A principal tool for the visual inspection of the middle ear in the hearing clinic is the surgical stereo-microscope. We have developed a compact accessory for the surgical microscope that enables volumetric optical coherence tomography (OCT) imaging of the middle ear as well as functional vibratory imaging with subnanometer sensitivity. The sensitivity to vibration is achieved by careful engineering of the microscope attachment and frequency-domain processing. The microscope attachment integrates the entire OCT interferometer onto a custom aluminum base that mounts directly to the accessory area at the foot of most surgical microscopes. This approach is effective at removing high-frequency phase-noise, thus enabling near shot-noise limited sensitivity above ~2 kHz even though the OCT system is suspended above the patient by a boom arm. We analyze the vibratory response in the frequency domain, hence our ability to measure vibrations in the tympanic membrane and ossicles is near the shot-noise limit. As a demonstration of this system we have recorded in vivo volumetric images of a healthy human patient as well as the vibratory response at the tympanic membrane down to the hearing threshold. We also show that it is possible to use time averaging to drive the noise floor down below 2 pm which allowed us to make the first measure of distortion product otoacoustic emissions (DPOAE) using OCT. Finally, the system can easily be taken on and off of the surgical microscope and when in use does not impinge on the normal view through the surgical microscope.
Zebrafish, an auditory specialist among fish, offer analogous auditory structures to vertebrates and is a model for hearing and deafness in vertebrates, including humans. Nevertheless, many questions remain on the basic mechanics of the auditory pathway. Phase-sensitive Optical Coherence Tomography has been proven as valuable technique for functional vibrometric measurements in the murine ear. Such measurements are key to building a complete understanding of auditory mechanics. The application of such techniques in the zebrafish is impeded by the high level of pigmentation, which develops superior to the transverse plane and envelops the auditory system superficially. A zebrafish double mutant for nacre and roy (mitfa-/- ;roya-/- [casper]), which exhibits defects for neural-crest derived melanocytes and iridophores, at all stages of development, is pursued to improve image quality and sensitivity for functional imaging. So far our investigations with the casper mutants have enabled the identification of the specialized hearing organs, fluid-filled canal connecting the ears, and sub-structures of the semicircular canals. In our previous work with wild-type zebrafish, we were only able to identify and observe stimulated vibration of the largest structures, specifically the anterior swim bladder and tripus ossicle, even among small, larval specimen, with fully developed inner ears. In conclusion, this genetic mutant will enable the study of the dynamics of the zebrafish ear from the early larval stages all the way into adulthood.
Coded excitation can improve the performance factors, such as SNR and CTR, of harmonic imaging with a low voltage transmit waveform. For these purposes, harmonic imaging methods using Golay codes with advantages in range sidelobe levels and implementation simplicity have been proposed. However, they require four transmit-receive (T/R) events to form each scan line. This work describes a new harmonic Golay coded excitation technique to overcome this problem. The proposed method can produce two scan lines through four T/R events using four pairs of codes. On the first T/R cycle, the first pair of codes is fired sequentially, one at a time, along each of the two scan lines, where the two codes are designed such that their second harmonic components are mutually orthogonal Golay codes. The same transmit sequence is carried out with the second pair of codes, each of which being 180 degrees out of phase with the corresponding one of the first pair of codes to remove the fundamental components by simply adding the two resulting received signals. The third and fourth T/R cycles are followed in the same manner, but with the codes whose harmonic components are composed of the complementary set of the mutually orthogonal Golay codes used in the first T/R cycle and their sign inverted codes, respectively. Consequently, the mutually orthogonal Golay codes and their complementary set of codes representing only the harmonic components are obtained after four T/R events. Finally, using the orthogonal and complementary properties, the coded harmonic signals along each scan line can easily be separated and compressed. Computer simulation results show that the proposed method can successfully perform pulse-inversion harmonic imaging to produce two scan lines simultaneously after four T/R events with coded sequences.
An efficient method for separating the harmonic component (2f0) from the fundamental component (f0) using harmonic
quadrature demodulation is presented. In the proposed method, the focused ultrasound signal is mixed with cosine and
sine signal waveforms of harmonic frequency 2f0 to produce the inphase and quadrature components, respectively. The
quadrature component is Hilbert-transformed and then added to the inphase component. This process cancels out both
the high and low frequency components of the mixed fundamental signal and the high frequency component of the
mixed harmonic signal, leaving only the envelope of the harmonic signal at the base band. This signal is then fed to a
low-pass filter to remove out of band noise. In summary, this method can extract the harmonic signal after a single
transmit-receive event even when there exists frequency overlap between the f0 and 2f0 components. Hence, the
proposed method is superior to the pulse inversion method which requires twice as many transmit-receive cycles as well
as the conventional filtering method which has a bandwidth limitation. Therefore, one can find the proposed method
useful not only for tissue harmonic imaging but also for contrast agent imaging in applications where high frame rate or
low motion artifact is important. The proposed method is verified by both the analytic and computer simulation studies.
For a stationary target, the difference between the estimated harmonic signals by the proposed and the pulse inversion
methods is within 0.1%.