Ultrasound-modulated fluorescence from rhodamine B aqueous solution

Fast ICCD-based temperature modulated fluorescence tomography

Fluorescence tomography (FT) has become a powerful preclinical imaging modality with a great potential for several clinical applications. Although it has superior sensitivity and utilizes low-cost instrumentation, the highly scattering nature of bio-tissue makes FT in thick samples challenging, resulting in poor resolution and low quantitative accuracy. To overcome the limitations of FT, we previously introduced a novel method, termed temperature modulated fluorescence tomography (TMFT), which is based on two key elements: (1) temperature-sensitive fluorescent agents (ThermoDots) and (2) high-intensity focused ultrasound (HIFU). The fluorescence emission of ThermoDots increases up to hundredfold with only several degree temperature elevation. The exceptional and reversible response of these ThermoDots enables their modulation, which effectively allows their localization using the HIFU. Their localization is then used as functional a priori during the FT image reconstruction process to resolve their distribution with higher spatial resolution. The last version of the TMFT system was based on a cooled CCD camera utilizing a step-and-shoot mode, which necessitated long total imaging time only for a small selected region of interest (ROI). In this paper, we present the latest version of our TMFT technology, which uses a much faster continuous HIFU scanning mode based on an intensified CCD (ICCD) camera. This new, to the best of our knowledge, version can capture the whole field-of-view (FOV) of 50×30m m 2 at once and reduces the total imaging time down to 30 min, while preserving the same high resolution (∼1.3m m) and superior quantitative accuracy (<7% error) as the previous versions. Therefore, this new method is an important step toward utilization of TMFT for preclinical imaging.

An ICCD camera-based time-domain ultrasound-switchable fluorescence imaging system

Fluorescence imaging in centimeter-deep tissues with high resolution is highly desirable for many biomedical applications. Recently, we have developed a new imaging modality, ultrasound-switchable fluorescence (USF) imaging, for achieving this goal. In our previous work, we successfully achieved USF imaging with several types of USF contrast agents and imaging systems. In this study, we introduced a new USF imaging system: an intensified charge-coupled device (ICCD) camera-based, time-domain USF imaging system. We demonstrated the principle of time-domain USF imaging by using two USF contrast agents. With a series of USF imaging experiments, we demonstrated the tradeoffs among different experimental parameters (i.e., data acquisition time, including CCD camera recording time and intensifier gate delay; focused ultrasound (FU) power; and imaging depth) and the image qualities (i.e., signal-to-noise ratio, spatial resolution, and temporal resolution). In this study, we also discussed several imaging strategies for achieving a high-quality USF image via this time-domain system.

Newly-Engineered Materials for Bio-Imaging Technology: A Focus on the Hybrid System of Ultrasound and Fluorescence

As an emerging technique, ultrasound-modulated fluorescence (UMF), or ultrasound switchable fluorescence (USF) bioimaging has shown promising features to produce deep-tissue and high-resolution fluorescence imaging for biomedical research and health diagnosis. The success of UMF or USF heavily relies on the design of their contrast agents (CAs). We herein surveyed recent advances in the development of such unique CAs, including configuration, mechanism, stability, sensitivity, and selectivity. Meanwhile, UMF or USF instrumentation has emerged as developmental breakthrough technologies to existing bio-imaging techniques. The best performance of UMF or USF bio-imaging requires an interactive response between CAs and the instrument. In this review, the description of UMF or USF instrumentation are also included for clarification and better understanding. Finally, the UMF and USF's performance in bioimaging is evaluated based on signal-to-noise ratio, resolution, imaging depth and speed, using photoacoustic imaging (PAI) as a standard, a well-developed technique of hybrid bio-imaging. Unlike PAI, UMF or USF is still in its early stage. Although results demonstrated a proof-of-concept landmark being reached, significant efforts are needed to improve the performance of UMF or USF.

Feasibility study of high spatial resolution multimodality fluorescence tomography in ex vivo biological tissue

Previously, we demonstrated that temperature-modulated fluorescence tomography (TM-FT) could provide fluorescence images with high quantitative accuracy and the spatial resolution of focused ultrasound. TM-FT is based on scanning the focused ultrasound across the medium to activate temperature-reversible fluorescent nanoprobes (ThermoDots). This technique can resolve small fluorescent targets located several centimeters deep in turbid media with millimeter resolution. Our past studies with this multimodality technique used agar phantoms, which could not represent the true heterogeneous nature of the acoustic and optical properties of biological tissue. In this work, we report the results of the first TM-FT study performed on ex vivo chicken breast tissue. In order to improve the spatial resolution of this technique, diffuse optical tomography is also used to better estimate the optical property maps of the tissue, which is utilized as functional a priori for the TM-FT reconstruction algorithm. These ex vivo results show that TM-FT can accurately recover the concentration and position of a 1.5  mm×5  mm inclusion filled with ThermoDots. Since the inclusion is embedded 2 cm deep in the chicken breast sample, these results demonstrate the great potential of TM-FT for future in vivo small animal imaging.

A Dual-Modality System for Both Multi-Color Ultrasound-Switchable Fluorescence and Ultrasound Imaging

Simultaneous imaging of multiple targets (SIMT) in opaque biological tissues is an important goal for molecular imaging in the future. Multi-color fluorescence imaging in deep tissues is a promising technology to reach this goal. In this work, we developed a dual-modality imaging system by combining our recently developed ultrasound-switchable fluorescence (USF) imaging technology with the conventional ultrasound (US) B-mode imaging. This dual-modality system can simultaneously image tissue acoustic structure information and multi-color fluorophores in centimeter-deep tissue with comparable spatial resolutions. To conduct USF imaging on the same plane (i.e., x-z plane) as US imaging, we adopted two 90°-crossed ultrasound transducers with an overlapped focal region, while the US transducer (the third one) was positioned at the center of these two USF transducers. Thus, the axial resolution of USF is close to the lateral resolution, which allows a point-by-point USF scanning on the same plane as the US imaging. Both multi-color USF and ultrasound imaging of a tissue phantom were demonstrated.

Simulation-based evaluation of the resolution and quantitative accuracy of temperature-modulated fluorescence tomography

Conventional fluorescence tomography (FT) can recover the distribution of fluorescent agents within a highly scattering medium. However, poor spatial resolution remains its foremost limitation. Previously, we introduced a new fluorescence imaging technique termed "temperature-modulated fluorescence tomography" (TM-FT), which provides high-resolution images of fluorophore distribution. TM-FT is a multimodality technique that combines fluorescence imaging with focused ultrasound to locate thermo-sensitive fluorescence probes using a priori spatial information to drastically improve the resolution of conventional FT. In this paper, we present an extensive simulation study to evaluate the performance of the TM-FT technique on complex phantoms with multiple fluorescent targets of various sizes located at different depths. In addition, the performance of the TM-FT is tested in the presence of background fluorescence. The results obtained using our new method are systematically compared with those obtained with the conventional FT. Overall, TM-FT provides higher resolution and superior quantitative accuracy, making it an ideal candidate for in vivo preclinical and clinical imaging. For example, a 4 mm diameter inclusion positioned in the middle of a synthetic slab geometry phantom (D:40  mm×W:100  mm) is recovered as an elongated object in the conventional FT (x=4.5  mm; y=10.4  mm), while TM-FT recovers it successfully in both directions (x=3.8  mm; y=4.6  mm). As a result, the quantitative accuracy of the TM-FT is superior because it recovers the concentration of the agent with a 22% error, which is in contrast with the 83% error of the conventional FT.

Ultrasound-modulated fluorescence based on donor-acceptor-labeled microbubbles

A fluorescence resonance energy transfer (FRET)-based microbubble contrast agent system was designed to experimentally demonstrate the concept of ultrasound-modulated fluorescence (UMF). Microbubbles were simultaneously labeled with donor and acceptor fluorophores on the surface to minimize self-quenching and maximize FRET. In response to ultrasound, the quenching efficiency was greatly modulated by changing the distance between the donor and acceptor molecules through microbubble size oscillations. Both donors and acceptors exhibited UMF on individual microbubbles. The UMF strength of the donor was more significant compared to that of the acceptor. Furthermore, the UMF of the donor was observed from a microbubble solution in a turbid media. This study exploits the feasibility of donor–acceptor labeled microbubbles as UMF contrast agents.

Detection of a novel mechanism of acousto-optic modulation of incoherent light

A novel form of acoustic modulation of light from an incoherent source has been detected in water as well as in turbid media. We demonstrate that patterns of modulated light intensity appear to propagate as the optical shadow of the density variations caused by ultrasound within an illuminated ultrasonic focal zone. This pattern differs from previous reports of acousto-optical interactions that produce diffraction effects that rely on phase shifts and changes in light directions caused by the acoustic modulation. Moreover, previous studies of acousto-optic interactions have mainly reported the effects of sound on coherent light sources via photon tagging, and/or the production of diffraction phenomena from phase effects that give rise to discrete sidebands. We aimed to assess whether the effects of ultrasound modulation of the intensity of light from an incoherent light source could be detected directly, and how the acoustically modulated (AOM) light signal depended on experimental parameters. Our observations suggest that ultrasound at moderate intensities can induce sufficiently large density variations within a uniform medium to cause measurable modulation of the intensity of an incoherent light source by absorption. Light passing through a region of high intensity ultrasound then produces a pattern that is the projection of the density variations within the region of their interaction. The patterns exhibit distinct maxima and minima that are observed at locations much different from those predicted by Raman-Nath, Bragg, or other diffraction theory. The observed patterns scaled appropriately with the geometrical magnification and sound wavelength. We conclude that these observed patterns are simple projections of the ultrasound induced density changes which cause spatial and temporal variations of the optical absorption within the illuminated sound field. These effects potentially provide a novel method for visualizing sound fields and may assist the interpretation of other hybrid imaging methods.

Ultrasound-modulated fluorescence based on fluorescent microbubbles

Ultrasound-modulated fluorescence (UMF) imaging has been proposed to provide fluorescent contrast while maintaining ultrasound resolution in an optical-scattering medium (such as biological tissue). The major challenge is to extract the weakly modulated fluorescent signal from a bright and unmodulated background. UMF was experimentally demonstrated based on fluorophore-labeled microbubble contrast agents. These contrast agents were produced by conjugating N-hydroxysuccinimide (NHS)-ester-attached fluorophores on the surface of amine-functionalized microbubbles. The fluorophore surface concentration was controlled so that a significant self-quenching effect occurred when no ultrasound was applied. The intensity of the fluorescent emission was modulated when microbubbles were oscillated by ultrasound pulses, presented as UMF signal. Our results demonstrated that the UMF signals were highly dependent on the microbubbles' oscillation amplitude and the initial surface fluorophore-quenching status. A maximum of ∼42% UMF modulation depth was achieved with a single microbubble under an ultrasound peak-to-peak pressure of 675 kPa. Further, UMF was detected from a 500-μm tube filled with contrast agents in water and scattering media with ultrasound resolution. These results indicate that ultrasound-modulated fluorescent microbubble contrast agents can potentially be used for fluorescence-based molecular imaging with ultrasound resolution in the future.

High-resolution imaging in a deep turbid medium based on an ultrasound-switchable fluorescence technique

The spatial resolution of fluorescence imaging techniques in deep optically turbid media such as tissues is limited by photon diffusion. To break the diffusion limit and achieve high-resolution and deep-tissue fluorescence imaging, a fundamentally different method was demonstrated based on a concept of ultrasound-switchable fluorescence. The results showed that a small fluorescent tube with a diameter of ∼180 μm at a depth of ∼20 mm in an optical scattering medium ([Formula: see text] and [Formula: see text] cm(-1)) can be clearly imaged with a size of ∼260 μm. The depth-to-resolution ratio is shown to be about one order of magnitude better than other deep-tissue fluorescence imaging techniques.

Effect of object size and acoustic wavelength on pulsed ultrasound modulated fluorescence signals

Detection of ultrasound (US)-modulated fluorescence in turbid media is a challenge because of the low level of fluorescent light and the weak modulation of incoherent light. A very limited number of theoretical and experimental investigations have been performed, and this is, to our knowledge, the first demonstration of pulsed US-modulated fluorescence tomography. Experimental results show that the detected signal depends on the acoustic frequency and the fluorescent target's size along the ultrasonic propagation axis. The modulation depth of the detected signal is greatest when the length of the object along the acoustic axis is an odd number of half wavelengths and is weakest when the object is an integer multiple of an acoustic wavelength. Images of a fluorescent tube embedded within a 22- by 13- by 30 mm scattering gel phantom (μ(s)∼15  cm(-1), g=0.93) with 1-, 1.5-, and 2 MHz frequency US are presented. The modulation depth of the detected signal changes by a factor of 5 depending on the relative size of the object and the frequency. The approach is also verified by some simple experiments in a nonscattering gel and using a theoretical model.

Temperature-modulated fluorescence tomography based on both concentration and lifetime contrast

It is challenging to image fluorescence objects with high spatial resolution in a highly scattering medium. Recently reported temperature-sensitive indocyanine green-loaded pluronic nanocapsules can potentially alleviate this problem. Here we demonstrate a frequency-domain temperature-modulated fluorescence tomography system that could acquire images at high intensity-focused ultrasound resolution with use of these nanocapsules. The system is experimentally verified with a phantom study, where a 3-mm fluorescence object embedded 2 cm deep in a turbid medium is successfully recovered based on both intensity and lifetime contrast.

State-of-the art of acousto-optic sensing and imaging of turbid media

Acousto-optic (AO) is an emerging hybrid technique for measuring optical contrast in turbid media using coherent light and ultrasound (US). A turbid object is illuminated with a coherent light source leading to speckle formation in the remitted light. With the use of US, a small volume is selected,which is commonly referred to as the "tagging" volume. This volume acts as a source of modulated light, where modulation might involve phase and intensity change. The tagging volume is created by focusing ultrasound for good lateral resolution; the axial resolution is accomplished by making either the US frequency, amplitude, or phase time-dependent. Typical resolutions are in the order of 1 mm. We will concentrate on the progress in the field since 2003. Different schemes will be discussed to detect the modulated photons based on speckle detection, heterodyne detection, photorefractive crystal (PRC) assisted detection, and spectral hole burning (SHB) as well as Fabry-Perot interferometers. The SHB and Fabry-Perot interferometer techniques are insensitive to speckle decorrelation and therefore suitable for in vivo imaging. However, heterodyne and PRC methods also have potential for in vivo measurements. Besides measuring optical properties such as scattering and absorption, AO can be applied in fluorescence and elastography applications.

Effect of fluorescent particle size on the modulation efficiency of ultrasound-modulated fluorescence

To investigate whether the size of fluorescent particles affects the modulation efficiency of ultrasound-modulated fluorescence (UMF), we measured UMF and DC (direct current) signals of the fluorescence emission from four different sized fluorescent particles: (1) three carboxylate-modified fluorescent microspheres (FM) with diameters of 20 nm, 200 nm, and 1.0 µm and (2) streptavidin-conjugated Alexa Fluor 647 with a diameter of approximately 5 nm. The UMF and DC signals were simultaneously measured using a broadband lock-in amplifier and a narrowband amplifier, respectively. The ratio of the UMF strength to the DC signal strength is defined as the modulation efficiency. This modulation efficiency was then used to evaluate the effects of fluorophore size and concentration. Results show that the modulation efficiency was improved by approximately a factor of two when the size of the fluorescent particles is increased from 5 nm to 1 µm. In addition, the linear relationship between the UMF strength and ultrasound pressure (observed in our previous study) were maintained regardless of the fluorescent particle sizes.