Temperature elevation measured in a tissue-mimicking phantom for transvaginal ultrasound at clinical settings

INTRODUCTION: This paper reports the results of an audit to assess the possible thermal hazard associated with the clinical use of ultrasound scanners in UK Hospitals for transvaginal ultrasound imaging. METHODS: An anatomically relevant phantom composed of a block of agar-based tissue mimicking material with embedded thermal sensors was developed. Seventeen hospitals around the UK were visited and a total of 64 configurations were tested. A representative typical scanning protocol was adopted, which primarily used B-mode with 30 s periods of colour-flow and pulsed Doppler modes for both gynaecology and obstetrics pre-sets. RESULTS: The results confirmed that the highest temperature increase is always at the surface. The greatest temperature rise measured across all the systems was 3.6℃, with an average of 2.0℃ and 2.16℃ for gynaecology and obstetrics pre-sets, respectively. For some systems, the temperature increased rapidly when selecting one of the Doppler modes, so using them for longer than 30 s will in many cases lead to greater heating. It is also shown that, in agreement with previous studies, the displayed thermal index greatly underestimates the temperature rise, particularly close to the transducer face but even to distances approaching 2 cm. CONCLUSIONS: Overall, the results of the audit for the temperature rise during transvaginal ultrasound at clinical settings fell within the limits indicated by the national and international standards, for the pre-sets tested and following a representative typical scanning protocol. Only selected pre-sets were tested and the scanner outputs were not maximised (for example by using zoom, greater depth or narrow sector angles). Consequently, higher temperatures than those measured can certainly be achieved

The effect of curing temperature and time on the acoustic and optical properties of PVCP

Polyvinyl chloride plastisol (PVCP) has been increasingly used as a phantom material for photoacoustic and ultrasound imaging. As one of the most useful polymeric materials for industrial applications, its mechanical properties and behaviour are well-known. Although the acoustic and optical properties of several formulations have previously been investigated, it is still unknown how these are affected by varying the fabrication method. Here, an improved and straightforward fabrication method is presented and the effect of curing temperature and curing time on PVCP acoustic and optical properties, as well as their stability over time, is investigated. Speed of sound and attenuation were determined over a frequency range from 2 to 15 MHz, while the optical attenuation spectra of samples was measured over a wavelength range from 500 to 2200 nm. Results indicate that the optimum properties are achieved at curing temperatures between 160 °C and 180 °C, while the required curing time decreases with increasing temperature. The properties of the fabricated phantoms were highly repeatable, meaning the phantoms are not sensitive to the manufacturing conditions provided the curing temperature and time are within the range of complete gelation-fusion (samples are optically clear) and below the limit of thermal degradation (indicated by the yellowish appearance of the sample). The samples’ long term stability was assessed over 16 weeks and no significant change was observed in the measured acoustic and optical properties

Measurement of the ultrasound attenuation and dispersion in 3D-printed photopolymer materials from 1 to 3.5 MHz

Over the past decade, the range of applications in biomedical ultrasound exploiting 3D printing has rapidly expanded. For wavefront shaping specifically, 3D printing has enabled a diverse range of new, low-cost approaches for controlling acoustic fields. These methods rely on accurate knowledge of the bulk acoustic properties of the materials; however, to date, robust knowledge of these parameters is lacking for many materials that are commonly used. In this work, the acoustic properties of eight 3D-printed photopolymer materials were characterised over a frequency range from 1 to 3.5 MHz. The properties measured were the frequency-dependent phase velocity and attenuation, group velocity, signal velocity, and mass density. The materials were fabricated using two separate techniques [PolyJet and stereolithograph (SLA)], and included Agilus30, FLXA9960, FLXA9995, Formlabs Clear, RGDA8625, RGDA8630, VeroClear, and VeroWhite. The range of measured density values across all eight materials was 1120–1180 kg · m−3, while the sound speed values were between 2020 to 2630 m · s−1, and attenuation values typically in the range 3–9 dB · MHz−1· cm−1

Measurement of the temperature-dependent output of lead zirconate titanate transducers

The effect of temperature and electrical drive conditions on the output of lead zirconate titanate (PZT) transducers is of particular interest in ultrasound metrology and medical ultrasound applications. In this work, the temperature-dependent output of two single-element PZT transducers was measured between 22 °C and 46 °C. Two independent measurement methods were used, namely radiation force balance measurements and laser vibrometry. When driven at constant voltage using a 50 matched signal generator and amplifier using continuous wave (CW) or quasi-CW excitation, the output of the two transducers increased on average by 0.6% per degree, largely due to an increase in transducer efficiency with temperature. The two measurement methods showed close agreement. Similar trends were observed when using single cycle excitation with the same signal chain. However, when driven using a pulser (which is not electrically matched), the two transducers exhibited different behaviour depending on their electrical impedance. Accounting for the temperature-dependent output of PZT transducers could have implications for many areas of ultrasound metrology, for example, in therapeutic ultrasound where a coupling fluid at an increased or decreased temperature is often used

The Feasibility of Thermal Imaging as a Future Portal Imaging Device for Therapeutic Ultrasound.

This technical note describes a prototype thermally based portal imaging device that allows mapping of energy deposition on the surface of a tissue mimicking material in a focused ultrasound surgery (FUS) beam by using an infrared camera to measure the temperature change on that surface. The aim of the work is to explore the feasibility of designing and building a system suitable for rapid quality assurance (QA) for use with both ultrasound- and magnetic resonance (MR) imaging-guided clinical therapy ultrasound systems. The prototype was tested using an MR-guided Sonalleve FUS system (with the treatment couch outside the magnet bore). The system's effective thermal noise was 0.02°C, and temperature changes as low as 0.1°C were easily quantifiable. The advantages and drawbacks of thermal imaging for QA are presented through analysis of the results of an experimental session

ISUOG Safety Committee updated recommendation on use of respirators by practitioners undertaking obstetric and gynecological ultrasound in context of SARS-CoV-2 Omicron variant of concern

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Tissue mimicking materials for imaging and therapy phantoms: a review

Tissue mimicking materials (TMMs), typically contained within phantoms, have been used for many decades in both imaging and therapeutic applications. This review investigates the specifications that are typically being used in development of the latest TMMs. The imaging modalities that have been investigated focus around CT, mammography, SPECT, PET, MRI and ultrasound. Therapeutic applications discussed within the review include radiotherapy, thermal therapy and surgical applications. A number of modalities were not reviewed including optical spectroscopy, optical imaging and planar x-rays. The emergence of image guided interventions and multimodality imaging have placed an increasing demand on the number of specifications on the latest TMMs. Material specification standards are available in some imaging areas such as ultrasound. It is recommended that this should be replicated for other imaging and therapeutic modalities. Materials used within phantoms have been reviewed for a series of imaging and therapeutic applications with the potential to become a testbed for cross-fertilization of materials across modalities. Deformation, texture, multimodality imaging and perfusion are common themes that are currently under development

Qualitative assessment of thermal effects in high intensity ultrasound thrombolysis experiments

Objective High Intensity Focused Ultrasounds (HIFU) demonstrated the ability to destroy blood clots without addiction of thrombolytic drugs. However, the involved physical principles are yet unclear, thus slowing a translation to clinical application. It is agreed that thermal effects must be avoided; however it is hard to directly measure the temperature because of the dynamics of the procedures. Methods We demonstrated the possibility to break human blood clots in an in-vitro system. We used a commercial HIFU transducer (Precision Acoustics) with focus dimensions (previously mapped) of 2.2mm and 23mm (-6dB focal radius and length). Acoustic parameters were: frequency 1MHz, pulse length 450μs, duty cycle (d.c.) 10%, output power 65W, therapy duration (t.d.) 120s. Results We used long segments of porcine clots (similar acoustic properties of human thrombi but easier to produce) to evaluate the presence of thermal effects, by varying the duty cycle of the sonication protocol. The following parameters were adopted in order to maintain the same amount of delivered energy: A. d.c. 100% t.d. 20s; B. d.c. 50% t.d. 40s; C. d.c. 10% (same as thrombolysis experiments) t.d. 200s; D. d.c. 0% (control). After sonication, porcine clots were cut in correspondence of the produced lesion and were observed under a digital microscope (HiroxKH7700, magnification 20x). The following features were observed: A. evidence of necrotic tissue, surface erosion and internal holes; B. moderate necrosis and surface erosion; C. presence of surface erosion, no evidence of thermal lesions; D. homogeneous and uniform surface. Conclusions The absence of visible thermal lesions enforces the hypothesis of mechanical effects in HIFU thrombolysis. Inertial cavitation should play an important role in the phenomenon and must be detected and quantified, and possibly enhanced by microbubbles. We implemented a system which detects acoustic emission of collapsing bubbles (i.e. broadband noise at high frequencies) in order to assess the influence of this phenomenon in thrombolysis