Parag V. Chitnis

Photoacoustic-guided convergence of light through optically diffusive media

We demonstrate that laser beams can be converged toward a light-absorbing target through optically diffusive media by using photoacoustic-guided interferometric focusing. The convergence of light is achieved by shaping the wavefront of the incident light with a deformable mirror to maximize the photoacoustic signal, which is proportional to the scattered light intensity at the light absorber.

Feasibility of using RH795 dye for photoacoustic imaging of neuro-electrical activity

Currently, the most researched noninvasive approach for monitoring neuro-electrical activity involves opticalfluorescence imaging, which suffers from limited imaging penetration. We propose an alternative approach, photoacoustic imaging (PAI) of biopotentials, that relies on transient absorption of light by voltage-sensitive probes and subsequent generation/detection of ultrasound. PAI-based voltage imaging approach can offer the same advantages as the fluorescence imaging in terms of sensitivity and molecular specificity, but it also can significantly extend the imaging depth. In this pilot study we are investigating the feasibility of photoacoustically visualizing biopotentials in rat pheochromocytoma (PC12) cells tagged with voltage-sensitive styrylpyridinium dye, RH795. A change in the intramembrane potential was induced in PC12 cells by adding tetraphenylborate (TPB) to the cell culture. A custommade absorption spectrophotometer was used to verify the change in optical absorption of RH795 dye as a result of TPBinduced electrical fields. Absorption spectra recorded before and after the addition of 100 μM TPB exhibited a wavelength shift of the absorption peak (approximately 510 nm to 550 nm) as well as an increase in the overall magnitude of absorption in the wavelength range of 500-1000 nm. The absorption spectral measurements indicated that RH795 is a good candidate as a voltage-sensitive dye for photoacoustically tracking changes in cell-membrane potential.

Correlation of rupture dynamics to the nonlinear backscatter response from polymer-shelled ultrasound contrast agents

Polymer-shelled ultrasound contrast agents (UCAs) may expel their encapsulated gas subject to ultrasound- induced shell buckling or rupture. Nonlinear oscillations of this gas bubble can produce a subharmonic component in the ultrasound backscatter. This study investigated the relationship between this gas-release mechanism and shell-thickness- to-radius ratios (STRRs) of polymer-shelled UCAs. Three types of polylactide-shelled UCAs with STRRs of 7.5, 40, and 100 nm/μm were studied. Each UCA population had a nominal mean diameter of 2 μm. UCAs were subjected to increasing static overpressure ranging from 2 to 330 kPa over a duration of 2 h in a custom-designed test chamber while being imaged using a 200× magnification video microscope at a frame rate of 5 frames/s. Digitized video images were binarized and processed to obtain the cross-sectional area of individual UCAs. Integration of the normalized cross-sectional area over normalized time, defined as buckling factor (Bf), provided a dimensionless parameter for quantifying and comparing the degree of pre-rupture buckling exhibited by the UCAs of different STRRs in response to overpressure. The UCAs with an STRR of 7.5 nm/μm exhibited a distinct shell-buckling phase before shell rupture (Bf <; 1), whereas the UCAs with higher STRRs (40 and 100 nm/μm) did not undergo significant prerupture buckling (Bf ≈ 1). The difference in the overpressure response was correlated with the subharmonic response produced by these UCAs. When excited using 20-MHz ultrasound, individual UCAs (N = 3000) in populations that did not exhibit a buckling phase produced a subharmonic response that was an order of magnitude greater than the UCA population with a prominent pre-rupture buckling phase. These results indicate the mechanism of gas expulsion from these UCAs might be a relevant factor in determining the level of subharmonic response in response to high-frequency ultrasound.

An investigation of contrast-agent shell-rupture threshold in response to overpressure

The stiffness of ultrasound contrast agent (UCA) shells is a critical parameter that determines responsiveness to acoustic excitation and half-life in circulation. The current study investigates the static pressure threshold for UCA rupture, which potentially can be used to infer the material properties of the UCA-shell. Polymer-shelled UCAs (POINT Biomedical) with a nominal mean diameter of 3 µm were subjected to overpressure from 3.5 to 155 kPa over a one hour duration while being imaged using a CCD-based video-microscope (50X magnification) at 1 frame/s. The static pressure corresponding to each image of the UCAs was recorded. A semi-automated post-processing algorithm was employed to determine the size and the static pressure threshold for rupture for each individual UCA. The majority of UCAs did not respond to static pressure change until the threshold for shell rupture was exceeded. Shell rupture resulted in abrupt destruction of the UCA. Eighteen trials resulted in a total of 976 valid UCAs, of which 851 ruptured when subjected to overpressure; 125 remained intact. The rupture pressures for 632 UCAs exhibited a Gaussian distribution (Gaussian group) centered around a mean pressure of 42.9 ± 18.7 kPa. The remaining 219 UCAs (residual group) ruptured at pressures ranging from 86 to 155 kPa with a mean of 118.2 ± 25.9 kPa. The distinctly different rupture behavior exhibited by the Gaussian, residual, and intact groups appeared to be unrelated to UCA diameter, but may have been related to surface defects.

A frequency-domain non-contact photoacoustic microscope based on an adaptive interferometer

A frequency-domain, non-contact approach to photoacoustic microscopy (PAM) that employs amplitude-modulated (0.1-1 MHz) laser for excitation (638-nm pump) in conjunction with a 2-wave mixing interferometer (532-nm probe) for non-contact detection of photoacoustic waves at the specimen surface is presented. A lock-in amplifier is employed to detect the photoacoustic signal. Illustrative images of tissue-mimicking phantoms, red-blood cells and retinal vasculature are presented. Single-frequency modulation of the pump beam directly provides an image that is equivalent to the 2-dimensional projection of the image volume. Targets located superficially produce phase modulations in the surface-reflected probe beam due to surface vibrations as well as direct intensity modulation in the backscattered probe light due to local changes in pressure and/or temperature. In comparison, the observed modulations in the probe beam due to targets located deeper in the specimen, for example, beyond the ballistic photon regime, predominantly consist of phase modulation.

Low-power ultrasound imaging systems using time delay spectrometry

Ultrasound (US) imaging systems have undergone substantial miniaturization leading to many clinical applications that rely on batteries. However, current clinical US systems utilizing pulse-echo imaging require a high voltage and short duration transmit pulse along with electronics that operate in the MHz frequency range. We are investigating a novel imaging method that employs time-delay spectrometry (TDS) as an alternative to pulse-echo and uses low-voltage (∼5V peak-to-peak) transmit pulses along with low-power electronics operating in the kHz frequency range.

Frequency domain non-contact photoacoustic microscopy

We present a non-contact “frequency domain photoacoustic microscope” (Fd-PAM) in which photoacoustic signal is generated by an amplitude modulated continuous-wave (CW) laser and detected at the sample surface using two wave mixing (interferometer) in a photorefractive crystal (PRC). The optical detection eliminates the need for a coupling medium, thus making the probe contactless and mitigates loss in signal-to-noise ratio (SNR) resulting from attenuation associated with wave propagation from the sample to the sensor. The single frequency excitation enables the use of extremely narrow band detection techniques like a lock-in amplifier for noise suppression. Our approach also can image multi-layered specimen and directly produce an image that is equivalent to the maximum-intensity projection of the 3D image volume.

Patient-Specific Studies of Pelvic Floor Biomechanics Using Imaging

Biomechanical modeling and simulation of the pelvic floor structures have caught much attention in the past decade. Many computational approaches were developed with the goal of advancing our understanding the mechanism of the pelvic floor pathologies and improving treatment clinically. In this chapter, we review some of the existing work on building three dimensional (3D) models of the pelvic floor and modeling its mechanics.

Ultrasound characterization of interface oscillation as a proxy for ventriculoperitoneal shunt function

Hydrocephalus, where cerebrospinal fluid (CSF) production rate is greater than reabsorption rate, leads to impaired neurological function if left untreated. Ventriculoperitoneal shunts (VPS) are implanted in the brain ventricles to route CSF. VPS systems have a high failure rate, and failure symptoms resemble symptoms of common maladies. The current gold standard for shunt diagnosis, surgical intervention, poses high risk and requires an expensive procedure for patients. Current non-invasive methods lack proper insight to assist physicians. We propose a noninvasive method of characterizing the oscillation of the shunts pressure-relief valve to assist physicians in shunt diagnosis. Brightness-mode and motion-mode ultrasound images can be used to determine fluid flow. Blockage in the system could be detected by observing the phase change of the ultrasound signal in different flow cases with or without perturbation. Future testing and implementation can allow for the use of this method in localizing and identifying the modality of failure.

Subharmonic Response of Polymer Contrast Agents Based on the Empirical Mode Decomposition

The subharmonic threshold for ultrasound contrast agents has been defined as a 20-25 dB difference between the fundamental and subharmonic (2/1) spectral components of the backscatter signal. However, this Fourier-based criterion assumes a linear time-invariant signal. A more appropriate criterion for short cycle and frequency-modulated waveforms is proposed with an adaptive signal-processing approach based on the empirical mode decomposition (EMD) method. The signal is decomposed into an orthogonal basis known as intrinsic mode functions (IMFs) and a subharmonic threshold is defined with respect to the energy ratio of the subharmonic IMF component to that of the incident signal. The method is applied to backscatter data acquired from two polymer-shelled contrast agents, Philips (#38, mean diameter 2.0 μm) and Point Biomedical (#12027, mean diameter 3.9 μm). The acoustic backscatter signals are investigated for a single contrast agent subjected to monofrequency (20 MHz, 20 cycles) and chirp (15-25 MHz, 20 cycles) forcing for incident pressures ranging from 0.5 to 2.4 MPa. In comparison to the spectral peak difference (20 dB) criterion, the EMD method is more sensitive in determining subharmonic signals.