Light diffraction by two parallel superposed ultrasonic waves of the frequency ratio 1:2

The paper constitutes a concise study of light diffraction by two superposed ultrasonic waves of the frequency ratio 1:2, beyond the Raman-Nath diffraction regime. Relevant numerical simulations of light intensity distribution within separate diffraction orders are shown based on the Nth order approximation method (NOA). A comparison is given between generalized theoretical predictions (N=7) and simplified formulas of Mertens (N=2) [R. Mertens, Diffraction of light by two parallel superposed supersonic waves, one being the nth harmonic of the other: A critical study of the methods leading to approximate solutions in finite form, Proc. Indian Acad. Sci. 55 (1962) 63-98.]. Moreover, the results of the performed experiment are presented which indicate limited applicability range of Mertens' pioneer description of 1962.

Reconstruction of ultrasonic fields by deconvolving the hydrophone aperture effects. I. Theory and simulation

In many ultrasound applications-especially in the area of medical diagnostics-conventional sensors and particularly piezoelectric hydrophones do not comply with the requirements for "point detectors". It therefore suggests itself to develop methods, which allow ultrasonic fields to be reconstructed from the spatially averaged measurement values with the necessary spatial resolution. In this publication, this task will be treated as an inverse problem. It will be shown by simulations that it is possible with the aid of reconstruction methods to invert the aperture effects of the sensor. Three different reconstruction methods are tested: (1) Wiener's method furnishes the best approximation of the field to be reconstructed in the sense of the minimum mean square error. (2) The power spectral equalization method is based on equalization between the spatial power spectral densities of the field to be reconstructed and that of its estimated value. (3) With the maximum a-posteriori method, the maximum a-posteriori probability density function for the reconstructed field is searched. The results of this paper show that the quality of reconstruction depends on the signal-to-noise ratio (SNR) and the spatial frequency bandwidth of the sound field investigated. Good quality reconstructions were obtained at correlation coefficients larger than 0.995 and an SNR of at least 40 dB. In general, the quality of reconstruction decreases with decreasing SNR and is unacceptable at SNR = 20 dB. The quality also decreases with decreasing axial distance from the source that is with increasing spectral frequency bandwidth.

Reconstruction of ultrasonic fields by deconvolving the hydrophone aperture effects. II. Experiment

The correction of the averaging effect of finite-size hydrophones is at present realized by various methods which are based on idealized models for the sound field and thus are valid only to a limited extent. In the present article three methods for inverting the aperture effects of the hydrophone viz. the Wiener method, the power spectral equalization (PSE) method and the maximum a posteriori (MAP) method, are experimentally tested and their efficiency is compared. It is shown that both for a rectangular and for a circular hydrophone aperture all three methods are suitable to reconstruct complex sound field structures from spatially averaged data. The results of this paper confirm the finding of Part I [Ultrasonics 39 (2002) 603], i.e. the quality of reconstruction depends on the spatial frequency bandwidth of the sound field investigated. Good quality reconstructions are obtained when the effective dimensions of the hydrophone's sensing area are directly derived from the spatial frequency spectrum of the measurement signal. The Wiener method and the MAP method are superior to the PSE method, since the PSE filter does not take phase information into account. For the Wiener and the MAP method normalized correlation coefficients, r(pp), between the reconstructed sound pressure field and that of a reference measurement of typically 0.97 are obtained provided that the axial distance is not too small. Although the working frequency used for the experiments is rather low, it can be expected that the results obtained are applicable at higher frequencies provided that the complex spatial transfer function is determined with the necessary high precision.

Improvement of the lateral resolution of finite-size hydrophones by deconvolution

In various fields of ultrasound applications, frequencies well above 10 MHz are used. As a consequence of this, ultrasound sensors, especially the piezoelectric hydrophones presently available which are used for the characterization of the respective fields, can no longer be considered as point receivers. By means of numerical deconvolution, the adverse averaging effect caused by the finite sensor size can be revoked. The efficiency of the deconvolution process is dealt with for both numerical simulations and experimental investigations. Best results were obtained using a reconstruction filter consisting of a combination of a Wiener filter, a pruning filter and an additional low-pass filter.

[Trichoadenoma of Nikolowski--review of four decades and seven new cases]

Trichoadenoma of Nikolowski is a rare, benign, well differentiated, slowly growing tumor of the hair follicle which was first described in 1958 by Nikolowski. 26 cases reported in the literature since then and our seven cases show the following features: a tumor of adults, mostly in the face (57.5%) or buttocks (24.2%), size in general 0.5-1.5 cm, no sex preference. According its morphological differentiation, trichoadenoma is situated between a trichofolliculoma and a trichoepithelioma. The clinical appearance is, misleading, suggesting the diagnosis of basal cell carcinoma or epidermal cyst.

Shock-wave measurement using a calibrated interferometric fiber-tip sensor

The results of shock-wave measurements using a calibrated fiber-tip sensor based on a Michelson interferometer are presented. A transfer function, obtained by an independent experiment that describes the properties of the sensor system, was used to correct the measured shock-wave data in the Fourier frequency domain. The phase of the transfer function was determined from its amplitude by a fitting procedure using minimum-phase terms. As an example of application, the acoustic output field of an electromagnetic lithotriptor was investigated, and the shock-wave source was reliably characterized. The measured data provide a basis for estimating the hazard to which a patient is exposed during shock-wave treatment and for optimizing a lithotriptor system to produce a sharply localized and effective acoustic field.

[Sudden fatalities in mechanically restrained patients]

We report on 7 nursing home or hospital patients who died suddenly and unexpectedly during physical restraint. Four of the patients were found dead hanging beside their beds, with their waist restraints displaced to the thorax. In spite of a variety of preexisting diseases, asphyxia by thorax compression was the most probable cause of death. Three other patients, when falling out of their beds, were strangulated by the head opening of a nursing bedcover fixed to the bed. In all instances the fatal accidents resulted from improper handling of the restraint devices, namely from the omission of bed rails as well as of the obligatory waist belt lateral fixations. The bedcover type involved in three fatalities is destined for care purposes but not licensed as a restraint device. Physical restraint fatalities can be avoided to a large extent if the producers' instructions are strictly observed, and only especially trained and supervised personnel is admitted to this field of duties.

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%.

Microwatt ultrasonic power determination using laser interferometry

A system using a water-immersed balance with laser interferometer for radiation force measurements has been developed which is capable of determining powers in the range of a few microwatts up to about 100 mW. High consistency of measuring values and low background noise levels are achieved by compensating thermal and atmospheric pressure drifts, shielding the balance system from convection currents and effective isolation from building vibrations. The sensitivity ascertained is 104 micrometer/mW or, expressed as interferometer steps (IS), 8740 IS/mW. The standard deviation for a power output of 100 mu W is less than 1% ; it increases to 11% for 3.5 mu W, the lowest value measured.