Cellular polypropylene polymer foam as air-coupled ultrasonic transducer materials.

Cellular polypropylene polymer foams, also known as ferroelectrets, are compelling candidates for air-coupled ultrasonic transducer materials because of their excellent acoustic impedance match to air and because they have a piezoelectric d(33) coefficient superior to that of PVDF. This study investigates the performance of ferroelectret transducers in the generation and reception of ultrasonic waves in air. As previous studies have noted, the piezoelectric coupling coefficients of these foams depend on the number, size, and distribution of charged voids in the microstructure. The present work studies the influence of these parameters both theoretically and experimentally. First, a three-dimensional model is employed to explain the variation of piezoelectric coupling coefficients, elastic stiffness, and dielectric permittivity as a function of void fraction based on void-scale physics and void geometry. Laser Doppler vibrometer (LDV) measurements of the effective d(33) coefficient of a specially fabricated prototype transmitting transducer are then shown which clearly indicate that the charged voids in the ferroelectret material are randomly distributed in the plane of the foam. The frequency-dependent dynamic d(33) coefficient is then reported from 50 to 500 kHz for different excitation voltages and shown to be largely insensitive to drive voltage. Lastly, two ferroelectret transducers are operated in transmit-receive mode and the received signal is shown to accurately represent the corresponding signal generated by the transmitting transducer as measured using LDV.

Application of a matched filter approach for finite aperture transducers for the synthetic aperture imaging of defects.

The suitability of the synthetic aperture imaging of defects using a matched filter approach on finite aperture transducers was investigated. The first part of the study involved the use a finite-difference time-domain (FDTD) algorithm to simulate the phased array ultrasonic wave propagation in an aluminum block and its interaction with side-drilled hole-like defects. B-scans were generated using the FDTD method for three active aperture transducer configurations of the phased array (a) single element and (b) 16-element linear scan mode, and (c) 16-element steering mode. A matched filter algorithm (MFA) was developed using the delay laws and the spatial impulse response of a finite size rectangular phased array transducer. The conventional synthetic aperture focusing technique (SAFT) algorithm and the MFA were independently applied on the FDTD signals simulated with the probe operating at a center frequency of 5 MHz and the processed B-scans were compared. The second part of the study investigated the capability of the MFA approach to improve the SNR. Gaussian white noise was added to the FDTD generated defect signals. The noisy B-scans were then processed using the SAFT and the MFA and the improvements in the SNR were estimated. The third part of the study investigated the application of the MFA to image and size surface-crack-like defects in pipe specimens obtained using a 45 degrees steered beam from a phased array probe. These studies confirm that MFA is an alternative to SAFT with little additional computational burden. It can also be applied blindly, like SAFT, to effect synthetic focusing with distinct advantages in treating finite transducer effects, and in handling steered beam inspections. Finally, limitations of the MFA in dealing with larger-sized transducers are discussed.

Magnitude of acute respiratory infections in under five.

A community-based study was carried out in a rural area of Delhi to measure the prevalence and incidence of acute respiratory infections among children below the age of 5 years. The prevalence of 12.1%, was similar in boys and girls and was seen to decline with age. The incidence of acute respiratory infections was 2.5 episodes per child per year; it was not different in boys and girls. There was a statistically significant decline in the incidence with age. Upper respiratory tract infections comprised 87.5% of total acute respiratory infection morbidity while lower respiratory tract infections were 12.5%. Both upper and lower respiratory tract infections declined with increasing age; while the former was similar among boys and girls, the incidence of latter was significantly greater in boys (0.4 episodes per year) as compared to girls (0.2 episodes per year). A total of 87.5% episodes were mild, 10.4% moderate and only 2.1% were severe. The results suggest that acute respiratory infections are a major community health problem and an acute respiratory infection control programme needs to be implemented urgently.

Tuberculous lymphadenitis: correlation of cellular components and necrosis in lymph-node aspirate with A.F.B. positivity and bacillary count.

Cytomorphologic features of tuberculous lymphadenitis cases as observed in lymph-node aspirates were analysed and correlated with AFB positivity and bacillary count. Cytologic features were categorized under three major groups, viz. epithelioid granuloma without necrosis, epithelioid granuloma with necrosis and necrosis without epithelioid granulomas. These three major groups showed a distinct trend in respect of their cellular constituents. While cases with appreciable lymphocytic and multinucleated giant cells component showed a significant decreasing trend, cases with neutrophilic infiltration showed an increasing trend (P less than 0.001). In the above three groups, 9.1 percent, 64.7 percent and 77.4 percent respectively showed AFB positivity, the difference being highly significant (P less than 0.001). All 14 cases with very high bacillary count (greater than 50 bacilli per 500 oil immersion field) were associated with necrosis and 71.4% of them neutrophilic infiltration. A univariate analysis revealed that in presence of lymphocytes, epithelioid cells and Langhan's giant cells, AFB positivity was significantly lower while the picture was just the reverse in presence of necrosis and neutrophilic infiltration (P less than 0.001). The odd's rations for all these variables were highly significant (P less than 0.001). However, a multivariate regression analysis revealed that necrosis was the only independent contributing factor towards AFB positivity.