Assessment of thermal index compliance in clinical ultrasound examinations

Introduction

The thermal index (TI) is critical for monitoring potential heating risks in ultrasound examinations. This study aims to analyse adherence to TI guidelines in clinical ultrasound exams, focusing on the thermal index for soft tissue (TIS) and thermal index for bone at the focus (TIB).

Methods

Clinical ultrasound images over a five working day period from a single NHS University Hospital Trust were analysed using a custom MATLAB script. TI values were extracted from DICOM headers, supplemented by optical character recognition for missing values. Time at each TI value (image interval) was measured assuming a constant TI between consecutive images. TI values and corresponding image intervals were compared to TI guidelines provided by the British Medical Ultrasound Society (BMUS).

Results

Analysis included 627 obstetric and 1023 non-obstetric examinations, comprising 4643 obstetric and 19,228 non-obstetric image intervals. TIS values across all examinations and image intervals adhered to recommended BMUS limits; however, breaches of the guidelines were observed for TIB. In obstetrics, 44 image intervals across 47 examinations exceeded the TIB limits (1.01% of TIB image intervals and 7.02% of examinations). For non-obstetrics, two image intervals across two examinations exceeded the TIB limits (0.02% of intervals and 0.44% of examinations).

Conclusion

The majority of ultrasound examinations considered for this analysis demonstrated compliance with BMUS guidelines for TI, except for a small fraction which showed breaches in the obstetric thermal index for bone at the focus limits. All breaches were associated with the use of pulsed-wave Doppler.

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.

Brain injury mobile diagnostic system: Applications in civilian medical service and on the battlefield-General concept and medical aspects

To present the concept of a portable ultrasound tomography device for diagnosing traumatic and vascular brain lesions. The device consisting of multiple transcranial ultrasound probes placed on the surface of the head, specifically but not exclusively in natural acoustic windows. An integral part of the mobile diagnostic system (MDS) is a decision support system based on artificial intelligence algorithms utilizing information from: head images, laboratory data, and assessment of the patient's clinical condition. The MDS can significantly reduce the time from stroke onset to rtPA therapy in civilian medical services and support therapeutic and evacuation strategies in instances of brain and skull trauma on the battlefield.

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.

Spectral Shift Originating from Non-linear Ultrasonic Wave Propagation and Its Effect on Imaging Resolution

An amplitude-dependent downshift in the fundamental wave spectrum of a propagating ultrasonic pulse caused by non-linear wave propagation is described. The effects of non-linearity and the associated downshift on spatial resolution are also studied. The amounts of downshift and spatial resolution are extracted from the numerically simulated beam profile based on the KZK equation. Results for a 25-MHz transducer reveal that non-linear effects can lead to 58% additional downshift in the centre frequency of a pulse compared with a linear case with downshift caused only by attenuation. This additional downshift causes about 50% degradation in axial resolution. However, as the beam becomes narrower from the non-linear effects, the overall effect of non-linearity still leads to improved lateral resolution (≤26%). Therefore, as non-linearity increases with wave pressure, it is concluded that the increase in source pressure improves lateral resolution and degrades axial resolution.

Transcranial ultrafast ultrasound localization microscopy of brain vasculature in patients

Changes in cerebral blood flow are associated with stroke, aneurysms, vascular cognitive impairment, neurodegenerative diseases and other pathologies. Brain angiograms, typically performed via computed tomography or magnetic resonance imaging, are limited to millimetre-scale resolution and are insensitive to blood-flow dynamics. Here we show that ultrafast ultrasound localization microscopy of intravenously injected microbubbles enables transcranial imaging of deep vasculature in the adult human brain at microscopic resolution and the quantification of haemodynamic parameters. Adaptive speckle tracking to correct for micrometric brain-motion artefacts and ultrasonic-wave aberrations induced during transcranial propagation allowed us to map the vascular network of tangled arteries to functionally characterize blood-flow dynamics at a resolution of up to 25 μm and to detect blood vortices in a small deep-seated aneurysm in a patient. Ultrafast ultrasound localization microscopy may facilitate the understanding of brain haemodynamics and of how vascular abnormalities in the brain are related to neurological pathologies.

Hydrophone Spatial Averaging Correction for Acoustic Exposure Measurements From Arrays-Part I: Theory and Impact on Diagnostic Safety Indexes

This article reports underestimation of mechanical index (MI) and nonscanned thermal index for bone near focus (TIB) due to hydrophone spatial averaging effects that occur during acoustic output measurements for clinical linear and phased arrays. TIB is the appropriate version of thermal index (TI) for fetal imaging after ten weeks from the last menstrual period according to the American Institute of Ultrasound in Medicine (AIUM). Spatial averaging is particularly troublesome for highly focused beams and nonlinear, nonscanned modes such as acoustic radiation force impulse (ARFI) and pulsed Doppler. MI and variants of TI (e.g., TIB), which are displayed in real-time during imaging, are often not corrected for hydrophone spatial averaging because a standardized method for doing so does not exist for linear and phased arrays. A novel analytic inverse-filter method to correct for spatial averaging for pressure waves from linear and phased arrays is derived in this article (Part I) and experimentally validated in a companion article (Part II). A simulation was developed to estimate potential spatial-averaging errors for typical clinical ultrasound imaging systems based on the theoretical inverse filter and specifications for 124 scanner/transducer combinations from the U.S. Food and Drug Administration (FDA) 510(k) database from 2015 to 2019. Specifications included center frequency, aperture size, acoustic output parameters, hydrophone geometrical sensitive element diameter, etc. Correction for hydrophone spatial averaging using the inverse filter suggests that maximally achievable values for MI, TIB, thermal dose ( t ₄₃ ), and spatial-peak-temporal-average intensity ( [Formula: see text]) for typical clinical systems are potentially higher than uncorrected values by (means ± standard deviations) 9% ± 4% (ARFI MI), 19% ± 15% (ARFI TIB), 50% ± 41% (ARFI t ₄₃ ), 43% ± 39% (ARFI [Formula: see text]), 7% ± 5% (pulsed Doppler MI), 15% ± 11% (pulsed Doppler TIB), 42% ± 31% (pulsed Doppler t ₄₃ ), and 33% ± 27% (pulsed Doppler [Formula: see text]). These values correspond to frequencies of 3.2 ± 1.3 (ARFI) and 4.1 ± 1.4 MHz (pulsed Doppler), and the model predicts that they would increase with frequency. Inverse filtering for hydrophone spatial averaging significantly improves the accuracy of estimates of MI, TIB, t ₄₃ , and [Formula: see text] for ARFI and pulsed Doppler signals.

An Automated ALARA Method for Ultrasound: An Obstetric Ultrasound Feasibility Study

OBJECTIVES

Ultrasound users are advised to observe the ALARA (as low as reasonably achievable) principle, but studies have shown that most do not monitor acoustic output metrics. We developed an adaptive ultrasound method that could suggest acoustic output levels based on real-time image quality feedback using lag-one coherence (LOC).

METHODS

Lag-one coherence as a function of the mechanical index (MI) was assessed in 35 healthy volunteers in their second trimester of pregnancy. While imaging the placenta or the fetal abdomen, the system swept through 16 MI values ranging from 0.15 to 1.20. The LOC-versus-MI data were fit with a sigmoid curve, and the ALARA MI was selected as the point at which the fit reached 98% of its maximum.

RESULTS

In this study, the ALARA MI values were between 0.35 and 1.03, depending on the acoustic window. Compared to a default MI of 0.8, the pilot acquisitions suggested a lower ALARA MI 80% of the time. The contrast, contrast-to-noise ratio, generalized contrast-to-noise ratio, and LOC all followed sigmoidal trends with an increasing MI. The R2 of the fit was statistically significantly greater for LOC than the other metrics (P < .017).

CONCLUSIONS

These results suggest that maximum image quality can be achieved with acoustic output levels lower than the US Food and Drug Administration limits in many cases, and an automated tool could be used in real time to find the ALARA MI for specific imaging conditions. Our results support the feasibility of an automated, LOC-based implementation of the ALARA principle for obstetric ultrasound.

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.

Non-linear Wave Propagation and Safety Standards for Diagnostic Ultrasound Devices

Safety standards for clinical diagnostic ultrasonic devices were developed for use in relatively low-frequency systems (1-10 MHz), under the assumption that non-linear effects would be negligible. This article reviews ways in which neglecting non-linear wave propagation affects the measurements and calculations required to comply with safety standards and U.S. Food and Drug Administration guidance that recognizes these standards. An attempt is made to evaluate whether ignoring non-linear effects could result in significant error in the exposure quantities defined in these standards at either low or high frequencies, based on published literature. This article maintains that although non-linear effects have been considered in some parts of safety standards related to hydrophone requirements, the coverage is inadequate, especially for modern equipment with high working frequencies. A new approach is required to assess the magnitude of thermal heating for recently developed high-frequency systems to incorporate non-linear effects. In contrast, the current approach for evaluating the risk of cavitation can be used after appropriate modifications.

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.

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.

[Principles of Doppler sonography]

The techniques of medical Doppler are spectral Doppler (contiuous-wave (CW) and pulse-wave (PW) Doppler) and color flow imaging (Color Doppler and Power Doppler). All are based on the fact that the frequency of an echo from a moving reflecting particle will be altered by a characteristic frequency shift determined by its velocity in relation to the source/detector. The CW Doppler will only detect flow within a pre-defined depth and will not be guided by an image, whereas the PW Doppler is carried out with B-mode guidance (Duplex doppler). The so derived curves permit to assess the temporal distribution of flow velocities and directions and flow disturbances as well. In the case of color flow imaging, a part of the interrogated tissue section is mapped for Doppler signals and then color-coded, resulting in a dynamic color map of flow, where the colors encode characteristic flow parameters (e. g. mean flow velocity plus direction). This article describes the technical and physical basics of medical Doppler techniques.

Survey of current practice in clinical transvaginal ultrasound scanning in the UK

During transvaginal ultrasound scanning, the fetus and other sensitive tissues are placed close to the transducer. Heating of these tissues occurs by direct conduction from the transducer and by absorption of ultrasound in the tissue. The extent of any heating will depend on the equipment and settings used, the duration of the scan, imaging modes and other aspects of scanning practice. To ensure that scans are performed with minimum risk, staff should have an appropriate knowledge of safety and follow guidelines issued by professional bodies. An online survey aiming to document current practice in transvaginal ultrasound in the UK was created and distributed to individuals performing this type of scanning. The survey posed questions about the respondents, the departments where scans were performed, the equipment used, knowledge of ultrasound safety, scanning practice and the frequency, duration and mode of transvaginal ultrasound scans for gynaecology, obstetrics and fertility applications. In all, 294 responses were obtained, mostly from sonographers (94%). From the analysis of the responses, it was clear that there was a good understanding of the general meaning of thermal and mechanical index and high awareness of guidelines issued by professional bodies. However, 40% of respondents stated that they rarely or never monitor Thermal or Mechanical indices during scanning. Scanning practice was consistent in terms of the duration of scans, scan protocols followed and use of imaging modes. The results highlight the importance of continued ultrasound safety training and promotion of safety guidelines to users.

Conditionally Increased Acoustic Pressures in Nonfetal Diagnostic Ultrasound Examinations Without Contrast Agents: A Preliminary Assessment

The mechanical index (MI) has been used by the US Food and Drug Administration (FDA) since 1992 for regulatory decisions regarding the acoustic output of diagnostic ultrasound equipment. Its formula is based on predictions of acoustic cavitation under specific conditions. Since its implementation over 2 decades ago, new imaging modes have been developed that employ unique beam sequences exploiting higher-order acoustic phenomena, and, concurrently, studies of the bioeffects of ultrasound under a range of imaging scenarios have been conducted. In 2012, the American Institute of Ultrasound in Medicine Technical Standards Committee convened a working group of its Output Standards Subcommittee to examine and report on the potential risks and benefits of the use of conditionally increased acoustic pressures (CIP) under specific diagnostic imaging scenarios. The term "conditionally" is included to indicate that CIP would be considered on a per-patient basis for the duration required to obtain the necessary diagnostic information. This document is a result of that effort. In summary, a fundamental assumption in the MI calculation is the presence of a preexisting gas body. For tissues not known to contain preexisting gas bodies, based on theoretical predications and experimentally reported cavitation thresholds, we find this assumption to be invalid. We thus conclude that exceeding the recommended maximum MI level given in the FDA guidance could be warranted without concern for increased risk of cavitation in these tissues. However, there is limited literature assessing the potential clinical benefit of exceeding the MI guidelines in these tissues. The report proposes a 3-tiered approach for CIP that follows the model for employing elevated output in magnetic resonance imaging and concludes with summary recommendations to facilitate Institutional Review Board (IRB)-monitored clinical studies investigating CIP in specific tissues.

A theoretical study of inertial cavitation from acoustic radiation force impulse imaging and implications for the mechanical index

The mechanical index (MI) attempts to quantify the likelihood that exposure to diagnostic ultrasound will produce an adverse biological effect by a non-thermal mechanism. The current formulation of the MI implicitly assumes that the acoustic field is generated using the short pulse durations appropriate to B-mode imaging. However, acoustic radiation force impulse (ARFI) imaging employs high-intensity pulses up to several hundred acoustic periods long. The effect of increased pulse durations on the thresholds for inertial cavitation was studied computationally in water, urine, blood, cardiac and skeletal muscle, brain, kidney, liver and skin. The results indicate that, although the effect of pulse duration on cavitation thresholds in the three liquids can be considerable, reducing them by, for example, 6%-24% at 1 MHz, the effect on tissue is minor. More importantly, the frequency dependence of the MI appears to be unnecessarily conservative; that is, the magnitude of the exponent on frequency could be increased to 0.75. Comparison of these theoretical results with experimental measurements suggests that some tissues do not contain the pre-existing, optimally sized bubbles assumed for the MI. This means that in these tissues, the MI is not necessarily a strong predictor of the probability of an adverse biological effect.

Trends in diagnostic ultrasound acoustic output from data reported to the US Food and Drug Administration for device indications that include fetal applications

OBJECTIVES

A survey was conducted of acoustic output data received by the US Food and Drug Administration for diagnostic ultrasound devices whose indications for use include fetal applications to assess trends in maximum available acoustic output over time.

METHODS

Data were collected from 124 regulatory submissions received between 1984 and 2010. Data collection excluded transducers not indicated for diagnostic fetal imaging. The output parameters of ultrasonic power, mean center frequency, and bone thermal index (TIB) were extracted or computed from the submissions for 3 periods: 1984-1989, 1992-1997, and 2005-2010. The data were stratified according to the following imaging modes: M-mode, B/M-mode, pulsed wave Doppler, color flow Doppler, and continuous wave Doppler.

RESULTS

Ultrasonic power and maximum TIB values have increased roughly an order of magnitude from pre-1991 to post-1991 periods; the center frequency has decreased somewhat (4.2 to 3.4 MHz). The percentage of Doppler-mode transducers has increased substantially over time, with the majority of the diagnostic fetal imaging transducers currently designed to operate in Doppler modes; this increase is particularly important, since Doppler modes generate much higher TIB levels than B/M-modes. Color flow Doppler ultrasound currently operates at the highest mean ultrasonic power level (with a 14-fold increase over time).

CONCLUSIONS

The observed trends in increased acoustic output for both Doppler and non-Doppler modes underscore the widely recognized importance of adherence to the ALARA (as low as reasonably achievable) principle and prudent use in fetal ultrasound imaging.

Quantitative ultrasonic computed tomography using phase-insensitive pyroelectric detectors

The principle of using ultrasonic computed tomography (UCT) clinically for mapping tissue acoustic properties was suggested almost 40 years ago. Despite strong research activity, UCT been unable to rival its x-ray counterpart in terms of the ability to distinguish tissue pathologies. Conventional piezoelectric detectors deployed in UCT are termed phase-sensitive (PS) and it is well established that this property can lead to artefacts related to refraction and phase-cancellation that mask true tissue structure, particularly for reconstructions involving attenuation. Equally, it has long been known that phase-insensitive (PI) detectors are more immune to this effect, although sufficiently sensitive devices for clinical use have not been available. This paper explores the application of novel PI detectors to UCT. Their operating principle is based on exploiting the pyroelectric properties of the piezoelectric polymer polyvinylidene difluoride. An important detector performance characteristic which makes it particularly suited to UCT, is the lack of directionality of the PI response, relative to the PS detector mode of operation. The performance of the detectors is compared to conventional PS detection methods, for quantitatively assessing the attenuation distribution within various test objects, including a two-phase polyurethane phantom. UCT images are presented for a range of single detector apertures; tomographic reconstruction images being compared with the known structure of phantoms containing inserts as small as 3 mm, which were readily imaged. For larger diameter inserts (>10 mm), the transmitter-detector combination was able to establish the attenuation coefficient of the insert to within ±10% of values determined separately from plane-wave measurements on representative material plaques. The research has demonstrated that the new PI detectors are significantly less susceptible to refraction and phase-cancellation artefacts, generating realistic images in situations where conventionally-employed through-transmission PS detection techniques were unable to do so. The implications of the study to the potential screening of breast disease are discussed.

Should the mechanical index be revised for ARFI imaging?

The mechanical index (MI) quantifies the likelihood that exposure to diagnostic ultrasound will produce an adverse biological effect by a nonthermal mechanism. The current formulation of the MI is based on inertial cavitation thresholds in two liquids, water and blood, as calculated by a formalism assuming very short pulse durations. Although tissue contains a high proportion of water, it is not a liquid but a viscoelastic solid. Further, acoustic radiation force impulse imaging employs high-intensity pulses up to several hundred acoustic periods long. The effect of these differences was studied in water, blood and five representative tissues.

Ultrasonic imaging: safety considerations

Modern ultrasound imaging for diagnostic purposes has a wide range of applications. It is used in obstetrics to monitor the progress of pregnancy, in oncology to visualize tumours and their response to treatment, and, in cardiology, contrast-enhanced studies are used to investigate heart function and physiology. An increasing use of diagnostic ultrasound is to provide the first photograph for baby's album-in the form of a souvenir or keepsake scan that might be taken as part of a routine investigation, or during a visit to an independent high-street 'boutique'. It is therefore important to ensure that any benefit accrued from these applications outweighs any accompanying risk, and to evaluate the existing ultrasound bio-effect and epidemiology literature with this in mind. This review considers the existing laboratory and epidemiological evidence about the safety of diagnostic ultrasound and puts it in the context of current clinical usage.