Category: Research Topics

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Single Pixel Imaging at MHz Rates via Cyclic Hadamard Patterns

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In this project we focused on speeding up single pixel imaging systems and until now managed to achieve modulation rates of 2.4 MHz. This is two orders of magnitudes above the currently available modulation rates! Also, we developed algorithms for correcting motion errors that occur when moving faster than the imaging rate, this way enabling even faster imaging.

Short Project Overview

Full details of our work are found in the following two papers:

 1. Fast cyclic coding and imaging system

Single pixel imaging at megahertz switching rates via cyclic Hadamard masks.

Hahamovich E*, Monin S*, Hazan Y, Rosenthal A. “Spatial light modulation at megahertz switch rates via cyclic Hadamard masks.” Nature Communications, 2021

 * Equal contribution

https://www.nature.com/articles/s41467-021-24850-x

Abstract

Optical imaging is commonly performed with either a camera and wide-field illumination or with a single detector and a scanning collimated beam; unfortunately, these options do not exist at all wavelengths. Single-pixel imaging offers an alternative that can be performed with a single detector and wide-field illumination, potentially enabling imaging applications in which the detection and illumination technologies are immature. However, single-pixel imaging currently suffers from low imaging rates owing to its reliance on configurable spatial light modulators, generally limited to 22 kHz rates. We develop an approach for rapid single-pixel imaging which relies on cyclic patterns coded onto a spinning mask and demonstrate it for in vivo imaging of C. elegans worms. Spatial modulation rates of up to 2.4 MHz, imaging rates of up to 72 fps, and image-reconstruction times of down to 1.5 ms are reported, enabling real-time visualization of dynamic objects.

2. Motion correction algorithms

Single-pixel imaging of dynamic objects using multi-frame motion estimation.

 Monin S, Hahamovich E, Rosenthal A. “Single-pixel imaging of dynamic objects using multi-frame motion estimation.” Scientific reports, 2021

https://doi.org/10.1038/s41598-021-83810-z

Abstract

Single-pixel imaging (SPI) enables the visualization of objects with a single detector by using a sequence of spatially modulated illumination patterns. For natural images, the number of illumination patterns may be smaller than the number of pixels when compressed-sensing algorithms are used. Nonetheless, the sequential nature of the SPI measurement requires that the object remains static until the signals from all the required patterns have been collected. In this paper, we present a new approach to SPI that enables imaging scenarios in which the imaged object, or parts thereof, moves within the imaging plane during data acquisition. Our algorithms estimate the motion direction from inter-frame cross-correlations and incorporate it in the reconstruction model. Moreover, when the illumination pattern is cyclic, the motion may be estimated directly from the raw data, further increasing the numerical efficiency of the algorithm. A demonstration of our approach is presented for both numerically simulated and measured data.

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Detection of ultrasound via optical interferometry

The detection of ultrasound is conventionally performed by using piezoelectric transducers. Despite the ubiquity of this approach, it suffers from several drawbacks that limit its application. In particular, in the field of optoacoustic imaging, the opacity of piezoelectric materials in use puts constraints on possible illumination patterns. In addition, piezoelectric transducers generally lose sensitivity upon miniaturization, hindering the development of minimally invasive optoacoustic endoscopes. At LBIS, we develop ultrasound detectors based on interferometric principles to enable new imaging devices.  Our approach relies on miniature optical resonators in silica and silicon platforms and on a unique interrogation approach called pulse interferometry. 

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Optoacoustic tomography: Systems and theory

optoacoustic

Optoacoustic tomography (OAT), also known as photoacoustic tomography, is a hybrid imaging modality that enables the visualization of optical contrast with high resolution at tissue depths in which light propagation is diffusive. In OAT, high-energy laser pulses are sent into the tissue, leading to local energy deposition and (almost) instantaneous thermal expansion of optically absorbing tissue constituents. This rapid expansion propagates outwards in the form of acoustic waves. To form an image of the optical absorption in the tissue, one needs to measure the acoustic waves emanating from the imaged object in a tomographic fashion and apply acoustic inversion. In multi-spectral optoacoustic tomography (MSOT), the optical excitation is performed in several wavelengths, enabling the detection of various chromophores based on their spectral signature and quantifying oxygen saturation in tissue. The additional dimension offered by MSOT makes it a highly promising technique for pre-clinical research and clinical applications.

The design of MSOT systems is a multi-facet problem that requires an understanding of the physics of light and ultrasound propagation in tissue, characteristics of ultrasound detectors, the mathematical properties of the inverse problems involved, and the technical constraints of the imaging scenario. At LBIS, our goal is to develop new system designs and algorithmic tools that enable imaging applications.

Fig (1): Experimental reconstruction of an optoacoustic point source when (a) the deterctor’s geometry is not modelled and (b) when it is. (taken from A. Rosenthal et al., Med. Phys., 2011)