Julian Haller

A comparative evaluation of three hydrophones and a numerical model in high intensity focused ultrasound fields.

The pressure fields of two different high intensity focused ultrasound (HIFU) transducers operated in burst mode were measured at acoustical power levels of 25 and 50 W (continuous wave equivalent) with three different hydrophones: A fiber-optic displacement sensor, a commercial HIFU needle hydrophone, and a prototype of a membrane hydrophone with a protective coating against cavitation effects. Additionally, the fields were modeled using a freely available simulations software package. The measured waveforms, the peak pressure profiles, as well as the spatial-peak temporal-average intensities from the different devices and from the modeling are compared and possible reasons for differences are discussed. The results clearly show that reliable pressure measurements in HIFU fields remain a difficult task concerning both the reliability of the measured values and the robustness of the sensors used: Only the fiber-optic hydrophone survived all four exposure regimes and the measured spatial-peak temporal-average intensities varied by a factor of up to 1.5 between the measurements and the modeling and between the measurements among themselves.

Characterization of a fiber-optic displacement sensor for measurements in high-intensity focused ultrasound fields

A fiber-optic sensor is presented that is capable of measuring the particle displacement in high-intensity focused ultrasound (HIFU) fields. For this probe, a secondary calibration was performed, and the resulting complex frequency response is discussed. As a first practical application, the setup was used to measure the pressure in the field of a weakly focusing ultrasound transducer. The result is compared with that of a membrane hydrophone measurement. The feasibility of measurements in HIFU fields is demonstrated by means of measurements of the spatial distribution of the peak particle velocity within the focus of a HIFU transducer and of the dependence of the peak values on the acoustical power level.

Towards a dosimetric framework for therapeutic ultrasound

Abstract There is a need for a coherent set of exposure and dose quantities to describe ultrasound fields in media other than water (including tissue and tissue-simulating materials). This paper proposes an outline dosimetry scheme, with quantities for free field exposure, in situ exposure, dose (both instantaneous and cumulative) and effect, to act as a structure for organising a more complete set of definitions. It also presents findings from a survey of the views of the therapeutic ultrasound community which generally supports the principle of using modified free field quantities to describe the in situ field, and the prioritising of dose quantities which are related to heating and thermal mechanisms. Although there is no one-to-one relationship between any known ultrasound dose quantity and a specific biological effect, this can also be said of radiotherapy and other modalities where weighting factors have been developed to calculate the degree of equivalence between different tissues and radiation types. This same separation is recommended for ultrasound, provided that an appropriate set of recognised ‘engineering’ quantities can be established for exposure and dose quantities.

Uncertainty estimation for temperature measurement with diagnostic ultrasound

BackgroundUltrasound therapies are promising, non-invasive applications with potential to significantly improve, e.g. cancer therapies like viro- or immunotherapy or surgical applications. However, a crucial step towards their breakthrough is still missing: affordable and easy-to-handle quality assurance tools for therapy devices and ways to verify treatment planning algorithms. This deficiency limits the safety and comparability of treatments.MethodsTo overcome this deficiency accurate spatial and temporal temperature maps could be used. In this paper, the suitability of temperature calculation based on time-shifts of diagnostic ultrasound backscattered signals (echo-time-shift) is investigated and associated uncertainties are estimated. Different analysis variations were used to calculate the time-shifts: discrete and continuous methods as well as different frames as a reference for temperature calculation (4 s before, 16 s before the frame of interest, base frame). A sigmoid function was fitted and used to calculate temperatures. Two-dimensional temperature maps recorded during and after therapeutic ultrasound sonication were examined. All experiments were performed in agar-graphite phantoms mimicking non-fatty tissue, with high-intensity focused ultrasound being the source of heating.ResultsContinuous methods are more accurate than discrete ones, and uncertainties of calculated temperatures are in general lower, the earlier the reference frame was recorded. Depending on the purpose of the measurement, a compromise has to be made between the following: calculation accuracy (early reference frame), tolerance towards small movements (late reference frame), reproducing large temperature changes or cooling processes (reference frame at a certain point in time), speed of the algorithm (discrete (fast) vs. continuous (slower) shift calculation), and spatial accuracy (interval size for index-shift calculation). Within the range from 20 °C to 44 °C, uncertainties as low as 12.4 % are possible, being mainly due to medium properties.ConclusionsTemperature measurements using the echo-time-shift method might be useful for validation of treatment plan algorithms. This might also be a comparatively accurate, fast, and affordable method for laboratory and clinical quality assessment. Further research is necessary to improve filter algorithms and to extend this method to multiple foci and the usage of temperature-dependent tissue quantities. We used an analytical approach to investigate the uncertainties of temperature measurement. Different analysis variations are compared to determine temperature distribution and development over time.

Metrology of high-intensity therapeutic ultrasound within the EMRP project ‘External Beam Cancer Therapy’. Characterization of sources

Within the framework of a European project several techniques for measurements in high-intensity therapeutic ultrasound (HITU) fields were improved in a collaboration of four national metrology institutes (NPL, UK; PTB, Germany; INRIM, Italy; UME, Turkey). This contribution focuses on measurement techniques for the characterization of HITU sources, in particular on measurements of the acoustic output power of HITU transducers and on the characterization of their pressure field.

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.

Determination of Acoustic Cavitation Probabilities and Thresholds Using a Single Focusing Transducer to Induce and Detect Acoustic Cavitation Events: II. Systematic Investigation in an Agar Material

In the accompanying article (Part I), a method is described to determine acoustic cavitation probabilities in tissue-mimicking materials (TMMs) using a high-intensity focused ultrasound (HIFU) transducer for both inducing and detecting the acoustic cavitation events, and its suitability for different sonication modes like continuous wave, single pulses (with pulse lengths from microseconds to milliseconds) and repeated burst signals is discussed. In Part II, the use of the method for a systematic study of the dependence of the acoustic cavitation thresholds in 3% (by weight) agar phantoms on the temporal sonication parameters is discussed. The values obtained at a frequency of 1.06 MHz, ranging from (0.58 ± 0.12) MPa for a 3-s continuous wave mode sonication to (5.2 ± 1.0) MPa for single shots with a length of 10 wave cycles, are discussed and interpreted on the basis of literature values and their self-consistency.

Determination of Acoustic Cavitation Probabilities and Thresholds Using a Single Focusing Transducer to Induce and Detect Acoustic Cavitation Events: I. Method and Terminology

A method to determine acoustic cavitation probabilities in tissue-mimicking materials (TMMs) is described that uses a high-intensity focused ultrasound (HIFU) transducer for both inducing and detecting the acoustic cavitation events. The method was evaluated by studying acoustic cavitation probabilities in agar-based TMMs with and without scatterers and for different sonication modes like continuous wave, single pulses (microseconds to milliseconds) and repeated burst signals. Acoustic cavitation thresholds (defined here as the peak rarefactional in situ pressure at which the acoustic cavitation probability reaches 50%) at a frequency of 1.06 MHz were observed between 1.1 MPa (for 1 s of continuous wave sonication) and 4.6 MPa (for 1 s of a repeated burst signal with 25-cycle burst length and 10-ms burst period) in a 3% (by weight) agar phantom without scatterers. The method and its evaluation are described, and general terminology useful for standardizing the description of insonation conditions and comparing results is provided. In the accompanying second part, the presented method is used to systematically study the acoustic cavitation thresholds in the same material for a range of sonication modes.

On the reliability of voltage and power as input parameters for the characterization of high power ultrasound applications

For power levels up to 200 W and sonication times up to 60 s, the electrical power, the voltage and the electrical impedance (more exactly: the ratio of RMS voltage and RMS current) have been measured for a piezocomposite high intensity therapeutic ultrasound (HITU) transducer with integrated matching network, two piezoceramic HITU transducers with external matching networks and for a passive dummy 50 Ω load. The electrical power and the voltage were measured during high power application with an inline power meter and an RMS voltage meter, respectively, and the complex electrical impedance was indirectly measured with a current probe, a 100:1 voltage probe and a digital scope. The results clearly show that the input RMS voltage and the input RMS power change unequally during the application. Hence, the indication of only the electrical input power or only the voltage as the input parameter may not be sufficient for reliable characterizations of ultrasound transducers for high power applications in some c...

A low-cost, easy-to-handle calibration phantom for MR thermometry in HIFU fields

In this work, a calibration phantom based on an electrical resistor heating rod is presented, which creates a temperature distribution similar to those in typical HIFU fields (diameter ∼2 mm, heating rate ∼10 K/s). The temperature distribution is measured first with a calibrated reference device and then with MR thermometry. Equal conditions for both measurements can be ensured by monitoring the voltage and current across the resistor during the heating. From the comparison of both measurements, the accuracy of the MR measurement can be assessed. The calibration phantom is MR compatible, and it has high reproducibility and low production costs. A wide variety of possible heating sequences can be employed and the phantom has a compact and easy setup. Thus, it can be used to quickly verify the results from newly developed MR thermometry sequences as well as to evaluate the uncertainties of existing sequences.