Feasibility of Reference Material Certification for Speed of Sound and Attenuation Coefficient Based on Standard Tissue-Mimicking Material.

Speed of sound and attenuation are essential for characterizing reference materials such as biological tissue-mimicking materials (TMMs) used in ultrasonic applications. There are many publications on the manufacture of TMMs and the measurement of their properties. However, no studies in the literature have applied the metrological approach of International Organization for Standardization (ISO) Guide 35 to certify biological ultrasound TMMs as candidates for reference materials (RMs). The work described here was aimed at studying the process for manufacturing fat, muscle and aorta artery TMMs, including the study of the homogeneity, stability, trend and characterization of TMMs. The properties of interest were the speed of sound (SoS) and attenuation coefficient (AttC) at 7.5 MHz, with target expanded uncertainty of 40 m/s and 0.3 dB/cm, respectively. The short-term stability study was 2 mo at 4°C and 40°C (simulating possible transportation conditions). The long-term stability study lasted an additional 4 mo with the TMM at 22°C (simulating possible storage conditions). Homogeneity was evaluated before the stability study. Uncertainties associated with homogeneity, stability, characterization and trend were duly calculated. No trend was observed in this study, but the AttC spread widely during the stability test, substantially enlarging the final uncertainty. Therefore, this property could not be used to certify TMM candidates as RMs. However, the SoSs for most TMMs lay within the target uncertainty, disclosing viability to certify TMMs as RMs for this property. Assigned values for SoS were 1560 m/s for aorta TMM with an average expanded uncertainty for certificate validity of 12 mo (Ue;12=20 m/s), 1552 m/s for muscle TMM (Ue;12=20 m/s) and 1494 m/s for fat TMM (Ue;12=11 m/s). Thus, TMMs were proved suitable to be certified as RMs for SoS.

Ultrasound-assisted transesterification of soybean oil using low power and high frequency and no external heating source.

In this work, high frequency and low power ultrasound without external heating source and mechanical stirring in biodiesel production were studied. Transesterification of soybean oil with methanol and catalyzed by KOH was investigated using ultrasound equipment and ultrasonic transducer. The effect of ultrasonic output power (3 W-9 W), ultrasonic frequency (1 MHz and 3 MHz), and alcohol to oil molar ratio (6:1 and 8:1) have been investigated. The increase in ultrasonic power provided higher conversion rates. In addition, higher conversion rates were obtained by increasing the ultrasonic frequency from 1 MHz to 3 MHz (48.7% to 79.5%) for the same reaction time. Results also indicate that the speed of sound can be used to evaluate the produced biodiesel qualitatively. Further, the ultrasound system presented electric consumption (46.2W∙h) four times lower than achieved using the conventional method (211.7W∙h and 212.3W∙h). Thus, biodiesel production using low power ultrasound in the MHz frequency range is a promising technology that could contribute to biodiesel production processes.

Non-invasive Estimation of Temperature during Physiotherapeutic Ultrasound Application Using the Average Gray-Level Content of B-Mode Images: A Metrological Approach.

Healing therapies that make use of ultrasound are based on raising the temperature in biological tissue. However, it is not possible to heal impaired tissue by applying a high dose of ultrasound. The temperature of the tissue is ultimately the physical quantity that has to be assessed to minimize the risk of undesired injury. Invasive temperature measurement techniques are easy to use, despite the fact that they are detrimental to human well being. Another approach to assessing a rise in tissue temperature is to derive the material's general response to temperature variations from ultrasonic parameters. In this article, a method for evaluating temperature variations is described. The method is based on the analytical study of an ultrasonic image, in which gray-level variations are correlated to the temperature variations in a tissue-mimicking material. The physical assumption is that temperature variations induce wave propagation changes modifying the backscattered ultrasound signal, which are expressed in the ultrasonographic images. For a temperature variation of about 15°C, the expanded uncertainty for a coverage probability of 0.95 was found to be 2.5°C in the heating regime and 1.9°C in the cooling regime. It is possible to use the model proposed in this article in a straightforward manner to monitor temperature variation during a physiotherapeutic ultrasound application, provided the tissue-mimicking material approach is transferred to actual biological tissue. The novelty of such approach resides in the metrology-based investigation outlined here, as well as in its ease of reproducibility.

Metrological Validation of a Measurement Procedure for the Characterization of a Biological Ultrasound Tissue-Mimicking Material.

The speed of sound and attenuation are important properties for characterizing reference materials such as biological phantoms used in ultrasound applications. There are many publications on the manufacture of ultrasonic phantoms and the characterization of their properties. However, few studies have applied the principles of metrology, such as the expression of the uncertainty of measurement. The objective of this study is to validate a method for characterizing the speed of sound and the attenuation coefficient of tissue-mimicking material (TMM) based on the expression of the measurement of uncertainty. Six 60-mm-diameter TMMs were fabricated, three 10 mm thick and three 20 mm thick. The experimental setup comprised two ultrasonic transducers, acting as transmitter or receiver depending on the stage of the measurement protocol, both with a nominal center frequency of 5 MHz and an element diameter of 12.7 mm. A sine burst of 20 cycles and 20-V peak-to-peak amplitude at 5 MHz excited the transmitter transducer, producing a maximum pressure of 0.06 MPa. The measurement method was based on the through-transmission substitution immersion technique. The speed of sound measurement system was validated using a calibrated stainless-steel cylinder as reference material, and normalized errors were <0.8. The attenuation coefficient measurement method was validated using replicated measurements under repeatability conditions. The normalized error between the two measurement sets was <1. The proposed uncertainty models for the measurements of the speed of sound and the attenuation coefficient can help other laboratories develop their own uncertainty models. These validated measurement methods can be used to certify a TMM as a reference material for biotechnological applications.

Thermochromic Phantom and Measurement Protocol for Qualitative Analysis of Ultrasound Physiotherapy Systems.

Thermochromic test bodies are promising tools for qualitatively evaluating the acoustic output of ultrasound physiotherapy systems. Here, a novel phantom, made of silicone mixed with thermochromic powder material, was developed. Additionally, a procedure was developed to evaluate the stability and homogeneity of the phantom in a metrologic and statistical base. Twelve phantoms were divided into three groups. Each group was insonated by a different transducer. An effective intensity of 1.0 W/cm(2) was applied to each phantom; two operators performed the procedure three times in all phantoms. The heated area was measured after image processing. No statistical difference was observed in the heated areas for different samples or in the results for different operators. The heated areas obtained using each transducer were statistically different, indicating that the thermochromic phantom samples had sufficient sensitivity to represent the heated areas of different ultrasonic transducers. Combined with the evaluation procedure, the phantom provides an approach not previously described in the literature. The proposed approach can be used to quickly assess changes in ultrasonic beam cross-sectional shape during the lifetime of ultrasound physiotherapy systems.

Induction of skeletal muscle differentiation in vitro by therapeutic ultrasound.

Therapeutic ultrasound (TU) has been used for the last 50 y in rehabilitation, including treatment of soft tissues. Ultrasound waves can be employed in two different modes of operation, continuous and pulsed, which produce both thermal and non-thermal effects. Despite the large-scale use of TU, there are few scientific studies on its biologic effects during skeletal muscle differentiation. To better analyze the cellular effects of TU, we decided to follow cells in vitro. The main purpose of this study was to evaluate the effects of TU in primary chick myogenic cell cultures using phase contrast optical microscopy and immunofluorescence microscopy, followed by image analysis and quantification. Our results indicate that TU can stimulate the differentiation of skeletal muscle cells in vitro, as measured by the thickness of multinucleated myotubes, the ratio of mononucleated cells to multinucleated cells and expression of the muscle-specific protein desmin. This study is a first step toward a metrologic and science-based protocol for cell treatment under different ultrasound field exposures.

Influence of subcutaneous fat in surface heating of ultrasonic diagnostic transducers.

The transducers of diagnostic ultrasonic equipment generate undesired local heating at the applied part of the transducer surface. The assessment of this heating is fundamental in warranting patient safety. On the standard IEC 60601-2-37, methods have been established for the reliable measurement of heating, where three tissue models based on tissue-mimicking materials are recommended: soft tissue mimic only, bone mimic close to the surface of soft tissue, and skin mimic at the surface of soft tissue. In the present work, we compared the last-mentioned tissue model with a new one using a layer of porcine subcutaneous fat inserted between the soft tissue and skin-mimicking materials. We verify significant statistical differences between models, with the average temperature rise measured for the tests without subcutaneous fat at 6.7 °C±1.7 °C and for the ones with subcutaneous fat at 8.9 °C±1.8 °C (k=2; p=0.95). For each model, the procedure was performed 10 times in repeatability conditions of measurement. It has been suggested that the influence of subcutaneous fat for external transducers heating evaluation should be considered, as the presence of many millimeters of subcutaneous fat is a common condition in patients. Otherwise, the transducer surface heating and, therefore, the risk to the patient may be underestimated.

Nondestructive testing ultrasonic immersion probe assessment and uncertainty evaluation according to EN 12668-2:2010.

Ultrasonic beam parameters from nondestructive testing probes and respective measurement uncertainties were calculated according to the Guide to the Expression of Uncertainty in Measurement (BIPM-JCGM-100:2008). The major conclusion of this work is that proper measurements of ultrasonic probes parameters are necessary because each probe has intrinsic construction particularities. Uncertainty evaluation was essential to properly assess the experimental results.

Effective radiating area and beam non-uniformity ratio of ultrasound transducers at 5MHz, according to IEC 61689:2007.

This paper discloses results of measuring the effective radiating area (A(ER)) and the beam non-uniformity ratio (R(BN)) for US transducers at 5.0 MHz. Measurements were carried out at Laboratory of Ultrasound of the Brazilian National Institute of Metrology, Standardization, and Industrial Quality. As reliability proof of system's adequacy, uncertainties were assessed. The calculation protocol was developed based on standard IEC 61689:2007. Type A uncertainty was estimated after four repetitions of the full procedure for the determination of A(ER) and R(BN), and Type B uncertainty was estimated from the mathematical model for both calculations, obtained from IEC 61689:2007 and the guide to the expression of uncertainty in measurement. The procedure presented herein represents the state of the art regarding metrology for testing therapeutic ultrasound devices, and its application results in fundamental aspects to support their evaluation regarding quality assurance, for instance, for a certification process due safety and performance.