A standard test phantom for the performance assessment of magnetic resonance guided high intensity focused ultrasound (MRgHIFU) thermal therapy devices.

Purpose: Test objects for High Intensity Focused Ultrasound (HIFU) are required for the standardization and definition of treatment, Quality Assurance (QA), comparison of results between centers and calibration of devices. This study describes a HIFU test object which provides temperature measurement as a function of time, in a reference material compatible with Magnetic Resonance (MR) and ultrasound.Materials and methods: T-Type fine wire thermocouples were used as sensors and 5 correction methods for viscous heating artifacts were assessed. The phantom was tested in a MR-HIFU Philips Sonalleve device over a period of 12 months, demonstrating stability and validity to evaluate the performance of the device.Results: The study furnished useful information regarding the MR-HIFU sessions and highlighted potential limitations of the existing QA and monitoring methods. The importance of temperature monitoring along the whole acoustic path was demonstrated as MR Thermometry readings differed in the three MR plane views (coronal, sagittal, transverse), in particular when the focus was near a soft-tissue/bone interface, where there can be an MR signal loss with significant temperature and thermal dose underestimation (138% variation between the three plane views).Conclusions: The test object was easy to use and has potential as a valid tool for training, QA, research and development for MR guided HIFU and potentially ultrasound guided devices.

Characterisation of a phantom for multiwavelength quantitative photoacoustic imaging.

Quantitative photoacoustic imaging (qPAI) has the potential to provide high- resolution in vivo images of chromophore concentration, which may be indicative of tissue function and pathology. Many strategies have been proposed recently for extracting quantitative information, but many have not been experimentally verified. Experimental phantom-based validation studies can be used to test the robustness and accuracy of such algorithms in order to ensure reliable in vivo application is possible. The phantoms used in such studies must have well-characterised optical and acoustic properties similar to tissue, and be versatile and stable. Polyvinyl chloride plastisol (PVCP) has been suggested as a phantom for quality control and system evaluation. By characterising its multiwavelength optical properties, broadband acoustic properties and thermoelastic behaviour, this paper examines its potential as a phantom for qPAI studies too. PVCP's acoustic properties were assessed for various formulations, as well as its intrinsic optical absorption, and scattering with added TiO2, over a range of wavelengths from 400-2000 nm. To change the absorption coefficient, pigment-based chromophores that are stable during the phantom fabrication process, were used. These yielded unique spectra analogous to tissue chromophores and linear with concentration. At the high peak powers typically used in photoacoustic imaging, nonlinear optical absorption was observed. The Grüneisen parameter was measured to be [Formula: see text] = 1.01 ± 0.05, larger than typically found in tissue, though useful for increased PA signal. Single and multiwavelength 3D PA imaging of various fabricated PVCP phantoms were demonstrated.

Twins and the paradox of dental-age estimations: a caution for researchers and clinicians.

The biological age difference among twins is frequently an issue in studies of genetic influence on various dental features, particularly dental development. The timing of dental development is a crucial issue also for many clinicians and researchers. The aim of this study was therefore to verify within groups of twins how dental development differs, by applying Demirjian's method, Mincer's charts of development of third molars and two of Cameriere's methods for dental age estimation, which are among the most popular methods both in the clinical and the forensic scenario. The sample consisted of 64 twin pairs: 21 monozygotic, 30 dizygotic same-sex and 13 dizygotic opposite-sex with an age range between 5.8 and 22.6 years. Dental age was determined from radiographs using the mentioned methods. Results showed that dental age of monozygotic twins is not identical even if they share all their genes. The mean intra-pair difference of monozygotic pairs was low and similar to the difference in dizygotic same-sex twins; the maximum difference between monozygotic twins, however, was surprisingly large (nearly two years). This should lead to some circumspection in the interpretation of systematic estimations of dental age both in the clinical and forensic scenario.

Characterisation and improvement of a reference cylindrical sonoreactor.

This paper describes theoretical and experimental methods for characterising the performance of a 25 kHz sonochemical reactor (RV-25), which is being developed as a reference facility for studying acoustic cavitation at the National Physical Laboratory (NPL). Field measurements, acquired in different locations inside the sonoreactor, are compared with finite element models at different temperatures, showing that relatively small temperature variations can result in significant changes in the acoustic pressure distribution (and consequent cavitation activity). To improve stability, a deeper insight into the way energy is transferred from the power supply to the acoustic field is presented, leading to criteria - based on modal analysis - to dimension and verify an effective temperature control loop. The simultaneous use of measurements and modelling in this work produced guidelines for the design of multi-frequency cylindrical sonoreactors, also described.

Measurements, phantoms, and standardization.

The last 25 years has seen a number of significant developments in the establishment of a measurement infrastructure supporting medical applications of ultrasound. This has allowed manufacturers and users of medical ultrasonic equipment to undertake and compare measurements of key parameters describing the magnitude or strength of the applied ultrasonic field in a meaningful and traceable way: for equipment development, standards compliance, and quality assurance purposes. This paper describes the current state of the art for measurement techniques used to determine the key properties of an ultrasonic field, principally acoustic pressure and acoustic power. Measurement tools and methodologies are described in detail, alongside considerations of how these are likely to develop, shaped by user need. The way that these measurement methods underpin a range of international and national specification standards enabling equipment manufacturers to demonstrate that their equipment is safe and fit for purpose is covered.

Utilization of carbon nanofibers for airborne ultrasonic acoustic field detection using heterodyne interferometry.

Carbon nanofibers and nanotubes are currently being utilized as active elements in acoustic sensors for emerging microelectromechanical systems and nanoelectromechanical systems technologies. A methodology for measuring the displacement of carbon nanofibers in combination with heterodyne interferometry is reported here. Experimental results show that ultrasonic field detection is possible using this technique, and results are presented for measurements in the ultrasonic frequency range. This approach could potentially lead to new calibration methods for ultrasonic sensors. A different approach to that of interferometry is also reported for future investigation.

Validated ultrasonic power measurements up to 20 W.

A project has been completed to develop reference methods for the measurement of ultrasonic power with a validated measurement uncertainty of < 7% at power levels of 1 to 20 W over the frequency range 1 to 3 MHz of collimated beams. The project is the result of collaborative research between the Physikalisch-Technische Bundesanstalt, Germany (PTB, DE), the National Physical Laboratory, UK (NPL, UK) and the Netherlands Organisation for Applied Scientific Research, Prevention and Health (TNO-PG, NL). The work has been undertaken under the 4th Framework Programme of the European Community (EC). Primary standard designs of radiation force balances based on both absorbing and reflecting targets have been constructed. To avoid heating effects, the measurements should be done relatively quickly (10 to 20 s). The methods have been validated using ultrasound (US) transducers that demonstrated an adequate short and long-term stability; a method to detect cavitation based on monitoring the acoustic signals produced by bubble oscillation and collapse has been confirmed. It has been shown that only the detection of the subharmonic can be used in practice as cavitation detector. Different procedures for obtaining degassed water have been investigated. A method showing significant promise to be used in a clinical or manufacturer's environment involves the addition of sodium sulphite (Na2SO3). During the validation process, commercially available radiation force balances and ultrasonic physiotherapy devices have also been evaluated. Limitations of current measurement methods and practices, including power measurements made on transducers exhibiting a diverging beam, have been identified. It has been shown that a reflecting target is not appropriate to measure powers of transducers with a ka-value < 30. Based on beam shape and target distance, it has been shown also that proper power measurements using a 45 degrees convex-conical reflecting target can never be performed for transducers with a ka-value < 17.4.

A new anechoic material for medical ultrasonic applications.

This paper describes a newly developed material with acoustic properties that make it ideal for applications as radiation force balance-absorbing targets. The material is now commercially available from National Physical Laboratory (NPL) and is based on a polyurethane rubber. It exhibits an echo reduction of 45 dB, and single-pass transmission loss of 30 dB, both determined at an acoustic frequency of 1 MHz. The composition and structure of the new NPL absorber are presented, along with values for the frequency and temperature variation of the echo reduction and transmission loss. Over the frequency range 1 to 10 MHz, its acoustic properties comply with the requirements for force balance-absorbing targets specified in IEC 61161.

A new method for measuring the effective radiating area of physiotherapy treatment heads.

This paper describes the investigation and validation of a new method for measuring the effective radiating area (AER) of physiotherapy ultrasound treatment heads. The method is based on the use of a conventional radiation force balance, but employs special attenuating apertures that are used to selectively mask off different areas of the treatment head. The resultant reduction in the radiating surface is accompanied by a decrease in output power that is measured using the force balance. The AER of the treatment head is derived from an analysis of the measurements, which essentially involves initially evaluating the minimum area through which 75% of the acoustic power is transmitted. AER values derived using the new method are presented for 17 treatment heads representative of the range of physiotherapy systems commonly used in clinical practice. These are compared to reference values derived using hydrophone scanning, according to the recently published International Standard, IEC 1689. Typical levels of agreement between values of AER derived using the two techniques are +/- 11%. The potential of the method as a rapid, relatively low-cost, means of measuring treatment head AER, applicable in both manufacturing and hospital environments, is assessed.

A strategy for the development and standardisation of measurement methods for high power/cavitating ultrasonic fields: review of high power field measurement techniques.

This review was compiled as part of a project to formulate a UK strategy for the development and standardisation of measurement methods for high power/cavitating ultrasonic fields. It reviews the scientific literature relating to various methods of measuring high power fields which have been developed for application in health care, sonochemistry and industrial ultrasonics, and compares these methods in terms of attributes such as spatial resolution, bandwidth and sensitivity.

Development of standard measurement methods for essential properties of ultrasound therapy equipment.

In a European collaborative project, partly funded by the EC Community Bureau of Reference (BCR), reliable methods of measurement for characterising the output and performance of ultrasound physiotherapy equipment have been developed. Experimental investigations using miniature hydrophones to scan the distribution of pressure in therapeutic fields have been undertaken in combination with theoretical simulations of the sound fields. Important parameters such as Beam Cross Sectional Area (BCSA), Effective Radiating Area (ERA) and Beam Nonuniformity Ratio (BNR) (characterising "Hot-spots": potentially harmful to patients) have been redefined, and these new definitions have been incorporated in a revision of IEC 150:1963. The reproducibility and accuracy of measurements of ERA based on these procedures are presented in detail for a variety of therapy fields. Furthermore, it is shown that the value of the BNR for any treatment head should not exceed 8. Values of effective intensity derived using the new procedures are shown to be significantly higher than those obtained using FDA (USA) definitions, a conclusion in agreement with the theoretical expectations. Measurements on four treatment heads were used to validate the procedures of the proposed revised standard. Values of ERA derived by the two laboratories were in agreement to within 2.5%.

Prediction of in situ exposure to ultrasound: an acoustical attenuation method.

The prediction of the acoustic levels occurring in a patient during ultrasound examination is important for the assessment of equipment safety. While considerable effort has been devoted to theoretical methods of predicting exposure levels, there is a need to develop simple experimental methods which are universally applicable to the wide range of ultrasonic fields generated by medical ultrasonic equipment. This article outlines a number of methods that have been proposed and explores in detail a new experimental method based on the use of acoustical attenuators, made of low-density polyethylene, which are placed in the ultrasound beam between the transducer and the measuring hydrophone. Measurements of important acoustical quantities have been made using a measurement system based on a multielement hydrophone and comparisons made between this new method and other methods such as electrical attenuation and linear derating theory. The acoustical attenuation technique has been systematically studied, and results are compared with reference levels obtained using tissue-mimicking material. A procedure involving making measurements with the attenuator placed first at the face of the transducer and then at the hydrophone has been developed. By taking the mean value of these two measurements, it has been shown that this new method can simulate within +/- 10% the acoustic levels obtained using the reference. It is concluded that it could be the basis of a universally applicable method.

Prediction of in situ exposure to ultrasound: a proposed standard experimental method.

This article presents results for the prediction of in situ exposure levels using a new experimental technique based on the use of acoustical attenuators. These are made of low-density polyethylene and are placed in the ultrasound beam between the transducer face and the measuring hydrophone. The method has been tested for a number of different ultrasonic fields including those generated by diagnostic medical ultrasonic equipment. Using a tissue-mimicking material as a reference, it is shown that it is possible to use this new method to simulate acoustic pressure levels to within +/- 10% and acoustic intensity and power levels within +/- 20%. The method is proposed as the basis for a standard test method with wide applicability.