Eleanor Martin

Infrared mapping of ultrasound fields generated by medical transducers: feasibility of determining absolute intensity levels.

Considerable progress has been achieved in the use of infrared (IR) techniques for qualitative mapping of acoustic fields of high intensity focused ultrasound (HIFU) transducers. The authors have previously developed and demonstrated a method based on IR camera measurement of the temperature rise induced in an absorber less than 2 mm thick by ultrasonic bursts of less than 1 s duration. The goal of this paper was to make the method more quantitative and estimate the absolute intensity distributions by determining an overall calibration factor for the absorber and camera system. The implemented approach involved correlating the temperature rise measured in an absorber using an IR camera with the pressure distribution measured in water using a hydrophone. The measurements were conducted for two HIFU transducers and a flat physiotherapy transducer of 1 MHz frequency. Corresponding correction factors between the free field intensity and temperature were obtained and allowed the conversion of temperature images to intensity distributions. The system described here was able to map in good detail focused and unfocused ultrasound fields with sub-millimeter structure and with local time average intensity from below 0.1 W/cm(2) to at least 50 W/cm(2). Significantly higher intensities could be measured simply by reducing the duty cycle.

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.

A comparison of three different types of temperature measurement in HITU fields

The spatial and temporal distribution of the temperature elevation caused by high-intensity therapeutic ultrasound (HITU) in a tissue-mimicking material (TMM) has been determined with magnetic resonance (MR) thermometry, infrared (IR) thermometry and a thermal test object with an integrated thin-film thermocouple at three different National Metrological Institutes (PTB/Germany, NPL/UK, INRIM/Italy). Results obtained from the different types of measurement are compared and some general aspects of the methods are discussed, particularly with regard to their suitability for the in vitro characterization of transducers for treatment planning.

The Cellular Bioeffects of Low Intensity Ultrasound

A wide range of non-lethal cellular effects of low intensity ultrasound have been demonstrated in the literature. In order to utilise these effects therapeutically, the underlying interaction mechanisms of ultrasound with cells, and the cellular processes involved in these interactions must be understood. This article reviews recent work on cellular bioeffects using both cell culture and in vivo animal models. We also discuss our current understanding of cellular effects and processes involved in thermosensing and mechanosensing.

Experimental study of beam distortion due to fiducial markers during salvage HIFU in the prostate

BackgroundProstate cancer is frequently treated using external beam radiation therapy (EBRT). Prior to therapy, the prostate is commonly implanted with a small number of permanent fiducial markers used to monitor the position of the prostate during therapy. In the case of local cancer recurrence, high-intensity focused ultrasound (HIFU) provides a non-invasive salvage treatment option. However, the impact of the fiducial markers on HIFU treatment has not been thoroughly studied to date. The objective of this study was to experimentally investigate the effect of a single EBRT fiducial marker on the efficacy of HIFU treatment delivery using a tissue-mimicking material (TMM).MethodsA TMM with the acoustic properties of the prostate was developed based on a polyacrylamide hydrogel containing bovine serum albumin. Each phantom was implanted with a cylindrical fiducial marker and then sonicated using a 3.3 MHz focused bowl HIFU transducer. Two sets of experiments were performed. In the first, a single lesion was created at different positions along either the anteroposterior or left-right axes relative to the marker. In the second, a larger ablation volume was created by raster scanning. The size and position of the ablated volume were assessed using a millimetre grid overlaid on the phantom.ResultsThe impact of the marker on the position and size of the HIFU lesion was significant when the transducer focus was positioned within 7 mm anteriorly, 18 mm posteriorly or within 3 mm laterally of the marker. Beyond this, the generated lesion was not affected. When the focus was anterior to the marker, the lesion increased in size due to reflections. When the focus was posterior, the lesion decreased in size or was not present due to shadowing.ConclusionsThe presence of an EBRT fiducial marker may result in an undertreated region beyond the marker due to reduced energy arriving at the focus, and an overtreated region in front of the marker due to reflections. Depending on the position of the targeted regions and the distribution of the markers, both effects may be undesirable and reduce treatment efficacy. Further work is necessary to investigate whether these results indicate the necessity to reconsider patient selection and treatment planning for prostate salvage HIFU after failed EBRT.

Sensitivity of simulated transcranial ultrasound fields to acoustic medium property maps

High intensity transcranial focused ultrasound is an FDA approved treatment for essential tremor, while low-intensity applications such as neurostimulation and opening the blood brain barrier are under active research. Simulations of transcranial ultrasound propagation are used both for focusing through the skull, and predicting intracranial fields. Maps of the skull acoustic properties are necessary for accurate simulations, and can be derived from medical images using a variety of methods. The skull maps range from segmented, homogeneous models, to fully heterogeneous models derived from medical image intensity. In the present work, the impact of uncertainties in the skull properties is examined using a model of transcranial propagation from a single element focused transducer. The impact of changes in bone layer geometry and the sound speed, density, and acoustic absorption values is quantified through a numerical sensitivity analysis. Sound speed is shown to be the most influential acoustic property, and must be defined with less than 4% error to obtain acceptable accuracy in simulated focus pressure, position, and volume. Changes in the skull thickness of as little as 0.1 mm can cause an error in peak intracranial pressure of greater than 5%, while smoothing with a 1 [Formula: see text] kernel to imitate the effect of obtaining skull maps from low resolution images causes an increase of over 50% in peak pressure. The numerical results are confirmed experimentally through comparison with sonications made through 3D printed and resin cast skull bone phantoms.

Rapid spatial mapping of the acoustic pressure in high intensity focused ultrasound fields at clinical intensities using a novel planar Fabry-Pérot interferometer

Measurement of high acoustic pressures is necessary in order to fully characterise clinical high-intensity focused ultrasound (HIFU) fields, and for accurate validation of computational models of ultrasound propagation. However, many existing methods are unable to withstand the extreme pressures generated in these fields, and those that can often have high noise levels. Here, a robust sensor, based on a planar Fabry-Pérot interferometer with hard dielectric spacer and mirrors, was used to measure acoustic pressure in the field of a 3.3 MHz single element spherically focused bowl transducer. In preliminary measurements, peak positive pressures of 27 MPa, and peak negative pressures of 14 MPa were measured. The noise equivalent pressure scaled with the adjustable dynamic range of the system between 50 kPa for pressures up to 8 MPa and 235 kPa for measurements up to 70 MPa. This makes the system suitable for measuring low pressure regions of the field as well as the high focal pressures. The -3 dB bandwidth of the sensor was 600 MHz, and the effective element size was 25 μm, which makes the sensor well suited to the measurement of the highly nonlinear and localised high-pressure focal regions generated in HIFU fields. Waveforms were acquired at a rate of 200 Hz, several orders of magnitude faster than can be achieved with a hydrophone scanning system. This sensor represents a critical improvement in measurement capability for HIFU fields in terms of dynamic range, bandwidth, noise equivalent pressure, and acquisition speed.

Ultrasound-induced contraction of the carotid artery in vitro.

Ultrasound is known to produce a range of nonlethal responses in cells and tissues. Frequencies in the kilohertz ultrasound range have been shown to produce relaxation in large arteries. The present work explores the effects of insonation at MHz frequencies, representative of those used diagnostically and therapeutically, in an in vitro preparation of the carotid artery. Fresh 12.7 mm wide rings of equine common carotid artery obtained from the abattoir were mounted in a purpose-made myograph. They were immersed in a bath of Krebs-Ringer buffer at 37 degrees C and were positioned at the focus of an ultrasound beam from a weakly focused 3.2 MHz source. Continuous wave insonation produced contraction. The tension increased rapidly over the first 2 min, followed by a slower increase for the duration of the exposure up to 15 min. At a power of 145 mW a maximum contractile stress of 0.04 +/- 0.03 mN/mm(2) (mean +/- SD, n = 77) was measured, which was approximately 4% of the maximum wall stress generated by noradrenaline (0.1 mM). The magnitude of the response was weakly dependent on power in the range 72-145 mW and was not significantly different for pulsed and continuous wave stimulation where time averaged power was constant. The response was unaffected by mechanical removal of the endothelium. The ultrasound beam generated insufficient radiation force to produce a measurable effect and streaming at the vessel surface was very small compared with flow rates known to produce physiologic effects. The temperature rise at the beam focus was approximately 0.3 degrees C and we hypothesise that this contributes to the observed response, probably through changes in ion channel activity in smooth muscle cell membranes. (E-mail: e.martin@exeter.ac.uk).

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.

Rapid Spatial Mapping of Focused Ultrasound Fields Using a Planar Fabry–Pérot Sensor

Measurement of high acoustic pressures is necessary in order to fully characterize clinical high-intensity focused ultrasound (HIFU) fields, and for accurate validation of computational models of ultrasound propagation. However, many existing measurement devices are unable to withstand the extreme pressures generated in these fields, and those that can often exhibit low sensitivity. Here, a planar Fabry–Pérot interferometer with hard dielectric mirrors and spacer was designed, fabricated, and characterized, and its suitability for measurement of nonlinear focused ultrasound fields was investigated. The noise equivalent pressure (NEP) of the scanning system scaled with the adjustable pressure detection range between 49 kPa for pressures up to 8 MPa and 152 kPa for measurements up to 25 MPa, over a 125 MHz measurement bandwidth. Measurements of the frequency response of the sensor showed that it varied by less than 3 dB in the range 1–62 MHz. The effective element size of the sensor was 65