to 255 +/- 5 mW. Brain perfusion was simulated by allowing distilled water to flow at various rates in a 2 mm diameter wall-less channel in the brain soft tissue phantom material. This study established that the cooling effect of the flowing water; 1. was independent of the acoustic source power, 2. was more effective close to the flow channel, for example, there was a marked cooling at a distance of 1 mm and negligible cooling at a distance of 3 mm from the channel; and 3. initially increased at low flow rates, but further increase above normal perfusion had very little effect.

Ultrasound-induced heating in a foetal skull bone phantom and its dependence on beam width and perfusion

The cooling effect of single and multiple perfusing channels has been measured in a model of human foetal skull bone heated by wide and narrow beams of simulated pulsed spectral Doppler ultrasound (US). A focussed transducer operating with a centre frequency of 3.5 MHz, that emitted pulses of 5.7 micros duration with a repetition frequency of 8 kHz, was used. This produced a beam of power 100 +/- 2 mW with -6 dB diameters of 3.1 mm and 7.8 mm at 9 cm and 6 cm, respectively, from the transducer face. Arterial perfusion was simulated by allowing distilled water to flow in a large single channel or a grid of fine channels near the heated bone target. This study has established that: 1. perfusion-induced cooling is significantly enhanced when the bone phantom is heated by a wide rather than a narrow beam; 2. irrespective of the US beam width, a grid of small channels is more effective in cooling a heated bone target than a single larger diameter channel with the same volume flow rate; 3. the measured temperature rise and rate of temperature rise support the prediction of inverse proportionality to the US beam width; and 4. the perfusion time constants determined in our phantom model are 2 to 30 times larger than that assumed for the thermal index (TIB) algorithm.

Time to threshold (TT), a safety parameter for heating by diagnostic ultrasound

Activities in the standardisation of medical ultrasonic imaging and monitoring devices have been undertaken to ensure the safe application and to provide the user with tools to perform risk-to-benefit analysis. Internationally, agreement has not been reached to implement a system based on a physical quantity such as temperature rise. Therefore, recently, the IEC Standard 60601-2-37 has adopted the thermal index (TI) as a safety parameter. TI indicates conditions that are more likely than others to produce thermal effects. The models used to calculate TI are based on the equilibrium temperature rise for a stationary transducer. Ultrasound (US) is often applied during a short time at one place. To characterise the situation before equilibrium is reached, this study proposes a new parameter: TT (time to threshold). TT indicates the time after which a threshold temperature rise is exceeded. TT is calculated for some tissue models to illustrate the meaning of TT. TT indicates how long one piece of tissue can be insonated safely, provided the safe threshold is known. Using TT, higher intensities can be applied safely during a limited time. This paper illustrates the effects of short insonation times and the influence of the beam diameter. Questions that are critical to the introduction of TT as a safety parameter are discussed.