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

Neuroinflammation in the Rat Brain After Exposure to Diagnostic Ultrasound

OBJECTIVE

To date there have been no studies exploring the potential for neuroinflammation as an intracranial bio-effect associated with diagnostic ultrasound during neonatal cranial scans in a mammalian in vivo model. The study described here was aimed at investigating the effects of B-mode and Doppler mode ultrasound on inflammation in the rat brain.

METHODS

Twelve Wistar rats (7-9 wk old) were divided into a control group and an ultrasound-exposed group (n = 6/group). A craniotomy was performed, followed by 10 min of B-mode and spectral Doppler interrogation of the middle cerebral artery. The control group was subjected to sham treatment, with the transducer held stationary over the craniotomy site, but the ultrasound machine switched off. Animals were euthanized 48 h after exposure, and the brains formalin fixed for immunohistochemical analysis using allograft inflammatory factor 1 (IBA-1) and glial fibrillary acidic protein (GFAP) as markers of microglia and astrocytes, respectively. The numbers of IBA-1- and GFAP-immunoreactive cells were manually counted and expressed as areal density (cells/mm2). Results were analyzed using Student's unpaired t-test and one-way repeated-measures analysis of variance.

RESULTS

The ultrasound-exposed brain exhibited significant increases in IBA-1 and GFAP immunoreactive cell density in all regions of B-mode and Doppler mode exposure compared with the control group (p < 0.001).

CONCLUSION

Ten minutes of B-mode and Doppler mode ultrasound may induce neuroinflammatory changes in the rat brain. This suggests that exposure of brain tissue to current diagnostic ultrasound intensities may not be completely without risk.

Australian sonographers' perceptions of patient safety in ultrasound imaging: Part 1 - identifying the main safety concerns, a qualitative study

Introduction

Patient safety has been an undervalued component of quality healthcare but is a challenging area of research.Ultrasound is the most common imaging modality in the world. Research on patient safety in ultrasound is generally focused on bioeffects and safe operation of ultrasound equipment. However, other safety issues exist in practice that warrant consideration.This paper forms the first part of a PhD study exploring patient safety in medical diagnostic ultrasound, beyond the notion of bioeffects.The ultimate aim of the study is to inform the final phase of the research 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. A thematic analysis categorised data into codes and generated final themes.

Results

A heterogeneous mix of 31 sonographers, who reflected the profile of the profession in Australia, were interviewed between September 2019 and January 2020. Seven themes emerged from the analysis. These were bioeffects, physical safety, workload, reporting, professionalism, intimate examinations and infection control.

Conclusion

This study presents a comprehensive analysis of sonographers' perceptions of patient safety in ultrasound imaging, not previously available in the literature. Consistent with the literature, patient safety in ultrasound tends to be viewed in technical terms through the potential for bioeffects of tissue damage or physical harm to the patient. However, other patient safety issues have emerged, and while not as well recognised, have the potential to negatively impact on patient safety.

Thermochromic Tissue-Mimicking Phantoms for Thermal Ablation Based on Polyacrylamide Gel

In recent years, thermal ablation has played an increasingly important role in treating various tumors in the clinic. A practical thermochromic phantom model can provide a favorable platform for clinical thermotherapy training of young physicians or calibration and optimization of thermal devices without risk to animals or human participants. To date, many tissue-mimicking thermal phantoms have been developed and are well liked, especially the polyacrylamide gel (PAG)-based phantoms. This review summarizes the PAG-based phantoms in the field of thermotherapy, details their advantages and disadvantages and provides a direction for further optimization. The relevant physical parameters (such as electrical, acoustic, and thermal properties) of these phantoms are also presented in this review, which can assist operators in a deeper understanding of these phantoms and selection of the proper recipes for phantom fabrication.

Acoustic Power Measurement and Thermal Bioeffect Evaluation of Therapeutic Langevin Transducers

We recently proposed an analytical design method of Langevin transducers for therapeutic ultrasound treatment by conducting parametric study to estimate the effect of compression force on resonance characteristics. In this study, experimental investigations were further performed under various electrical conditions to observe the acoustic power of the fully equipped transducer and to assess its heat-related bioeffect. Thermal index (TI) tests were carried out to examine temperature rise and thermal damage induced by the acoustic energy in fatty porcine tissue. Acoustic power emission, TI values, temperature characteristics, and depth/size of thermal ablation were measured as a function of transducer’s driving voltage. By exciting the transducer with 300 V_pp sinusoidal continuous waveform, for instance, the average power was 23.1 W and its corresponding TI was 4.1, less than the 6 specified by the Food and Drug Administration (FDA) guideline. The maximum temperature and the depth of the affected site were 74.5 °C and 19 mm, respectively. It is shown that thermal ablation is likely to be more affected by steep heat surge for a short duration rather than by slow temperature rise over time. Hence, the results demonstrate the capability of our ultrasonic transducer intended for therapeutic procedures by safely interrogating soft tissue and yet delivering enough energy to thermally stimulate the tissue in depth.

Effects of long-term Doppler ultrasound exposure on cochlea and cochlear nucleus in prenatal period in an experimental model

BACKGROUND

New generation Doppler ultrasonography (DUSG) application effects on cochlea and cochlear nucleus (CN) are unclear. We aimed to investigate the effects of new generation DUSG application at different frequencies in prenatal period on cochlea and CN in rats.

OBJECTIVE

Twenty-four pregnant female rats were divided into three groups (n = 8). Group 1 was the control group and was not subjected to any treatment. Group 2 was determined as the USG every day (USGED) treatment group. Group 2 has received DUSG application every day from the 4th to 18th day (20 min/15 per day). Group 3 has received DUSG application as "2 days/one dose as every other day application" (USG2D1) from the 4th to 18th day (20 min/8 every other day). Twenty-four female rats were sacrificed in 21 days. Also, 24 pups were sacrificed after two days. First day after born, the cochlear activities of the right ears of all pups were examined using DPOAEs. Second day, neural tissues from CN were evaluated histopathologically and immunohistochemically.

RESULTS

There was no any statistical difference between the groups in respect of histopathologically. USGED group showed mild caspase-3 positive neurons and glial cells. However, there was no significant difference between the USGED and other groups (p>.05). Similarly, the rats applied with USG2D1 had mild caspase-3 expression, but no significant difference between the USG2D1 and other groups (p>.05). Differences in DPOAE amplitudes, and therefore in cochlear activity, between the groups were revealed. The decrease in cochlear activity between the groups involved frequencies at 2, 8, 16, and 32 kHz (p<.05).

CONCLUSIONS

Multiple administration of new generation DUSG to pregnant rats has not shown harmful effects on the cochlear neural tissue. High frequencies are more sensitive in cochlea to apply DUSG.

Management of ureteric stone in pregnancy: a review

Background

Urolithiasis in pregnancy is a major health concern and is one of the most common causes for non-obstetrical abdominal pain and subsequent hospital admission during pregnancy. The incidence of urinary calculi during pregnancy varies in the range of 1/200 to 1/2000. Acute ureteric colic in pregnancy is associated with significant potential risks to both mother and fetus. Significant anatomic and functional changes occur in pregnancy which not only lead to stone formation but also create diagnostic dilemma. The diagnosis of ureteric calculi can be incorrect in about 28% of pregnant patients.

Main body

Management of ureteric stone during pregnancy is remaining to be a challenge for the treating urologist. Because of the inability to use good imaging options for the diagnosis confirmation and more invasive approach for the treatment, management continues to be difficult. The main threats are preterm labor with delivery and premature rupture of membranes. Other pregnancy complications are obstructive uropathy, gestational diabetes mellitus, recurrent abortions and pre-eclampsia. Management of diagnosed ureteric stone is unique in the pregnant population and requires multi-disciplinary care. It should be individualized for each patient and moves preferably from conservative to invasive approaches sequentially. With continued advancements in endourological techniques, few definitive treatment options are also available for such patients.

Conclusion

There are several lacunae related with the diagnostic imaging, medical expulsive therapy, reliability of ureteral stent/percutaneous nephrostomy insertions and safety of ureteroscopy during pregnancy. Herein, we review the management of ureteric stone during pregnancy, the various diagnostic modalities and treatment options with their advantages and disadvantages. We also proposed our management algorithm to deal with such clinical scenario in this particular population.

Surveillance Practice for Sonographic Detection of Intracranial Abnormalities in Premature Neonates: A Snapshot of Current Neonatal Cranial Ultrasound Practice in Australia

There are no publications reporting on scan duration and Doppler use during neonatal cranial ultrasound scans. We investigated current practice of neonatal cranial ultrasound at four large tertiary neonatal intensive care units in Australia. Cranial scans were prospectively recorded between March 2015 and November 2016. Variables, including total number of scans, scan duration and frequency and duration of colour and spectral Doppler mode, were extracted. A total of 196 scans formed the final cohort. The median (range) number of scans for each neonate was 1 (1-12). The median (range) overall total scan duration was 309 (119-801) s. Colour mode with or without spectral Doppler mode was used in approximately half of the cohort (106/196, 54%). Our findings comport with our hypotheses. Operators performing neonatal cranial scans in Australia have low overall scan durations. Although the use of Doppler mode during neonatal cranial scans is not standard practice in all neonatal intensive care units, it is used widely irrespective of the degree of prematurity or the presence of brain pathology. Further efforts are required to incorporate recommendations on scan duration and the routine use of Doppler mode during neonatal cranial scans. This is especially imperative given that the most vulnerable neonates with the greater neural tissue sensitivity are likely to be scanned more often.

Tissue-mimicking thermochromic phantom for characterization of HIFU devices and applications

PURPOSE

Tissue-mimicking phantoms (TMPs) are synthetic materials designed to replicate properties of biological tissues. There is a need to quantify temperature changes following ultrasound or magnetic resonance imaging-guided high intensity focused ultrasound (MR-HIFU). This work describes development, characterization and evaluation of tissue-mimicking thermochromic phantom (TMTCP) for direct visualization and quantification of HIFU heating. The objectives were to (1) develop an MR-imageable, HIFU-compatible TMTCP that reports absolute temperatures, (2) characterize TMTCP physical properties and (3) examine TMTCP color change after HIFU.

METHODS AND MATERIALS

A TMTCP was prepared to contain thermochromic ink, silicon dioxide and bovine serum albumin (BSA) and its properties were quantified. A clinical MRI-guided and a preclinical US-guided HIFU system were used to perform sonications in TMTCP. MRI thermometry was performed during HIFU, followed by T2-weighted MRI post-HIFU. Locations of color and signal intensity change were compared to the sonication plan and to MRI temperature maps.

RESULTS

TMTCP properties were comparable to those in human soft tissues. Upon heating, the TMTCP exhibited an incremental but permanent color change for temperatures between 45 and 70 °C. For HIFU sonications the TMTCP revealed spatially sharp regions of color change at the target locations, correlating with MRI thermometry and hypointense regions on T2-weighted MRI. TMTCP-based assessment of various HIFU applications was also demonstrated.

CONCLUSIONS

We developed a novel MR-imageable and HIFU-compatible TMTCP to characterize HIFU heating without MRI or thermocouples. The HIFU-optimized TMTCP reports absolute temperatures and ablation zone geometry with high spatial resolution. Consequently, the TMTCP can be used to evaluate HIFU heating and may provide an in vitro tool for peak temperature assessment, and reduce preclinical in vivo requirements for clinical translation.

Ultrasound-Enhanced Ciclopirox Delivery for Treatment of Onychomycosis

The study aim was to determine ultrasound's efficacy in increasing the permeability of the nail in order to improve treatment outcomes in onychomycosis. Three sets of ultrasound experiments were performed - the luminosity experiment and two sets of diffusion cell experiments. The luminosity experiments assessed dye levels inside the nail after ultrasound application as compared to sham treatments, and the diffusion cell experiments compared changes in nail permeability due to the application of ultrasound. All in vitro experiments used planar ultrasound transducers, frequencies of 400 kHz, 600 kHz, 800 kHz, and 1 MHz, an intensity of 1 W/$cm^{2}$ and a duration of 5 min in a continuous mode. The safety of applying ultrasound to the toe was assessed by performing modeling studies. It was found that application of ultrasound at higher frequencies (800 kHz and 1 MHz) resulted in more (and statistically significant) permeation of the nail, as compared to the control trials.

No. 359-Obstetric Ultrasound Biological Effects and Safety

OBJECTIVE

To review the biological effects and safety of obstetric ultrasound.

OUTCOME

Outline the circumstances in which safety may be a concern with obstetric ultrasound.

EVIDENCE

The 2005 version of this guideline was used as a basis and updated following a Medline search and review of relevant publications. Sources included guidelines and reports by Health Canada and the American Institute of Ultrasound in Medicine.

VALUES

Review by principal author and the Diagnostic Imaging Committee of the SOGC. The quality of evidence and classification of recommendations have been adapted from the Report of the Canadian Task Force on the Periodic Health Examination.

BENEFITS, HARMS, AND COSTS

Obstetric ultrasound should only be done for medical reasons, and exposure should be kept as low as reasonably achievable because of the potential for tissue heating. Higher energy is of particular concern in the following scenarios: Doppler studies (pulsed, colour, and power), first trimester ultrasound with a long trans-vesical path (>5 cm), second or third trimester exams when bone is in the focal zone, when scanning tissue with minimal perfusion (embryonic), or in patients who are febrile. Operators can minimize risk by limiting dwell time and exposure to critical structures. It is also important to be aware of equipment-generated exposure information.

RECOMMENDATIONS

Low-intensity ultrasound neuromodulation: An overview of mechanisms and emerging human applications

BACKGROUND

There is an emerging need for noninvasive neuromodulation techniques to improve patient outcomes while minimizing adverse events and morbidity. Low-intensity focused ultrasound (LIFUS) is gaining traction as a non-surgical experimental approach of modulating brain activity. Several LIFUS sonication parameters have been found to potentiate neural firing, suppress cortical and epileptic discharges, and alter behavior when delivered to cortical and subcortical mammalian brain regions.

OBJECTIVE

This review introduces the elements of an effective sonication protocol and summarizes key preclinical studies on LIFUS as a neuromodulation modality. The state of the art in human ultrasound neuromodulation is then comprehensively summarized, and current hypotheses regarding the underlying mechanism of action on neural activity are presented.

METHODS

Peer-reviewed literature on human ultrasound neuromodulation was obtained by searching several electronic databases. The abstracts of all reports were read and publications which examined low-intensity transcranial ultrasound applied to human subjects were selected for review.

RESULTS

LIFUS can noninvasively influence human brain activity by suppressing cortical evoked potentials, influencing cortical oscillatory dynamics, and altering outcomes of sensory/motor tasks compared to sham sonication. Proposed mechanisms include cavitation, direct effects on neural ion channels, and plasma membrane deformation.

CONCLUSIONS

Though optimal sonication paradigms and transcranial delivery methods are still being established, future applications may include non-invasive human brain mapping experiments, and nonsurgical treatments for functional neurological disorders.

Ultrasound-Enhanced Drug Delivery for Treatment of Onychomycosis

OBJECTIVES

The aim of our study was to determine the effectiveness of using ultrasound (US) to increase the permeability of the nail, with the goal of improving outcomes in the treatment of onychomycosis.

METHODS

Porcine nails were used because of their similarity to human nails. A hydrophilic blue dye was used as a drug-mimicking compound. Two sets of experiments were performed: luminosity experiments to assess the dye levels inside the nail after US and sham treatments and diffusion cell experiments for determination of changes in nail permeability due to US application. In both sets of experiments, planar US transducers were used to sonicate the nails at frequencies of 400, 600, and 800 kHz and 1 MHz, an intensity of 1 W/cm² , and a duration of 5 min in a continuous mode. Modeling studies were also performed to assess the safety of US application to the human toe for later clinical studies.

RESULTS

In the luminosity experiments, application of US at frequencies of 600 and 800 kHz led to statistically significant results (P < .05), with an increase in dye delivery into the nail of up to 95% compared to control values. The diffusion cell results found statistical significance (P < .05) at all applied frequencies, with up to a 70% increase in the nail permeability compared to the control. Safety modeling studies found a maximal temperature increase of 4.4 °C in the bone.

CONCLUSIONS

Our proposed US method may offer an alternative for improved treatment of onychomycosis. The current maximal temperature increase was found to be at the safety limit, and so pulsing and other alternatives will be investigated to minimize this temperature increase.

Knowledge of Safety, Training, and Practice of Neonatal Cranial Ultrasound: A Survey of Operators

OBJECTIVES

Ultrasound can lead to thermal and mechanical effects in interrogated tissues. This possibility suggests a potential risk during neonatal cranial ultrasound examinations. The aim of this study was to explore safety knowledge and training of neonatal cranial ultrasound among Australian operators who routinely perform these scans.

METHODS

An online survey was administered on biosafety and training in neonatal cranial ultrasound, targeting all relevant professionals who can perform neonatal cranial ultrasound examinations in Australia: namely, radiologists, neonatologists, sonographers, and pediatricians. The survey was conducted between November 2013 and May 2014.

RESULTS

A total of 282 responses were received. Twenty of 208 (10%) answered all ultrasound biosafety questions correctly, and 49 of 169 (29%) correctly defined the thermal index. Two-thirds (134 of 214 [63%]) of respondents failed to recognize that reducing the overall scanning time is the most effective method of reducing the total power exposure. Only 13% (31 of 237) indicated that a predetermined fixed period of training or that a specified minimum number of supervised scans was used during training. The reported number of supervised scans during training was highly variable. Almost half of the participants (82 of 181 [45%]) stated that they had received supervision for 10 to 50 scans (median, 20 scans).

CONCLUSIONS

There is a need to educate operators on biosafety issues and approaches to minimize power outputs and reduce the overall duration of cranial ultrasound scans. Development of standardized training requirements may be warranted.

Measured acoustic intensities for clinical diagnostic ultrasound transducers and correlation with thermal index

OBJECTIVES

To investigate if the thermal index for bone (TIB) displayed on screen is an adequate predictor for the derated spatial-peak temporal-average (I_SPTA.3 ) and spatial-peak pulse-average (I_SPPA.3 ) acoustic intensities in a selection of clinical diagnostic ultrasound machines and transducers.

METHODS

We calibrated five clinical diagnostic ultrasound scanners and 10 transducers, using two-dimensional grayscale, color Doppler and pulsed-wave Doppler, both close to and far from the transducer, with a TIB between 0.1 and 4.0, recording 103 unique measurements. Acoustic measurements were performed in a bespoke three-axis computer-controlled scanning tank, using a 200-μm-diameter calibrated needle hydrophone.

RESULTS

There was significant but poor correlation between the acoustic intensities and the on-screen TIB. At a TIB of 0.1, the I_SPTA.3 range was 0.51-50.49 mW/cm² and the I_SPPA.3 range was 0.01-207.29 W/cm² . At a TIB of 1.1, the I_SPTA.3 range was 19.02-309.44 mW/cm² and the I_SPPA.3 range was 3.87-51.89 W/cm² .

CONCLUSIONS

TIB is a poor predictor for I_SPTA.3 and I_SPPA.3 and for the potential bioeffects of clinical diagnostic ultrasound scanners. Copyright © 2016 ISUOG. Published by John Wiley & Sons Ltd.

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.

Measurement of Thermal Effects of Doppler Ultrasound: An In Vitro Study

Objective

Ultrasound is considered a safe imaging modality and is routinely applied during early pregnancy. However, reservations are expressed concerning the application of Doppler ultrasound in early pregnancy due to energy emission of the ultrasound probe and its conversion to heat. The objective of this study was to evaluate the thermal effects of emitted Doppler ultrasound of different ultrasound machines and probes by means of temperature increase of in-vitro test-media.

Methods

We investigated the energy-output of 5 vaginal and abdominal probes of 3 ultrasound machines (GE Healthcare, Siemens, Aloka). Two in-vitro test objects were developed at the Center for Medical Physics and Biomedical Engineering, Medical University Vienna (water bath and hydrogel bath). Temperature increase during Doppler ultrasound emission was measured via thermal sensors, which were placed inside the test objects or on the probes’ surface. Each probe was emitting for 5 minutes into the absorbing test object with 3 different TI/MI settings in Spectral Doppler mode.

Results

During water bath test, temperature increase varied between 0.1 and 1.0°C, depending on probe, setting and focus, and was found highest for spectral Doppler mode alone. Maximum temperature increase was found during the surface heating test, where values up to 2.4°C could be measured within 5 minutes of emission.

Conclusions

Activation of Doppler ultrasound in the waterbath model causes a significant increase of temperature within one minute. Thermally induced effects on the embryo cannot be excluded when using Doppler ultrasound in early pregnancy.

Prenatal exposure to ultrasound affects learning and memory in young rats

Prenatal exposure to ultrasound may cause cognitive impairments in experimental animals; however, the exact mechanisms remain unknown. In this study, we exposed pregnant rats (or sham-exposed controls) to different intensities of ultrasound repeatedly on days 6, 12 and 18 of pregnancy for 4 min (3.5 MHz, spatial peak time average intensity = 7.6 mW/cm(2), mechanical index = 0.1, thermal index bone = 0.1: 4-min group) or 20 min (3.5 MHz, spatial peak time average intensity = 106 mW/cm(2), mechanical index = 1.4, thermal index bone = 1.0: 20-min group). The Morris water maze was used to assess learning and memory function in pups at 2 mo of age. Noticeable deficits in behavior occurred in the group exposed to ultrasound for 20 min. Using real-time polymerase chain reaction and Western blot, we also determined that both the mRNA and protein expression levels of hippocampal N-methyl-D-aspartate (NMDA) receptor units 1 (NR1) and 2B (NR2B) and brain-derived neurotrophic factor (BDNF) were significantly lower in pups exposed to ultrasound for 20 min than in controls. Furthermore, the morphology of the synapses in the hippocampus was partially damaged. Compared with the control group, the 4-min group had better spatial learning and memory abilities, as well as higher mRNA and protein levels of NR1, NR2B and BDNF. Our study suggests that high-intensity ultrasound irradiation can decrease learning and memory abilities by reducing the expression of NR1, NR2B and BDNF in the hippocampal regions and damaging the structure of synapses. In contrast, low-intensity ultrasound irradiation can enhance the learning and memory abilities of the offspring rats by increasing the expression of NR1, NR2B and BDNF receptor in the hippocampal regions.

Ultrasound safety in early pregnancy: reduced energy setting does not compromise obstetric Doppler measurements

OBJECTIVES

We hypothesized that first-trimester Doppler ultrasonography can be carried out at lower output energies than the currently advocated limits without compromising clinically important information.

METHODS

We recruited 42 pregnant women for an ultrasound examination at 12 weeks' gestation. Twenty-one women were examined with a transvaginal transducer, the rest with a transabdominal transducer. We used pulsed Doppler to measure pulsatility index (PI) and peak systolic velocity (PSV) in five clinically relevant fetal and maternal blood vessels. The energy indicator thermal index for bone (TIb) was set at 1.0, 0.5 and 0.1. Each measurement was repeated three times. A mixed linear regression model accounting for correlation between measurements was used to assess the effect of different TIb levels and transducers.

RESULTS

We were able to visualize the vessels by color Doppler and measure PI and PSV in all vessels at all energy levels in all the participants with the exception of the ductus venosus in two participants, yielding 1872 recordings for statistical analysis. A reduction in TIb from 1.0 to 0.5 and 0.1 had no effect on the PI or PSV values, nor was there any trend towards higher parameter variance with decreasing TIb. There was no difference between measured values of PI and PSV between the transducers, but the transabdominal technique was associated with a greater parameter variance.

CONCLUSION

Reliable first-trimester Doppler data can be obtained with output energy reduced to a TIb of 0.5 or 0.1.

Improving image quality of diagnostic ultrasound by using the safe use time model with the dynamic safety factor and the effect of the exposure time on the image quality

Resolution and penetration are primary criteria for image quality of diagnostic ultrasound. In theory (and usually in practice), the maximum depth of imaging in a tissue increases as power (pressure) is increased. Alternatively, at a particular effective penetration, an increased power may be used to allow a higher ultrasound frequency for higher resolution and tissue contrast. Recently, Karagoz and Kartal proposed a safety parameter for thermal bioeffects of diagnostic ultrasound; that is, SUT (safe use time). The SUT model is constructed to determine how long one piece of tissue can be insonated safely according to a threshold exposure. Also, Karagoz and Kartal suggested that an increase in acoustic intensity beyond the current US Food and Drug Administration (FDA) limit of intensity can be theoretically possible by using SUT model while staying within the safe limit. The present study was motivated particularly by the goals of higher resolution and/or deeper penetration by using SUT model. The results presented here suggest that the safe use of higher exposure levels than currently allowed by the FDA may be possible for obtaining substantial improvements in penetration depth and/or resolution. Also, the study reveals that image quality can be functionally related to exposure time in addition to acoustic energy and frequency.

Ultrasound bioeffects and safety

The main mechanisms by which ultrasound can induce biological effects as it passes through the body are thermal and mechanical in nature. The mechanical effects are primarily related to the presence of gas, whether drawn out of solution by the negative going ultrasound pressure wave (acoustic cavitation), a naturally occurring gas body (such as lung alveoli), or deliberately introduced into the blood stream to increase imaging contrast (microbubble contrast agents). Observed biological effects are discussed in the context of these mechanisms and their relevance to ultrasound safety is discussed.

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.

Temperature measurement by thermal strain imaging with diagnostic power ultrasound, with potential for thermal index determination

Over the years, there has been a substantial increase in acoustic exposure in diagnostic ultrasound as new imaging modalities with higher intensities and frame rates have been introduced; and more electronic components have been packed into the probe head, so that there is a tendency for it to become hotter. With respect to potential thermal effects, including those which may be hazardous occurring during ultrasound scanning, there is a correspondingly growing need for in vivo techniques to guide the operator as to the actual temperature rise occurring in the examined tissues. Therefore, an in vivo temperature estimator would be of considerable practical value. The commonly-used method of tissue thermal index (TI) measurement with a hydrophone in water could underestimate the actual value of TI (in one report by as much as 2.9 times). To obtain meaningful results, it is necessary to map the temperature elevation in 2-D (or 3-D) space. We present methodology, results and validation of a 2-D spatial and temporal thermal strain ultrasound temperature estimation technique in phantoms, and its apparently novel application in tracking the evolution of heat deposition at diagnostic exposure levels. The same ultrasound probe is used for both transmission and reception. The displacement and thermal strain estimation methods are similar to those used in high-intensity focused ultrasound thermal monitoring. The use of radiofrequency signals permits the application of cross correlation as a similarity measurement for tracking feature displacement. The displacement is used to calculate the thermal strain directly related to the temperature rise. Good agreement was observed between the temperature rise and the ultrasound power and scan duration. Thermal strain up to 1.4% was observed during 4000-s scan. Based on the results obtained for the temperature range studied in this work, the technique demonstrates potential for applicability in phantom (and possibly in vivo tissue) temperature measurement for the determination of TI.

Pulsed high intensity focused ultrasound mediated nanoparticle delivery: mechanisms and efficacy in murine muscle

High intensity focused ultrasound (HIFU) is generally thought to interact with biological tissues in two ways: hyperthermia (heat) and acoustic cavitation. Pulsed mode HIFU has recently been demonstrated to increase the efficacy of a variety of drug therapies. Generally, it is presumed that the treatment acts to temporarily increase the permeability of the tissue to the therapeutic agent, however, the precise mechanism remains in dispute. In this article, we present evidence precluding hyperthermia as a principal mechanism for enhancing delivery, using a quantitative analysis of systemically administered fluorescent nanoparticles delivered to muscle in the calves of mice. Comparisons were carried out on the degree of enhancement between an equivalent heat treatment, delivered without ultrasound, and that of the pulsed-HIFU itself. In the murine calf muscle, Pulsed-HIFU treatment resulted in a significant increase in distribution of 200 nm particles (p < 0.016, n = 6), while the equivalent thermal dose showed no significant increase. Additional studies using this tissue/agent model also demonstrated that the pulsed HIFU enhancing effects persist for more than 24 h, which is longer than that of hyperthermia and acoustic cavitation, and offers the possibility of a novel third mechanism for mediating delivery.

Fetal thermal effects of diagnostic ultrasound

Processes that can produce a biological effect with some degree of heating (ie, about 1 degrees C above the physiologic temperature) act via a thermal mechanism. Investigations with laboratory animals have documented that pulsed ultrasound can produce elevations of temperature and damage in biological tissues in vivo, particularly in the presence of bone (intracranial temperature elevation). Acoustic outputs used to induce these adverse bioeffects are within the diagnostic range, although exposure times are usually considerably longer than in clinical practice. Conditions present in early pregnancy, such as lack of perfusion, may favor bioeffects. Thermally induced teratogenesis has been shown in many animal studies, as well as several controlled human studies; however, human studies have not shown a causal relationship between diagnostic ultrasound exposure during pregnancy and adverse biological effects to the fetus. All human epidemiologic studies, however, were conducted with commercially available devices predating 1992, that is, with acoustic outputs not exceeding a spatial-peak temporal-average intensity of 94 mW/cm2. Current limits in the United States allow a spatial-peak temporal-average intensity of 720 mW/cm2 for fetal applications. The synergistic effect of a raised body temperature (febrile status) and ultrasound insonation has not been examined in depth. Available evidence, experimental or epidemiologic, is insufficient to conclude that there is a causal relationship between obstetric diagnostic ultrasound exposure and obvious adverse thermal effects to the fetus. However, very subtle effects cannot be ruled out and indicate a need for further research, although research in humans may be extremely difficult to realize.

Quantification of risk from fetal exposure to diagnostic ultrasound

Biomedical ultrasound may induce adverse effects in patients by either thermal or non-thermal means. Temperatures above normal can adversely affect biological systems, but effects also may be produced without significant heating. Thermally induced teratogenesis has been demonstrated in many animal species as well as in a few controlled studies in humans. Various maximum 'safe' temperature elevations have been proposed, although the suggested values range from 0.0 to 2.5 degrees C. Factors relevant to thermal effects are considered, including the nature of the acoustic field in situ, the state of perfusion of the embryo/fetus, and the variation of sensitivity to thermal insult with gestational stage of development. Non-thermal mechanisms of action considered include acoustic cavitation, radiation force, and acoustic streaming. While cavitation can be quite destructive, it is extremely unlikely in the absence of stabilized gas bodies, and although the remaining mechanisms may occur in utero, they have not been shown to induce adverse effects. For example, pulsed, diagnostic ultrasound can increase fetal activity during exposure, apparently due to stimulation of auditory perception by radiation forces on the fetal head or auditory structures. In contrast, pulsed ultrasound also produces vascular damage near developing bone in the late-gestation mouse, but by a unknown mechanism and at levels above current US FDA output limits. It is concluded that: (1) thermal rather than nonthermal mechanisms are more likely to induce adverse effects in utero, and (2) while the probability of an adverse thermal event is usually small, under some conditions it can be disturbingly high.

Evaluation of nonscanned mode soft-tissue thermal index in the presence of the residual temperature rise

Previously, the temperature rise (deltaT) caused by diagnostic ultrasound and the AUIM/NEMA-defined thermal indices were examined to evaluate whether these indices were reasonable indicators of potential bioeffects due to ultrasound heating in the absence of a residual temperature rise (RTR). In our study, deltaT induced by diagnostic ultrasound exposures was estimated in the presence of an RTR using the Bioheat Transfer Equation. To evaluate deltaT/TIS in the presence of an RTR, 11 frequencies, eight cooling times, eight insonation times for the second ultrasound examination, and three source powers for a circular aperture (A(aprt)< or = 1 cm2) were investigated. In our comparison of the ratios of deltaT/TIS in the absence and presence of an RTR, a higher deltaT/TIS value was obtained in the examination with the RTR. We showed that the deltaT/TIS value is equal to 2.88 in the presence of an RTR, whereas the deltaT/TIS value without the RTR equals 1.90. In the presence of the RTR, although the TIS does not inform the user of higher ultrasound heating due to TIS values that do not exceed 1.00, deltaT reaches 2.62 degrees C, and the deltaT without the RTR reaches 1.68 degrees C in the case of a TIS value that does not exceed 1.00. These results suggest that, for nonscanned mode situations where soft tissue is insonated, the TIS should not be regarded as a reliable indicator of potential bioeffects due to ultrasound heating in the presence of the RTR. Our study also indicates the necessity for a new indicator that provides the clinical user with accurate in vivo temperature rise feedback (possibly even true deltaT), and includes adding an exposure time component to the Bio-Heat Equation model.

The effects of residual temperature rise on ultrasound heating

In recent theoretical studies, the temperature rise produced by diagnostic ultrasound was estimated by solving the Bioheat Transfer Equation (BHTE) but ignoring the initial temperature rise. The temperature rise was determined in our study by the BHTE including an initial temperature rise. We discuss how the initial temperature rise occurs during an ultrasound examination, and how the initial temperature rise affects subsequent ultrasound heating. We theoretically show that the temperature rise produced by the ultrasound examination (exposure time of 500 s) in a tissue sample having an initial temperature rise was higher than that in a tissue sample with no initial temperature rise that was exposed to ultrasound (exposure time of 1200 s). The theoretical results for these two cases were 5.64 degrees C and 3.58 degrees C, respectively. In our experimental study, the highest temperature rise was measured in the presence of an initial temperature rise as in the theoretical study under the same exposure conditions. Mean temperature rises for tissue without an initial temperature rise and for tissue with an initial temperature rise were 2.42 +/- 0.13 degrees C and 3.62 +/- 0.17 degrees C, respectively. Both theoretical and experimental studies show that unless the initial temperature rise produced by the first ultrasound examination decreases to 0 degrees C, the next ultrasound examination on the same tissue sample may cause the temperature rise to be higher than expected.

Obstetric Ultrasound Biological Effects and Safety

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Thermal assessment of 40-MHz ultrasound at soft tissue-bone interfaces

Tissue exposure to diagnostic ultrasound (US) can cause significant temperature rises. However, little has been reported on thermal effects of high-frequency US, and guidelines for the use of US do not necessarily apply to higher frequencies. Temperature rise induced by US biomicroscopy (UBM) was measured in phantoms containing mouse skulls and in anesthetized mice and mice post mortem, with a 50-microm K-type thermocouple. The operating frequency was 40 MHz with a free field I(SPTA) of 2.6 mW/cm(2) (B-mode) and 11.9 W/cm(2) (Doppler). Peak negative pressures were 5.22 MPa (B mode) and 7.32 MPa (Doppler), resulting in a mechanical index (MI) of 0.83 (B-mode) and 1.05 (Doppler mode). In Doppler mode, mean temperature rises of 1.80 degrees C and 1.73 degrees C were measured for proximal and distal skull phantom surfaces after a 3-min insonation. In vivo, the proximal mouse skull surface showed a mean temperature rise of 2.1 degrees C, with no statistically significant differences post mortem. Our results indicate temperature rise from insonation of bone interfaces using similar exposure parameters should not cause adverse bioeffects.

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.

Human brain temperature in vivo: lack of heating during color transcranial Doppler ultrasonography

This study was undertaken to assess the effect of ultrasound on human brain temperature in vivo. The investigation consisted of direct recording of intracranial temperature during color transcranial Doppler (TCD) sonography in a neurosurgical patient. The temperature was recorded from 3 thermocouples. One was implanted together with an intracranial pressure sensor into a surgically reduced intraparenchymal hematoma, the second was placed within the subdural space close to the temporal acoustic window, and the third was located extracranially at the outer surface of the temporal bone. Tympanic temperatures were also measured to give an approximation of global brain temperature. A 2.5-MHz transducer was used, and the system settings were as follows: spatial peak temporal average intensity = 234 mW/cm2 in B-mode at a maximum power of 32.3 mW and 2132 mW/cm2 in Doppler mode at a maximum power of 149.3 mW. Neither increase in the intraparenchymal brain temperature nor increase in the temperature at the bone/soft tissue interface was observed during 30 minutes of insonation. The ipsilateral tympanic temperature increased by only 0.06 degree C, and this value may be regarded as a measure of the overall increase in brain temperature. Passive cooling effect produced by the transducer, which was at ambient temperature, was found to reach the brain surface and to surpass any possible heating caused by the ultrasound. The results indicate that no noticeable increases in human brain temperature occur in response to ultrasound emitted by a color TCD device at high transmitter power settings within the diagnostic range.

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.

Biological effects of ultrasound: development of safety guidelines. Part I: personal histories

After the end of World War II, advances in ultrasound (US) technology brought improved possibilities for medical applications. The first major efforts in this direction were in the use of US to treat diseases. Medical studies were accompanied by experiments with laboratory animals and other model systems to investigate basic biological questions and to obtain better understanding of mechanisms. Also, improvements were made in methods for measuring and controlling acoustical quantities such as power, intensity and pressure. When diagnostic US became widely used, the scope of biological and physical studies was expanded to include conditions for addressing relevant safety matters. In this historical review, a major part of the story is told by 21 investigators who took part in it. Each was invited to prepare a brief personal account of his/her area(s) of research, emphasizing the "early days," but including later work, showing how late and early work are related, if possible, and including anecdotal material about mentors, colleagues, etc.

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).