Improving the penetration depth in multiphoton excitation laser scanning microscopy

Vision with pulsed infrared light is mediated by nonlinear optical processes

When the eye is exposed to pulsed infrared (IR) light, it is perceived as visible of the corresponding half wavelength. Previous studies have reported evidence that this is due to a non-linear two-photon absorption process. We have carried out a study which provides additional support to this nonlinear hypothesis. To this end, we have measured the spectral sensitivity at 2 different pulse repetition rates and have developed a theoretical model to account for the experimental observations. This model predicts a ratio between the minimum powers needed to detect the visual stimulus at the 2 pulse repetition rates employed of 0.45 if the stimulus were detected through a nonlinear effect and 1 if it were caused by a linear effect as in normal vision. The value experimentally found was 0.52 ± 0.07, which supports the hypothesis of a nonlinear origin of the two-photon vision phenomena.

Label-free multiphoton microscopic imaging as a novel real-time approach for discriminating colorectal lesions: A preliminary study

BACKGROUND AND AIM

Although various endoscopic technologies have been developed to increase the visual diagnostic accuracy of colorectal precancerosis and early carcinoma, the biopsy-dependent pathology still remains the golden standard. During routine endoscopy, real-time optical histological diagnosis is desired. Without fluorescent label, multiphoton microscopy (MPM) imaging directly reveals live cellular morphology and tissue microenvironment based on intrinsic two-photon excited fluorescence and second harmonic generation signals. Its high-imaging resolution and performance are comparable with the histopathology. We thus aimed to initially investigate the original features of colorectal diseases under MPM and evaluate its potential for real-time diagnosis.

METHODS

Experimental and diagnostic cohorts were designed. Multiphoton images of 40 ex vivo fresh tissues confirmed pathologically of colorectal normal tissues, hyperplastic polyps, adenomas, and adenocarcinomas were collected. Features were recorded to establish diagnostic standards with MPM. For the second cohort with 92 fresh tissues, we distinguished the various colorectal diseases with conclusive MPM features.

RESULTS

Through the investigation, the colorectal diseases were presented differences in the crypt opening, gland structure, epithelial cells, and collagen fibers. With the typical features, we preliminarily tested the diagnostic efficiency and found that its sensitivity for distinguishing normal, hyperplastic polyps, adenoma and adenocarcinoma was 88.89%, 76.47%, 83.33%, and 97.92%, while the specificity was 99.32%, 94.00%, 94.92%, and 94.12%, respectively.

CONCLUSION

The real-time multiphoton microscopic imaging can be effective to identify the colorectal lesions with high resolution. Via integrating with the endoscopes in the future, it could promote precise optical diagnosis in clinics.

Multiphoton microscopy for label-free identification of intramural metastasis in human esophageal squamous cell carcinoma

For complete removal of cancerous tissue in esophageal squamous cell carcinoma (ESCC), intramural metastasis (IM) should be identified preoperatively or intraoperatively. Here, multiphoton microscopy (MPM) was introduced for label-free identification of IM in the esophageal wall, by a combination of two-photon excited fluorescence (TPEF), second harmonic generation (SHG) imaging, and spectral analysis. Three-dimensional (3D) imaging of the IM region was also performed. Quantitative parameters, including 3D fiber orientation, were measured by 3D-weighted orientation vector summation. Overall, MPM showed the potential to identify IM. With the development of the advanced MPM endoscope, clinical identification of IM by MPM will be possible.

Periscope for noninvasive two-photon imaging of murine retina in vivo

Two-photon microscopy allows visualization of subcellular structures in the living animal retina. In previously reported experiments it was necessary to apply a contact lens to each subject. Extending this technology to larger animals would require fitting a custom contact lens to each animal and cumbersome placement of the living animal head on microscope stage. Here we demonstrate a new device, periscope, for coupling light energy into mouse eye and capturing emitted fluorescence. Using this periscope we obtained images of the RPE and their subcellular organelles, retinosomes, with larger field of view than previously reported. This periscope provides an interface with a commercial microscope, does not require contact lens and its design could be modified to image retina in larger animals.

Human infrared vision is triggered by two-photon chromophore isomerization

Vision relies on photoactivation of visual pigments in rod and cone photoreceptor cells of the retina. The human eye structure and the absorption spectra of pigments limit our visual perception of light. Our visual perception is most responsive to stimulating light in the 400- to 720-nm (visible) range. First, we demonstrate by psychophysical experiments that humans can perceive infrared laser emission as visible light. Moreover, we show that mammalian photoreceptors can be directly activated by near infrared light with a sensitivity that paradoxically increases at wavelengths above 900 nm, and display quadratic dependence on laser power, indicating a nonlinear optical process. Biochemical experiments with rhodopsin, cone visual pigments, and a chromophore model compound 11-cis-retinyl-propylamine Schiff base demonstrate the direct isomerization of visual chromophore by a two-photon chromophore isomerization. Indeed, quantum mechanics modeling indicates the feasibility of this mechanism. Together, these findings clearly show that human visual perception of near infrared light occurs by two-photon isomerization of visual pigments.

Characterization, comparison, and choice of a commercial double-clad fiber for nonlinear endomicroscopy

Several endomicroscope prototypes for nonlinear optical imaging were developed in the last decade for in situ analysis of tissue with cellular resolution by using short infrared light pulses. Fourier-transform-limited pulses at the tissue site are necessary for optimal excitation of faint endogenous signals. However, obtaining these transform-limited short pulses remains a challenge, and previously proposed devices did not achieve an optimal pulse delivery. We present a study of fibered endomicroscope architecture with an efficient femtosecond pulse delivery and a high excitation level at the output of commercially available double-clad fibers (DCFs). The endomicroscope incorporates a module based on a grism line to compensate for linear and nonlinear effects inside the system. Simulations and experimental results are presented and compared to the literature. Experimentally, we obtained short pulses down to 24 fs at the fiber output, what represents to the best of our knowledge the shortest pulse duration ever obtained at the output of a nonlinear endoscopic system without postcompression. The choice of the optimal DCF among four possible commercial components is discussed and evaluated in regard to multiphoton excitation and fluorescence emission.

Two-photon excited fluorescence enhancement with broadband versus tunable femtosecond laser pulse excitation

The inverse relationship between two-photon excited fluorescence (TPEF) and laser pulse duration suggests that two-photon microscopy (TPM) performance may be improved by decreasing pulse duration. However, for ultrashort pulses of sub-10 femtosecond (fs) in duration, its spectrum contains the effective gain bandwidth of Ti:Sapphire and its central wavelength is no longer tunable. An experimental study was performed to explore this apparent tradeoff between untuned sub-10 fs transform-limited pulse (TLP) and tunable 140 fs pulse for TPEF. Enhancement factors of 1.6, 6.7, and 5.2 are measured for Indo-1, FITC, and TRITC excited by sub-10 fs TLP compared with 140 fs pulse tuned to the two-photon excitation (TPE) maxima at 730 nm, 800 nm, and 840 nm, respectively. Both degenerate (v(1) = v(2)) and nondegenerate (v(1) ≠ v(2)) mixing of sub-10 fs TLP spectral components result in its broad second-harmonic (SH) power spectrum and high spectral density, which can effectively compensate for the lack of central wavelength tuning and lead to large overlap with dye TPE spectra for TPEF enhancements. These pulse properties were also exploited for demonstrating its potential applications in multicolor imaging with TPM.

Pulse compression in two-photon excitation fluorescence microscopy

The use of shorter pulses is a practical way to improve the signal in two-photon excitation fluorescence microscopy. We report on the theoretical and experimental results of pulse compression in a two-photon excitation fluorescence microscope by using approximately 100-fs Ti:Sapphire laser and highly nonlinear photonic crystal fiber. Effects of the fiber parameters, transmitted power, and group-delay dispersion provided by the gratings have been investigated to optimize the compressor performance. By using a 20-mm-long photonic crystal fiber with a zero dispersion wavelength of 850 nm, a compressed pulse of 23.6 fs starting from 94 fs at 790 nm is experimentally demonstrated as a verification of our simulations. By integrating the compressor with a two-photon excitation fluorescence microscope, 5.6 times increase in autofluorescence intensity of NAD(P)H in Nasopharyngeal carcinoma cells is demonstrated, showing its potential in enhanced imaging and sensing for disease diagnosis.

In vivo high-resolution structural imaging of large arteries in small rodents using two-photon laser scanning microscopy

In vivo (molecular) imaging of the vessel wall of large arteries at subcellular resolution is crucial for unraveling vascular pathophysiology. We previously showed the applicability of two-photon laser scanning microscopy (TPLSM) in mounted arteries ex vivo. However, in vivo TPLSM has thus far suffered from in-frame and between-frame motion artifacts due to arterial movement with cardiac and respiratory activity. Now, motion artifacts are suppressed by accelerated image acquisition triggered on cardiac and respiratory activity. In vivo TPLSM is performed on rat renal and mouse carotid arteries, both surgically exposed and labeled fluorescently (cell nuclei, elastin, and collagen). The use of short acquisition times consistently limit in-frame motion artifacts. Additionally, triggered imaging reduces between-frame artifacts. Indeed, structures in the vessel wall (cell nuclei, elastic laminae) can be imaged at subcellular resolution. In mechanically damaged carotid arteries, even the subendothelial collagen sheet (approximately 1 microm) is visualized using collagen-targeted quantum dots. We demonstrate stable in vivo imaging of large arteries at subcellular resolution using TPLSM triggered on cardiac and respiratory cycles. This creates great opportunities for studying (diseased) arteries in vivo or immediate validation of in vivo molecular imaging techniques such as magnetic resonance imaging (MRI), ultrasound, and positron emission tomography (PET).

Beyond the 1/Tp limit: two-photon-excited fluorescence using pulses as short as sub-10-fs

Two-photon-excited fluorescence and second-harmonic generation are characterized as a function of laser pulse duration as short as sub-10-fs. A comparative study is performed where pulse duration is varied by introducing dispersion, as reported previously, and by tailoring pulse spectral width and minimizing its time-bandwidth product (transform-limited pulses). Experimental data and calculations show that characterizing a two-photon signal with the two schemes to vary pulse duration measures different phenomena. Two-photon signal characterization using dispersion-broadened pulses measures only the effect of chirp on the pulse two-photon-excitation spectrum and is independent of molecular response. Transform-limited pulses are used to measure the dependence of two-photon signal generation on pulse duration. Calculations show that deviation from the 1T(p) relationship would be expected as the transform-limited pulse spectral width approaches the molecular two-photon absorption linewidth and exhibits asymptotic behavior for pulse spectral widths 10 times greater than the absorption linewidth. Experimental measurements are consistent with the predicted behavior. The impact of using ultrashort laser pulses on the performance characteristics of nonlinear optical microscopy is discussed.

Extended-working-distance multiphoton micromanipulation microscope for deep-penetration imaging in live mice and tissue

We constructed a multiphoton (2-P) microscope with space to mount and operate microphysiology hardware, and still acquire high quality 2-P images of tumor cells deep within tissues of live mice. We reconfigured for nondescanned 2-P imaging, a dedicated electrophysiology microscope, the Nikon FN1. This microscope is compact, with retractable objectives, allowing more stage space. The instrument is fitted with long-working-distance objectives (2.5- to 3.5-mm WD) with a narrow bore, high NA, and efficient UV and IR light transmission. The system is driven by a powerful 3.5-W peak power pulsed Ti-sapphire laser with a broad tuning range. This 2-P system images a fluorescent standard to a depth of 750 to 800 microm, acquires images of murine pancreatic tumors in vivo, and also images fluorescently labeled T-cells inside live, externalized mouse lymph nodes. Effective imaging depths range between 100 and 500 microm. This compares favorably with the 100- to 300 microm micron depth attained by many 2-P systems, especially descanned 2-P instruments, and 40-microm-deep imaging with confocal microscopes. The greater depth penetration is attributable to the use of high-NA long-working-distance water-dipping lenses incorporated into a nondescanned instrument with carefully configured laser beam introduction and image-acquisition optics. Thus the new system not only has improved imaging capabilities, but allows micromanipulation and maintenance of tissues and organs.

Two-photon imaging of the immune system: a custom technology platform for high-speed, multicolor tissue imaging of immune responses

Modern imaging approaches are proving important for addressing contemporary issues in the immune system. These approaches are especially useful for characterizing the complex orchestration of immune responses in vivo. Multicolor, two-photon imaging has been proven to be especially enabling for such studies because of its superior tissue penetration, reduced image degradation by light scattering leading to better resolution, and its high image quality deep inside tissues. Here, we examine the functional requirements of two-photon imaging instruments necessary for such immune studies. These requirements include frame rate, spatial resolution and the number of emission channels. We use this discussion as a starting point to compare commercial systems and to introduce a custom technology platform that meets those requirements. This platform is noteworthy because it is very cost-effective, flexible and experimentally useful. Representative data collected with this instrument is used to demonstrate the utility of this platform. Finally, as the field is rapidly evolving, consideration is given to some of the cutting-edge developments in multiphoton microscopy that will likely improve signal strength, depth penetration and/or the experimental usefulness of this approach.

Two-photon microscopy on vital carotid arteries: imaging the relationship between collagen and inflammatory cells in atherosclerotic plaques

We used two-photon laser scanning microscopy (TPLSM) to demonstrate for the first time its potential in studying relational details at the cellular level of atherogenesis in intact, viable mouse carotid arteries. Isolated and mounted arteries of ApoE-/-mice, aged 15 or 21 weeks (7 and 13 weeks on western diet), were imaged after labeling with specific fluorescent markers for cell nuclei, inflammatory cells, collagen, and lipids. Data were compared with C57BL6/J mice fed a chow diet. Control vessels had intact endothelium without adhering blood cells or significant intimal collagen labeling. In ApoE-/-mice already at 15 weeks, inflammatory cells adhered to the endothelium and increased labeling of collagen was observed in tunica intima at both lesion-prone and non-lesion-prone sites, indicating endothelium activation. In plaques, internalized inflammatory cell density increased with age and plaque progression in tunicae adventitia and intima, but not media. In the whole plaque, aging or plaque progression did not alter the direct relationship between inflammatory cells and collagen. However, within the fibrous caps specifically, direct contact between inflammatory cells and collagen increased with age. This study demonstrates the potential of TPLSM in determining detailed information regarding the complex relationship between inflammatory cells and collagen during atherogenesis.

Three-dimensional random access multiphoton microscopy for functional imaging of neuronal activity

The dynamic ability of neuronal dendrites to shape and integrate synaptic responses is the hallmark of information processing in the brain. Effectively studying this phenomenon requires concurrent measurements at multiple sites on live neurons. Substantial progress has been made by optical imaging systems that combine confocal and multiphoton microscopy with inertia-free laser scanning. However, all of the systems developed so far restrict fast imaging to two dimensions. This severely limits the extent to which neurons can be studied, as they represent complex three-dimensional structures. Here we present a new imaging system that utilizes a unique arrangement of acousto-optic deflectors to steer a focused, ultra-fast laser beam to arbitrary locations in three-dimensional space without moving the objective lens. As we demonstrate, this highly versatile random-access multiphoton microscope supports functional imaging of complex three-dimensional cellular structures such as neuronal dendrites or neural populations at acquisition rates on the order of tens of kilohertz.

Using magnetic resonance microscopy to study the growth dynamics of a glioma spheroid in collagen I: A case study

BACKGROUND

Highly malignant gliomas are characterized by rapid growth, extensive local tissue infiltration and the resulting overall dismal clinical outcome. Gaining any additional insights into the complex interaction between this aggressive brain tumor and its microenvironment is therefore critical. Currently, the standard imaging modalities to investigate the crucial interface between tumor growth and invasion in vitro are light and confocal laser scanning microscopy. While immensely useful in cell culture, integrating these modalities with this cancer's clinical imaging method of choice, i.e. MRI, is a non-trivial endeavour. However, this integration is necessary, should advanced computational modeling be able to utilize these in vitro data to eventually predict growth behaviour in vivo. We therefore argue that employing the same imaging modality for both the experimental setting and the clinical situation it represents should have significant value from a data integration perspective. In this case study, we have investigated the feasibility of using a specific form of MRI, i.e. magnetic resonance microscopy or MRM, to study the expansion dynamics of a multicellular tumor spheroid in a collagen type I gel.

METHODS

An U87mEGFR human giloblastoma multicellular spheroid (MTS) containing approximately 4.103 cells was generated and pipetted into a collagen I gel. The sample was then imaged using a T2-weighted 3D spoiled gradient echo pulse sequence on a 14T MRI scanner over a period of 12 hours with a temporal resolution of 3 hours at room temperature. Standard histopathology was performed on the MRM sample, as well as on control samples.

RESULTS

We were able to acquire three-dimensional MR images with a spatial resolution of 24 x 24 x 24 microm3. Our MRM data successfully documented the volumetric growth dynamics of an MTS in a collagen I gel over the 12-hour period. The histopathology results confirmed cell viability in the MRM sample, yet displayed distinct patterns of cell proliferation and invasion as compared to control.

CONCLUSION

In this study, we demonstrate that a specific form of MRI, i.e. magnetic resonance microscopy or MRM, can be used to study the dynamic growth of a multicellular tumor spheroid (MTS) with a single cell scale spatial resolution that approaches the level of light microscopy. We argue that MRM can be employed as a complementary non-invasive tool to characterize microscopic MTS expansion, and thus, together with integrative computational modeling, may allow bridging of the experimental and clinical scales more readily.