A proposal to clarify the relationship between the thermal index and the corresponding risk to the patient

The thermal index (TI) displayed on the screens of most modern diagnostic ultrasound machines is linearly proportional to the absorbed power or, equivalently, to the in-situ intensity or temperature rise. Users are instructed to interpret the TI as a "relative indication of bioeffect risk." The thermal dose is a well-known empirical relationship between the temperature T of a biological system and the time t needed for that temperature to induce a deleterious effect. For any two temperatures, T1 and T2, and the corresponding times t1 and t2, required to produce the same level of effect, this general relation holds: t1/t2=RT2-T1, where R is the thermal normalization constant. Hence, it is experimentally determined that the rate of induction, or risk, of a thermal effect increases exponentially with temperature. Because exponential relationships are not intuitive to many users, there is a significant potential for underestimation of the thermal risk associated with exposure to diagnostic ultrasound. To better quantify this risk and thereby make the displayed information more useful, the current linear display of the calculated value of the thermal index, i.e., of TIcur, should be altered to an exponential form based on the thermal dose and representing the excess risk associated with the exposure: TInew=(RTIcur-1)/(R-1). This expression has the advantage that for the usual choice of R=4 for T<or=43 degrees C, TInew approximately TIcur in the range most often seen onscreen, i.e., TIcur<1.2, minimizing any confusion during a transition from TIcur to TInew. For the relatively rare but potentially much more serious circumstances when TIcur>3.5, the displayed TInew>>TIcur, consistent with empirical observations of the likelihood of harm. Additional advantages also obtain.

Nephron injury induced by diagnostic ultrasound imaging at high mechanical index with gas body contrast agent

The right kidney of anesthetized rats was imaged with intermittent diagnostic ultrasound (1.5 MHz; 1-s trigger interval) under exposure conditions simulating those encountered in human perfusion imaging. The rats were infused intravenously with 10 microL/kg/min Definity (Bristol-Myers Squibb Medical Imaging, Inc., N. Billerica, MA, USA) while being exposed to mechanical index (MI) values of up to 1.5 for 1 min. Suprathreshold MI values ruptured glomerular capillaries, resulting in blood filling Bowman's space and proximal convoluted tubules of many nephrons. The re-establishment of a pressure gradient after hemostasis caused the uninjured portions of the glomerular capillaries to resume the production of urinary filtrate, which washed some or all of the erythrocytes out of Bowman's space and cleared blood cells from some nephrons into urine within six hours. However, many of the injured nephrons remained plugged with tightly packed red cell casts 24 h after imaging and also showed degeneration of tubular epithelium, indicative of acute tubular necrosis. The additional damage caused by the extravasated blood amplified that caused by the original cavitating gas body. Human nephrons are virtually identical to those of the rat and so it is probable that similar glomerular capillary rupture followed by transient blockage and/or epithelial degeneration will occur after clinical exposures using similar high MI intermittent imaging with gas body contrast agents. The detection of blood in postimaging urine samples using standard hematuria tests would confirm whether or not clinical protocols need to be developed to avoid this potential for iatrogenic injury.

Towards evidence based emergency medicine: best BETs from the Manchester Royal Infirmary. Using the ultrasound compression test for deep vein thrombosis will not precipitate a thromboembolic event

A short cut review was carried out to establish whether patients with deep vein thrombosis (DVT) are at risk of embolism during ultrasound compression testing. No papers were found that directly answered the clinical question. The clinical bottom line is that currently there is no evidence to suggest that compressing vessels in order to identify a DVT could cause an embolic event. Therefore we can consider ultrasound assessment a safe reliable investigation for the diagnosis of DVT with no evidence of causing harm.

Blisters on the anterior shin in 3 research subjects after a 1-MHz, 1.5-W/cm , continuous ultrasound treatment: a case series

CONTEXT

Clinicians should consider multiple factors when estimating tissue-heating rates.

OBJECTIVE

To report 3 separate occurrences of blisters during an ultrasound treatment experiment.

BACKGROUND

While we were conducting a research experiment comparing the measurement capabilities of 2 different intramuscular temperature devices, 3 female participants (age = 26.33 +/- 3.79 years, height = 169.34 +/- 3.89 cm, mass = 63.39 +/- 3.81 kg) out of 16 healthy volunteers (7 men: age = 22.83 +/- 1.17 years, height = 170.61 +/- 7.77 cm, mass = 74.62 +/- 19.24 kg; 9 women: age = 24.22 +/- 2.73 years, height = 171.88 +/- 6.35 cm, mass = 73.99 +/- 18.55 kg) developed blisters on the anterior shin after a 1-MHz, 1.5-W/cm (2) continuous ultrasound treatment delivered to the triceps surae muscle.

DIFFERENTIAL DIAGNOSIS

Allergies; chemical reaction with cleaning agents; sunburn; negative interaction between the temperature measurement instruments and the ultrasound field; the ultrasound transducer not being calibrated properly, producing a nonuniform field and creating a hot spot or heating differently when compared with other ultrasound devices; the smaller anatomy of our female subjects; or a confounding interaction among these factors.

TREATMENT

Participants were given standard minor burn care by a physician.

UNIQUENESS

(1) The development of blisters on the anterior aspect of the shin as a result of an ultrasound treatment to the posterior aspect of the triceps surae muscle and (2) muscle tissue heating rates ranging from 0.19 degrees C to 1.1 degrees C/min, when ultrasound researchers have suggested tissue heating in the range of 0.3 degrees C/min with these settings.

CONCLUSIONS

These adverse events raise important questions regarding treatment application and potential differences in heating and quality control among different ultrasound devices from different manufacturers.

Enhancement of vascular permeability with low-frequency contrast-enhanced ultrasound in the chorioallantoic membrane model

PURPOSE

To characterize the effect of low-frequency contrast material-enhanced ultrasound on the vascular endothelium and to determine the parameters and techniques required to deliver a therapeutic agent by using the chorioallantoic membrane (CAM) model.

MATERIALS AND METHODS

All in vivo animal procedures were conducted with institutional Animal Care and Use Committee approval. Extravasation of 8.5-nm-diameter fluorescein isothiocyanate-labeled dextran was evaluated in the vasculature of a chick CAM model. Intravital microscopy was performed during contrast-enhanced ultrasound exposure (1.00 or 2.25 MHz); results were compared with results of electron microscopy of the insonated regions. Data acquired after insonation with greater mechanical stress (n = 30 animals) (mechanical index [MI] > 1.3) and with lower mechanical stress (n = 86 animals) (MI < 1.13) were compared with measurements in control conditions (n = 46 animals). The diameter of affected vessels; number of extravasation sites; extravasation rate, area, and location; and changes in endothelial cells and basement membrane were evaluated. Differences were tested with analysis of variance or the Student t test.

RESULTS

After ultrasound application, convective transport of the model drug was observed through micron-sized openings with a mean fluid velocity of 188.6 microm/sec in the low-stress class and 362.5 microm/sec in the high-stress class. Electron microscopy revealed micron-sized focal endothelial gaps and disseminated blebs, vacuoles, and filopodia extending across tens of microns. The threshold pressure for extravasation was 0.5 MPa for a transmitted center frequency of 1.00 MHz (MI = 0.5) and 1.6 MPa for a frequency of 2.25 MHz (MI = 1.06); thus, the frequency dependence of the threshold was not predicted simply by the MI.

CONCLUSION

Low-frequency contrast-enhanced ultrasound can increase vascular permeability and result in convective extravasation of an 8.5-nm-diameter model drug.

Evaluation of the threshold for lung hemorrhage by diagnostic ultrasound and a proposed new safety index

In a recent report (O'Brien et al. (2006b), it was suggested that the current expression for the mechanical index (MI) was not well suited to its function of quantifying the likelihood of an adverse biological effect after exposure of the gas-filled lung to diagnostic ultrasound. The purpose of this study was to analyze the relatively large database of experimental thresholds for the induction of lung hemorrhage to: (i) determine which variable(s) best describe the data and (ii) use the resulting equation to obtain a new formulation for the MI for lung exposures. Data from 14 studies of lung hemorrhage in four common laboratory animals (mouse, rat, rabbit and pig) were tabulated with regard to five common acoustic variables: center frequency (f(c)), pulse repetition frequency (PRF), pulse duration (PD), exposure duration (ED) and the threshold in situ peak rarefactional pressure (p(r)). The 34 threshold data points were fit by linear regression to: (i) a multiplicative model of the other variables, p(r) = Af(c)(B)PRF(C)PD(D)ED(E), where A is a constant; (ii) 14 "reduced" models in which one or more variables were not included in the analysis; (iii) four models in which a multiplicative combination of variables has a common name e.g., duty factor; and (iv) the general form of the current expression for the MI. The MI was shown to provide a poor fit to the threshold data (r(2) = 0.382), as were three of the four named models. The best fits were found for the complete model and for three reduced models, all of which contain the exposure duration. Because the implementation of a time-dependent safety parameter would present significant practical difficulties, a different model, p(r) = Af(c)(B)PRF(C)PD(D), was chosen as the basis for the new MI. Thus, the expression for the lung-specific mechanical index, MI(Lung), includes several, rather than only one, of the relevant acoustic variables. This is the first potential safety index developed as a direct result of experimental measurements rather than theoretical analysis.

Optical and acoustic monitoring of bubble cloud dynamics at a tissue-fluid interface in ultrasound tissue erosion

Short, high-intensity ultrasound pulses have the ability to achieve localized, clearly demarcated erosion in soft tissue at a tissue-fluid interface. The primary mechanism for ultrasound tissue erosion is believed to be acoustic cavitation. To monitor the cavitating bubble cloud generated at a tissue-fluid interface, an optical attenuation method was used to record the intensity loss of transmitted light through bubbles. Optical attenuation was only detected when a bubble cloud was seen using high speed imaging. The light attenuation signals correlated well with a temporally changing acoustic backscatter which is an excellent indicator for tissue erosion. This correlation provides additional evidence that the cavitating bubble cloud is essential for ultrasound tissue erosion. The bubble cloud collapse cycle and bubble dissolution time were studied using the optical attenuation signals. The collapse cycle of the bubble cloud generated by a high intensity ultrasound pulse of 4-14 micros was approximately 40-300 micros depending on the acoustic parameters. The dissolution time of the residual bubbles was tens of ms long. This study of bubble dynamics may provide further insight into previous ultrasound tissue erosion results.

Rapporteur report: Roundup, discussion and recommendations

There is a relative paucity of recent information regarding the long-term health effects associated with exposure to ultrasound, and to infrasound and low-frequency noise (LFN). For ultrasound, further epidemiological studies are recommended, and priority should be given to studies investigating the effects of handedness and to studies assessing possible subtle effects on brain function. These studies should reflect contemporary practises in diagnostic ultrasound and have sufficiently long follow-up periods to examine the possibility of effects into late adolescence or beyond. In the absence of a non-exposed control group, it would be advisable to make comparisons between a highly exposed group with a less exposed group, and to compare groups exposed at differing gestational stages. The effects associated with ultrasound contrast agents should also be studied, and the appropriateness of the thermal index (TI) and mechanical index (MI) should be reviewed. It is recommended that animal models should be used to investigate the effects of exposure at differing gestational ages on development, and modern cellular and molecular techniques used to investigate potential mechanisms of interaction. Although explicit morphological changes have been reported following occupational and experimental exposures to infrasound and low LFN, it was recommended that a thorough review of the relevant biological and health effects literature was necessary before specific proposals could be made. Uncertainties about the characterisation of these low frequencies also indicated the need to develop appropriate measurement techniques and protocols.

[Contemporary hygienic requirements to work conditions of medical professionals performing ultrasound investigations]

The article covers work conditions and health state of medical professionals performing ultrasound investigations on modern medical diagnostic equipment. The authors suggest a guidelines project including complex of measures to prevent unfavorable influence of contact ultrasound on medical professionals.

"Just scanning around" with diagnostic medical ultrasound: should states regulate the non-diagnostic uses of this technology?

This article examines how the non-diagnostic uses of medical ultrasound may violate the prudent use of this technology and supports the proposal of state-based legislative efforts to protect consumers from abuse. The author identifies the potential health risks to consumers and reviews the existing federal and state regulations, ultimately recommending increased legislation and mandated control of this technology.

Threshold estimation of ultrasound-induced lung hemorrhage in adult rabbits and comparison of thresholds in mice, rats, rabbits and pigs

The objective of this study was to assess the threshold and superthreshold behavior of ultrasound (US)-induced lung hemorrhage in adult rabbits to gain greater understanding about species dependency. A total of 99 76 +/- 7.6-d-old 2.4 +/- 0.14-kg New Zealand White rabbits were used. Exposure conditions were 5.6-MHz, 10-s exposure duration, 1-kHz PRF and 1.1-micros pulse duration. The in situ (at the pleural surface) peak rarefactional pressure, p(r(in situ)), ranged between 1.5 and 8.4 MPa, with nine acoustic US exposure groups plus a sham exposure group. Rabbits were assigned randomly to the 10 groups, each with 10 rabbits, except for one group that had nine rabbits. Rabbits were exposed bilaterally with the order of exposure (left then right lung, or right then left lung) and acoustic pressure both randomized. Individuals involved in animal handling, exposure and lesion scoring were blinded to the exposure condition. Probit regression analysis was used to examine the dependence of the lesion occurrence on in situ peak rarefactional pressure and order of exposure (first vs. second). Likewise, lesion depth and lesion root surface area were analyzed using Gaussian tobit regression analysis. Neither probability of a lesion nor lesion size measurements was found to be statistically dependent on the order of exposure after the effect of p(r(in situ)) was considered. Also, a significant correlation was not detected between the two exposed lung sides on the same rabbit in either lesion occurrence or size measures. The p(r(in situ)) threshold estimates (in MPa) were similar to each other across occurrence (3.54 +/- 0.78), depth (3.36 +/- 0.73) and surface area (3.43 +/- 0.77) of lesions. Using the same experimental techniques and statistical approach, great consistency of thresholds was demonstrated across three species (mouse, rat and rabbit). Further, there were no differences in the biologic mechanism of injury induced by US and US-induced lesions were similar in morphology in all species and age groups studied. The extent of US-induced lung damage and the ability of the lung to heal led to the conclusion that, although US can produce lung damage at clinical levels, the degree of damage does not appear to be a significant medical problem.

Vascular lesions and s-thrombomodulin concentrations from auricular arteries of rabbits infused with microbubble contrast agent and exposed to pulsed ultrasound

Arterial injury resulting from the interaction of contrast agent (CA) with ultrasound (US) was studied in rabbit auricular arteries and assessed by histopathologic evaluation and s-thrombomodulin concentrations. Three sites on each artery were exposed (2.8 MHz, 5-min exposure duration, 10-Hz pulse repetition frequency, 1.4-mus pulse duration) using one of three in situ peak rarefactional pressures (0.85, 3.9 or 9.5 MPa). Saline, saline/CA, and saline/US infusion groups (n = 28) did not have histopathologic damage. The saline/CA/US infusion group (n = 10) at exposure conditions below the FDA mechanical index limit of 1.9 did not have histopathologic damage, whereas the saline/CA/US infusion group (n = 9) at exposure conditions above the FDA limit did have damage (5 of 9 arteries). Lesions were characteristic of acute coagulative necrosis. Mean s-thrombomodulin concentrations, a marker for endothelial cell injury, were highest in rabbits exposed to US at 0.85 and 3.9 MPa, suggesting that vascular injury may be physiological and not accompanied by irreversible cellular injury.

Overview of experimental studies of biological effects of medical ultrasound caused by gas body activation and inertial cavitation

Ultrasound exposure can induce bioeffects in mammalian tissue by the nonthermal mechanism of gas body activation. Pre-existing bodies of gas may be activated even at low-pressure amplitudes. At higher-pressure amplitudes, violent cavitation activity with inertial collapse of microbubbles can be generated from latent nucleation sites or from the destabilization of gas bodies. Mechanical perturbation at the activation sites leads to biological effects on nearby cells and structures. Shockwave lithotripsy was the first medical ultrasound application for which significant cavitational bioeffects were demonstrated in mammalian tissues, including hemorrhage and injury in the kidney. Lithotripter shockwaves can also cause hemorrhage in lung and intestine by activation of pre-existing gas bodies in these tissues. Modern diagnostic ultrasound equipment develops pressure amplitudes sufficient for inertial cavitation, but the living body normally lacks suitable cavitation nuclei. Ultrasound contrast agents (UCAs) are suspensions of microscopic gas bodies created to enhance the echogenicity of blood. Ultrasound contrast agent gas bodies also provide nuclei for inertial cavitation. Bioeffects from contrast-aided diagnostic ultrasound depend on pressure amplitude, UCA dose, dosage delivery method and image timing parameters. Microvascular leakage, capillary rupture, cardiomyocyte killing, inflammatory cell infiltration, and premature ventricular contractions have been reported for myocardial contrast echocardiography with clinical ultrasound machines and clinically relevant agent doses in laboratory animals. Similar bioeffects have been reported in intestine, skeletal muscle, fat, lymph nodes and kidney. These microscale bioeffects could be induced unknowingly in diagnostic examinations; however, the medical significance of bioeffects of diagnostic ultrasound with contrast agents is not yet fully understood in relation to the clinical setting.