Vijitha Periyasamy

Monte Carlo simulation of light transport in turbid medium with embedded object—spherical, cylindrical, ellipsoidal, or cuboidal objects embedded within multilayered tissues

Abstract. Monte Carlo modeling of light transport in multilayered tissue (MCML) is modified to incorporate objects of various shapes (sphere, ellipsoid, cylinder, or cuboid) with a refractive-index mismatched boundary. These geometries would be useful for modeling lymph nodes, tumors, blood vessels, capillaries, bones, the head, and other body parts. Mesh-based Monte Carlo (MMC) has also been used to compare the results from the MCML with embedded objects (MCML-EO). Our simulation assumes a realistic tissue model and can also handle the transmission/reflection at the object-tissue boundary due to the mismatch of the refractive index. Simulation of MCML-EO takes a few seconds, whereas MMC takes nearly an hour for the same geometry and optical properties. Contour plots of fluence distribution from MCML-EO and MMC correlate well. This study assists one to decide on the tool to use for modeling light propagation in biological tissue with objects of regular shapes embedded in it. For irregular inhomogeneity in the model (tissue), MMC has to be used. If the embedded objects (inhomogeneity) are of regular geometry (shapes), then MCML-EO is a better option, as simulations like Raman scattering, fluorescent imaging, and optical coherence tomography are currently possible only with MCML.

Monte Carlo simulation of light transport in tissue for optimizing light delivery in photoacoustic imaging of the sentinel lymph node

Abstract. Noninvasive or minimally invasive identification of sentinel lymph node (SLN) is essential to reduce the surgical effects of SLN biopsy. Photoacoustic (PA) imaging of SLN in animal models has shown its promise for clinical use in the future. Here, we present a Monte Carlo simulation for light transport in the SLN for various light delivery configurations with a clinical ultrasound probe. Our simulation assumes a realistic tissue layer model and also can handle the transmission/reflectance at SLN-tissue boundary due to the mismatch of refractive index. Various light incidence angles show that for deeply situated SLNs the maximum absorption of light in the SLN is for normal incidence. We also show that if a part of the diffused reflected photons is reflected back into the skin using a reflector, the absorption of light in the SLN can be increased significantly to enhance the PA signal.

Optimizing light delivery through fiber bundle in photoacoustic imaging with clinical ultrasound system: Monte Carlo simulation and experimental validation

Abstract. Translating photoacoustic (PA) imaging into clinical setup is a challenge. We report an integrated PA and ultrasound imaging system by combining the light delivery to the tissue with the ultrasound probe. First, Monte Carlo simulations were run to study the variation in absorbance within tissue for different angles of illumination, fiber-to-probe distance (FPD), and fiber-to-tissue distance (FTD). This is followed by simulation for different depths of the embedded sphere (object of interest). Several probe holders were designed for different light launching angles. Phantoms were developed to mimic a sentinel lymph node imaging scenario. It was observed that, for shallower imaging depths, the variation in signal-to-noise ratio (SNR) values could be as high as 100% depending on the angle of illumination at a fixed FPD and FTD. Results confirm that different light illumination angles are required for different imaging depths to get the highest SNR PA images. The results also validate that one can use Monte Carlo simulation as a tool to optimize the probe holder design depending on the imaging needs. This eliminates a trial-and-error approach generally used for designing a probe holder.

Multiple Spectral Peak Tracking for Heart Rate Monitoring from Photoplethysmography Signal During Intensive Physical Exercise

We propose a multiple initialization based spectral peak tracking (MISPT) technique for heart rate monitoring from photoplethysmography (PPG) signal. MISPT is applied on the PPG signal after removing the motion artifact using an adaptive noise cancellation filter. MISPT yields several estimates of the heart rate trajectory from the spectrogram of the denoised PPG signal which are finally combined using a novel measure called trajectory strength. Multiple initializations help in correcting erroneous heart rate trajectories unlike the typical SPT which uses only single initialization. Experiments on the PPG data from 12 subjects recorded during intensive physical exercise show that the MISPT based heart rate monitoring indeed yields a better heart rate estimate compared to the SPT with single initialization. On the 12 datasets MISPT results in an average absolute error of 1.11 BPM which is lower than 1.28 BPM obtained by the state-of-the-art online heart rate monitoring algorithm.

Non‐invasive sentinel lymph node mapping and needle guidance using clinical handheld photoacoustic imaging system in small animal

Translating photoacoustic imaging (PAI) into clinical setup is a challenge. Handheld clinical real-time PAI systems are not common. In this work, we report an integrated photoacoustic (PA) and clinical ultrasound imaging system by combining light delivery with the ultrasound probe for sentinel lymph node imaging and needle guidance in small animal. The open access clinical ultrasound platform allows seamless integration of PAI resulting in the development of handheld real-time PAI probe. Both methylene blue and indocyanine green were used for mapping the sentinel lymph node using 675 and 690 nm wavelength illuminations, respectively. Additionally, needle guidance with combined ultrasound and PAI was demonstrated using this imaging system. Up to 1.5 cm imaging depth was observed with a 10 Hz laser at an imaging frame rate of 5 frames per second, which is sufficient for future translation into human sentinel lymph node imaging and needle guidance for fine needle aspiration biopsy.

Importance sampling-based Monte Carlo simulation of time-domain optical coherence tomography with embedded objects.

Monte Carlo simulation for light propagation in biological tissue is widely used to study light-tissue interaction. Simulation for optical coherence tomography (OCT) studies requires handling of embedded objects of various shapes. In this work, time-domain OCT simulations for multilayered tissue with embedded objects (such as sphere, cylinder, ellipsoid, and cuboid) was done. Improved importance sampling (IS) was implemented for the proposed OCT simulation for faster speed. At first, IS was validated against standard and angular biased Monte Carlo methods for OCT. Both class I and class II photons were in agreement in all the three methods. However, the IS method had more than tenfold improvement in terms of simulation time. Next, B-scan images were obtained for four types of embedded objects. All the four shapes are clearly visible from the B-scan OCT images. With the improved IS B-scan OCT images of embedded objects can be obtained with reasonable simulation time using a standard desktop computer. User-friendly, C-based, Monte Carlo simulation for tissue layers with embedded objects for OCT (MCEO-OCT) will be very useful for time-domain OCT simulations in many biological applications.

Hand-held, clinical dual mode ultrasound - photoacoustic imaging of rat urinary bladder and its applications

Urinary bladder imaging is critical to diagnose urinary tract disorders, and bladder cancer. There is a great need for safe, non-invasive, and sensitive imaging technique which enables bladder imaging. Photoacoustic imaging is a rapidly growing imaging technique for various biological applications. It can be combined with clinical ultrasound imaging system for hand-held, dual modal ultrasound-photoacoustic real-time imaging. Structural (bladder wall) and functional (accretion of nanoparticles) bladder imaging is shown here with combined ultrasound and photoacoustic imaging in rats. Photoacoustic images of bladder wall is shown using black ink as the contrast agent. Chicken tissues were stacked on the abdomen of the animal to demonstrate the feasibility of photoacoustic imaging till a depth of 2 cm. Also, the feasibility of photoacoustic imaging for a common bladder disorder, vesicoureteral reflux is studied using urinary tract mimicking phantom. It is also shown that a clinical ultrasound system can be used for photoacoustic imaging of non-invasive clearance study of gold nanorods from circulation by monitoring the gradual accumulation of the gold nanorods in the bladder. The time taken for accumulation of nanorods in the bladder can be used as an indicator of the clearance rate of the nanoparticle circulation from the body.

Estimation of the invariant and variant characteristics in speech articulation and its application to speaker identification

Speech articulation varies across speakers for producing a speech sound due to the differences in their vocal tract morphologies, though the speech motor actions are executed in terms of relatively invariant gestures [1]. While the invariant articulatory gestures are driven by the linguistic content of the spoken utterance, the component of speech articulation that varies across speakers reflects speaker-specific and other paralinguistic information. In this work, we present a formulation to decompose the speech articulation from multiple speakers into the variant and invariant aspects when they speak the same sentence. The variant component is found to be a better representation for discriminating speakers compared to the speech articulation which includes the invariant part. Experiments with real-time magnetic resonance imaging (rtMRI) videos of speech production from multiple speakers reveal that the variant component of speech articulation yields a better frame-level speaker identification accuracy compared to the speech articulation as well as acoustic features by 29.9% and 9.4% (absolute) respectively.

Review on Heart-Rate Estimation from Photoplethysmography and Accelerometer Signals During Physical Exercise

Non-invasive monitoring of physiological signals during physical exercise is essential to customize the exercise module. Photoplethysmography (PPG) signal has often been used to non-invasively monitor heart-rate, respiratory rate, and blood-pressure among other physiological signals. Typically, PPG signal is acquired using pulse oximeter from finger-tip or wrist. Advantage of wrist-based PPG sensors is that it is more convenient to wear. Other sensors such as accelerometer can also be integrated with it due to large area on the wrist. This article provides a review of the algorithms developed for heart rate estimation during physical exercise from the PPG signals and accelerometer signals. The datasets used to develop these techniques are described. Algorithms for denoising of PPG signals using accelerometer signals are either in time domain or frequency domain.

A High-performance compact photoacoustic tomography system for in vivo small-animal brain imaging

In vivo small-animal imaging has an important role to play in preclinical studies. Photoacoustic tomography (PAT) is an emerging hybrid imaging modality that shows great potential for both preclinical and clinical applications. Conventional optical parametric oscillator-based PAT (OPO-PAT) systems are bulky and expensive and cannot provide high-speed imaging. Recently, pulsed-laser diodes (PLDs) have been successfully demonstrated as an alternative excitation source for PAT. Pulsed-laser diode PAT (PLD-PAT) has been successfully demonstrated for high-speed imaging on photoacoustic phantoms and biological tissues. This work provides a visualized experimental protocol for in vivo brain imaging using PLD-PAT. The protocol includes the compact PLD-PAT system configuration and its description, animal preparation for brain imaging, and a typical experimental procedure for 2D cross-sectional rat brain imaging. The PLD-PAT system is compact and cost-effective and can provide high-speed, high-quality imaging. Brain images collected in vivo at various scan speeds are presented.