Gopal B. Avinash

Laser doppler velocimetry of basilar membrane vibration

A method is described for the measurement of basilar membrane (BM) vibration velocimeter (LDV). The instrumentation was coupled to a compound microscope which served to visualize reflective glass microbeads placed on the BM. The laser beam of the LDV was focused in the microscope object plane and positioned over the reflective bead. We show examples of frequency tuning curves and displacement input/output intensity functions obtained with the technique.

Asynchronous neural activity recorded from the round window.

Voltage recorded from an electrode on the round window (RW) of guinea pig has characteristics that reflect the activity of auditory-nerve fibers in the absence of acoustic stimulation. Fast Fourier transformation (FFT) of the noise recorded from the RW electrode shows a broad spectral peak from 0.8-1.0 kHz. The magnitude of the biological noise is increased by high-frequency, bandlimited acoustic noise stimulation. Pure tones can suppress or enhance the spectral components around 0.8-1.0 kHz depending on frequency and intensity. Kainic acid applied to the intact RW membrane eliminates the biological noise (and the evoked cochlear whole-nerve responses) without alteration of the cochlear microphonic or the summating potential. The spectral characteristics of the biological noise seem to be related to the elemental waveform contributed by the individual auditory-nerve fibers to the voltage recorded at the RW electrode [Kiang et al., Electrocochleography, edited by R. J. Ruben, C. Elbering, and G. Solomon (University Park, Baltimore, 1976)].

The influence of loud sound on red blood cell velocity and blood vessel diameter in the cochlea

Using intravital microscopy, we observed both decreases in red blood cell velocity and possible vasoconstriction in stria vascularis capillaries of the rat cochlea in response to loud sound (Quirk et al., 1991). However, our observation of vasoconstriction was subject to error in measurements from the two dimensional images obtained with our silicon intensified (SIT) camera due to the influence of focus causing image blur. The purpose of the current study was to apply an extended focus microscopy technique to obtain quantitative assessment of vessel diameter changes (Avinash et al., 1992), as well as to extend these studies to the guinea pig model. Broad-band sound stimulation at intensities of 84 dB SPL and 110 dB SPL were used. The results show that loud sound induces a sequence of changes in cochlear blood flow. Stimulation with 110 dB SPL resulted in a mean increase (maximum = 27%) in red blood cell velocity for the first 20 min of exposure followed by a gradual decrease below baseline (minimum = -12%) prior to termination of the signal. This velocity decrease and subsequent recovery were associated with significant changes in vessel diameters of selected and measured capillaries. In contrast, the 84 dB SPL stimulus caused an increase in red blood cell velocity (maximum = 20%) and vessel diameter (mean = 7.5) during the stimulation period. No recovery was observed during the 10 min observation period following sound. Several possible mechanisms responsible for these changes are discussed.

Measurements of Basilar Membrane Tuning and Distortion with Laser Doppler Velocimetry

A method is described for the measurement of basilar membrane (BM) vibration. It makes use of commercially made laser Doppler velocimetry (LDV) instrumentation which is adapted to a compound microscope. This allows visualization reflective objects placed on the EM and the focusing of the laser beam onto the reflective object (a glass microbead). We show examples of frequency tuning curves and displacement input/output intensity functions obtained with the technique and demonstrate that cubic distortion of both the local and propagated kind are present in mechanical responses of the membrane.

3-D analysis of F-actin in stereocilia of cochlear hair cells after loud noise exposure☆

Fluorescence microscopy can be a useful tool in the early detection of pathological changes in the stereocilia of outer hair cells which have undergone acoustic overstimulation. Fluorescent phalloidin, a highly specific F-actin stain, can be used to label F-actin in stereocilia. In this study, phalloidin label is used to determine quantitative changes of F-actin in the stereocilia of guinea pigs exposed to loud noise (117 dB; octave band noise, centered at 1 kHz; 4 h). Reliably determining three-dimensional (3-D) structural changes in stereocilia is a challenging problem in optical microscopy since stereocilia diameter is close to the optical resolution limit. In order to alleviate the problem, a computational 3-D microscopy technique is used (Avinash et al., 1992). Whole-mounts of the cochlear second and third turns were examined in a Leitz Orthoplan microscope through a Leitz Plan Apo objective lens (100 x; 1.32 N.A.; 170/0.17). Images were acquired with a charge-coupled device camera where the focus was shifted in 0.2 microns steps using a piezoelectric translator. Images were processed with the appropriate point spread function of the optical system. Analysis of control cochleas indicate that our technique can resolve single stereocilia and distinguish between various intensities of label along each stereocilia. In noise-exposed cochleas, our data show length and intensity changes in the phalloidin label. These results suggest that both depolymerization and polymerization of F-actin can occur in stereocilia of outer hair cells after acoustic overstimulation. Our findings demonstrate the applicability of computational 3-D microscopy to quantitative and qualitative analysis of stereocilia.

Novel method for automated determination of the cancellation parameter in dual-energy imaging: evaluation using anthropomorphic phantom images

Dual-energy imaging shows increased conspicuity and specificity of lung nodule detection through the removal of undesired contrast resulting from overlying bone structures. We have developed an algorithm that automatically determines the optimal cancellation parameters for a log-subtraction technique for a pair of high- and low-energy images. The core algorithm involves shrinking the data, extracting bone features, extracting salient edge from these bone features, calculating a tissue-cancellation map, computing the maximum-likelihood bone contrast cancellation parameter, and finally, calculating the soft-tissue cancellation parameter using an empirical relationship. We verified the performance of the algorithm using observer studies, in which the value of the tissue-cancellation parameter calculated by the algorithm was compared to the value manually selected by nineteen trained observers. A number of dual-energy images were acquired with a modified GE Revolution XQ/i, flat-panel-detector chest imaging system, using an anthropomorphic phantom. The effects of variables such as patient size, kVp, mAs, lung texture, patient motion, and the presence of foreign objects in field-of-view on algorithmic performance were evaluated. We found that the algorithm-selected parameter values had less variability than those selected by the observers. Furthermore, the algorithm-selected parameter was within the limits of the variability of the observers for all cases.