Temperature rise generated by diagnostic ultrasound in a transcranial phantom

Correlation between quantitative parameters of CEUS and Ki-67 labeling index in soft-tissue sarcoma

BACKGROUND

Apart from the immunohistochemical Ki-67 labeling index (LI), clinicians need a non-invasive and convenient way to predict the prognosis of patients with soft-tissue sarcoma (STS).

PURPOSE

To investigate the correlation between quantitative parameters of contrast-enhanced ultrasound (CEUS) and Ki-67 LI in STS.

MATERIAL AND METHODS

A total of 25 patients diagnosed with STS who underwent CEUS examination using SonoVue®, between January 2019 to November 2020, were included in the study. They were then divided into a high-proliferation group and low-proliferation group according to 30% Ki-67 positive tumor cells. The quantitative parameters in the semi-automatic time intensity curve analysis software, including arrival time, time to peak, peak intensity, rise time (RT), rise slope, 50% wash-out time, and 50% wash-out intensity, were extracted from the time intensity curve of CEUS by two independent observers. Statistical evaluation of the correlation and difference between CEUS quantitative parameters and Ki-67 LI between the two groups was performed. According to the area under the curve (AUC) analysis, optimal cutoff points of parameters with significant difference were determined.

RESULTS

CEUS RT of the high-proliferation group in STS was significantly higher than that of the low-proliferation group (ρ = 0.509, P = 0.01). The most reasonable cutoff to distinguish between low- and high-proliferation groups was 10.84 s. The sensitivity, specificity, and the AUC were 86.7%, 80%, and 0.80, respectively.

CONCLUSION

CEUS RT was correlated with Ki-67 LI of STS, which can be used as a minimally invasive auxiliary tool to predict the prognosis of STS in clinical practice.

Diagnostic Value of Contrast-Enhanced Ultrasound for Differential Diagnosis of Malignant and Benign Soft Tissue Masses: A Meta-Analysis

This meta-analysis was aimed at investigating the value of using contrast-enhanced ultrasound (CEUS) in the differential diagnosis of benign and malignant soft tissue masses (STMs). Relevant studies published before March 24, 2020 were identified through a comprehensive search of PubMed, Ovid, Cochrane and Web of Science. According to the inclusion criteria, five studies were selected comprising 746 patients. In the differential diagnosis of benign and malignant STMs, the pooled sensitivity and specificity of CEUS were 76% (95% confidence interval [CI]: 71%-81%; heterogeneity [I²] = 74.5%) and 67% (95% CI: 62%-71%; I² = 36.5%), respectively. The diagnostic odds ratio was 7.37 (95% CI: 3.78%-14.35; I² = 66.6%). The overall area under the curve was 0.77 (standard error: 0.0392). Subgroup analysis revealed that different index tests of CEUS resulted in different diagnostic performance. Importantly, CEUS is an effective method for the differential diagnosis between benign and malignant STMs.

Comparison of Thermal Safety Practice Guidelines for Diagnostic Ultrasound Exposures

This article examines the historical evolution of various practice guidelines designed to minimize the possibility of thermal injury during a diagnostic ultrasound examination, including those published by the American Institute of Ultrasound in Medicine, British Medical Ultrasound Society and Health Canada. The guidelines for prenatal/neonatal examinations are in general agreement, but significant differences were found for postnatal exposures. We propose sets of thermal index versus exposure time for these examination categories below which there is reasonable assurance that an examination can be conducted without risk of producing an adverse thermal effect under any scanning conditions. If it is necessary to exceed these guidelines, the occurrence of an adverse thermal event is still unlikely in most situations because of mitigating factors such as transducer movement and perfusion, but the general principle of "as low as reasonably achievable" should be followed. Some limitations of the biological effects studies underpinning the guidelines also are discussed briefly.

Temperature changes caused by the difference in the distance between the ultrasound transducer and bone during 1 mhz and 3 mhz continuous ultrasound: a phantom study

[Purpose] This study aimed to use a thermograph to observe temperature changes caused by different distances between an ultrasound transducer and bone during 1 MHz and 3 MHz continuous ultrasound emission on a phantom. [Materials and Methods] We observed the distribution of temperature elevations on a phantom consisting of pig ribs and tissue-mimicking material. One megahertz and 3 MHz ultrasound were delivered at 2.0 W/cm(2) for 5 minutes. To record the temperature changes on the phantom, we took a screenshot of the thermograph with a digital camera every 20 seconds. [Results] With 1 MHz ultrasound at the distances of 2 and 3 cm, the temperature elevation near the bone was higher than that near the transducer. However, with 3 MHz ultrasound, the temperature elevation was higher near the transducer rather than near the bone. At this point, we consider that there is a possibility of heat injury to internal organs in spite of there being no elevation of skin temperature. [Conclusion] When performing ultrasonic therapy, not only should the frequency be taken into consideration, but also the influence of the absorption coefficient and the reflection of the tissue. We visually confirmed the thermal ultrasound effect by thermography. Special attention to the temperature elevation of the internal organs is necessary to avoid injuries.

Development of a thermal test object for the measurement of ultrasound intracavity transducer self-heating

The elevated surface temperature of diagnostic ultrasound transducers imposes an important limitation to their safe use in clinical situations. Moreover, particular care should be taken if transvaginal transducers are to be used during routine scans in the first few weeks of pregnancy as the transducer surface can be very close to embryonic/fetal tissues. Published results have shown that the heating of tissue due to transducer self-heating can equal and often exceed the acoustic heating contribution. In this article, we report the development of a portable self contained thermal test object (TTO) capable of assessing the self-heating of intracavity diagnostic ultrasound transducers. The thermal conductivity and volumetric heat capacity of the tissue mimicking material (TMM) used in the TTO were measured, yielding values of (0.56 +/- 0.01) W m(-1) K(-1) and (3.5 +/- 0.8) MJ m(-3) K(-1). The speed of sound of the TMM was measured as 1540 m s(-1) and the attenuation over a frequency range of 2 to 10 MHz was found to be (0.50 +/- 0.01) dB cm(-1) MHz(-1). These results are in excellent agreement with the International Electrotechnical Commission (IEC 60601-2-37) requirements and the previously published properties of biological soft tissue. The temperature stability and uniformity, and suitability of the TTO for the measurement of transducer self-heating were tested and found to be satisfactory. The TTO reached a stable temperature of 37 degrees C in 3 h and the spatial variation in temperature was less than +/- 0.2 degrees C. Lastly, transducer self-heating measurements from a transvaginal transducer exceeded the IEC temperature limit of 43 degrees C in less than 5 min and the temperature reached after 30 min was 47.3 degrees C.

Does Long-Term Continuous Transcranial Doppler Monitoring Require a Pause for Safer Use?

BACKGROUND

Transcranial Doppler sonography (TCD) has been used widely for long-term monitoring of cerebral blood flow without adverse reports. However, attention has not been adequately paid to the fact that an increase in the time period of TCD insonation causes brain temperature to rise due to ultrasound absorption by tissue and the skull. We measured the actual temperature rise in local brain tissue induced by TCD insonation over a long time period during in vivo animal experiments in order to verify whether or not a pause is required in long-term, continuous TCD monitoring.

METHODS

We inserted thermocouples into the skull-brain interface (SBI) of 15 New Zealand White rabbits (10: TCD application group; 5: control group, TCD non-application group). The TCD probe was placed on the parietal bone, and changes in SBI temperature (SBIT) were measured for 90 min. TCD was set at maximum output level (0.2 W, 2 MHz).

RESULTS

SBIT in the TCD group increased rapidly to 3.47 degrees C within 25 min and then reached a plateau. The maximum time for safe continuous TCD application is estimated to be 33 min.

CONCLUSIONS

Even though there are large differences in factors, such as brain volume and environmental conditions, between rabbits and humans, there is less difference in their cerebral blood flow per brain weight, which is the parameter that is mainly associated with heat reduction. Accordingly, the findings of the present experiment suggest that long-term TCD monitoring in clinical use should include a pause after every 30 min of insonation to avoid thermal damage to the brain surface.

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

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

The effects of low-intensity ultrasound on growing bone after sciatic neurectomy

The purpose of this study was to investigate the therapeutic effects of low-intensity ultrasound (US) on the sciatic-neurectomy-induced bone mass decrement in growing rats. A total of 20 male Sprague-Dawley rats (166.7+/-11.3 g) underwent right leg sciatic neurectomy. They were randomly assigned into two groups, US treatment group (UST) and control group (CON). The rats receiving US treatment were treated with a 125 mW/cm2 continuous low-intensity US stimulation for 15 min/day on the lateral site of the right leg. The control rats did not receive any US treatment. All the animals were euthanized after 4-week US treatment. Both the original data and bilateral difference ratio of femur or tibia weight, histomorphometry data and bone densitometry data showed that the sciatic neurectomy obviously reduced bone mass in the operated limbs of both groups. However, the continuous low-intensity US treatment did not ameliorate the neurectomy-induced loss of bone mass. Thus, the low-intensity US generated micromechanical strains might not induce enough bone formation activity to reverse the bone loss in this model of intact bones.

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.

Tofu as a tissue-mimicking material

The acoustic properties of one kind of tofu (soft, firm and extra-firm types) commercially available in grocery markets were measured. It was found that density, speed of sound and attenuation coefficient of tofu were close to those of some soft tissues. It is suggested that tofu may serve as a tissue-mimicking material for some biomedical ultrasound (US) through transmission applications in vitro.

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.

Contrast agent specific imaging modes for the ultrasonic assessment of parenchymal cerebral echo contrast enhancement

Previous work has demonstrated that cerebral echo contrast enhancement can be assessed by means of transcranial ultrasound using transient response second harmonic imaging (HI). The current study was designed to explore possible advantages of two new contrast agent specific imaging modes, contrast burst imaging (CBI) and time variance imaging (TVI), that are based on the detection of destruction or splitting of microbubbles caused by ultrasound in comparison with contrast harmonic imaging (CHI), which is a broadband phase-inversion-based implementation of HI. Nine healthy individuals with adequate acoustic temporal bone windows were included in the study. Contrast harmonic imaging, CBI, and TVI examinations were performed in an axial diencephalic plane of section after an intravenous bolus injection of 4 g galactose-based microbubble suspension in a concentration of 400 mg/mL. Using time-intensity curves, peak intensities and times-to peak-intensity (TPIs) were calculated off-line in anterior and posterior parts of the thalamus, in the region of the lentiform nucleus, and in the white matter. The potential of the different techniques to visualize cerebral contrast enhancement in different brain areas was compared. All techniques produced accurate cerebral contrast enhancement in the majority of investigated brain areas. Contrast harmonic imaging visualized signal increase in 28 of 36 regions of interest (ROIs). In comparison, TVI and CBI examinations were successful in 32 and 35 investigations, respectively. In CHI examinations, contrast enhancement was most difficult to visualize in posterior parts of the thalamus (6 of 9) and the lentiform nucleus (6 of 9). In TVI examinations, anterior parts of the thalamus showed signal increase in only 6 of 9 examinations. For all investigated imaging modes, PIs and TPIs in different ROIs did not differ significantly, except that TVI demonstrated significantly higher PIs in the lentiform nucleus as compared with the thalamus and the white matter (P < 0.05). The current study demonstrates for the first time that CBI and TVI represent new ultrasonic tools that allow noninvasive assessment of focal cerebral contrast enhancement and that CBI and TVI improve diagnostic sensitivity as compared with CHI.

Comparison of finite element and heated disc models of tissue heating by ultrasound

This paper compares different techniques used to model the heating caused by ultrasound (US) in a phantom containing a layer of bone mimic covered by agar gel. Results from finite element (FE) models are compared with those from two techniques based on the point-source solution to the bioheat transfer equation (BHTE): one in which the bone mimic is considered to be an absorbing disc of infinitesimal thickness and the other in which the region through which the US travels is considered to be a volume heat source. The FE results are also compared with experimental measurements. The results from the models differed by up to 40% compared with those from the FE model. Furthermore, for the intensity distribution considered, which corresponds to that in the focal zone of a single-element transducer, the top hat distribution predicts a temperature rise 1.8 times greater than that for a more realistic one based on measured values.

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

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

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

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

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

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

Application of low-intensity ultrasound to growing bone in rats

Low-intensity pulsed ultrasound recently has been shown to accelerate long bone fracture healing, but its effect on bone growth and development is unknown. The longitudinal growth and bone density of the femur and tibia in young rats was measured after application of an ultrasound transducer emitting 1.5-MHz pulsed ultrasound (30 mW/cm2, SATA) for 20 min/day. After 28 days, no length difference was detected (< or = 2%) compared to the sham-treated leg or to unexposed controls. Also, no significant difference in bone mineral density (BMD) of the femur or tibia was found (< or = 6%). In a repeated experiment in which a periosteal trauma stimulus was created in the femoral diaphysis, the ultrasound also had no effect on growth or BMD. This results suggests that physeal bone growth is far less sensitive to this level of ultrasound application than is fracture repair. This may be related to the cascade of cellular events and regulatory factors that are present after a fracture.

Temperature rise generated by ultrasound in the presence of contrast agent

Temperature elevation vs. time generated by a focused Gausssian ultrasound beam in the presence of contrast agents has been calculated using a perfect absorbing disc model. The results suggest that, if the contrast agent (Albunex) is introduced into the body intravenously, the temperature rise in the heart, which is 4.5 cm from the transducer, generated by 110-mW (the corresponding acoustic intensity at the transducer front surface is 0.4 W/cm2) 2-MHz ultrasound is about 2 degrees C in 10 s. The relationship between temperature rise and the blood perfusion, acoustic power and focal length is discussed.