Multiparametric evaluation of hindlimb ischemia using time-series indocyanine green fluorescence imaging

Mixing Matrix-corrected Whole-body Pharmacokinetic Modeling Using Longitudinal Micro-computed Tomography and Fluorescence-mediated Tomography

PURPOSE

Pharmacokinetic modeling can be applied to quantify the kinetics of fluorescently labeled compounds using longitudinal micro-computed tomography and fluorescence-mediated tomography (μCT-FMT). However, fluorescence blurring from neighboring organs or tissues and the vasculature within tissues impede the accuracy in the estimation of kinetic parameters. Contributions of elimination and retention activities of fluorescent probes inside the kidneys and liver can be hard to distinguish by a kinetic model. This study proposes a deconvolution approach using a mixing matrix to model fluorescence contributions to improve whole-body pharmacokinetic modeling.

PROCEDURES

In the kinetic model, a mixing matrix was applied to unmix the fluorescence blurring from neighboring tissues and blood vessels and unmix the fluorescence contributions of elimination and retention in the kidney and liver compartments. Accordingly, the kinetic parameters of the hepatobiliary and renal elimination routes and five major retention sites (the kidneys, liver, bone, spleen, and lung) were investigated in simulations and in an in vivo study. In the latter, the pharmacokinetics of four fluorescently labeled compounds (indocyanine green (ICG), HITC-iodide-microbubbles (MB), Cy7-nanogels (NG), and OsteoSense 750 EX (OS)) were evaluated in BALB/c nude mice.

RESULTS

In the simulations, the corrected modeling resulted in lower relative errors and stronger linear relationships (slopes close to 1) between the estimated and simulated parameters, compared to the uncorrected modeling. For the in vivo study, MB and NG showed significantly higher hepatic retention rates (P<0.05 and P<0.05, respectively), while OS had smaller renal and hepatic retention rates (P<0.01 and P<0.01, respectively). Additionally, the bone retention rate of OS was significantly higher (P<0.01).

CONCLUSIONS

The mixing matrix correction improves pharmacokinetic modeling and thus enables a more accurate assessment of the biodistribution of fluorescently labeled pharmaceuticals by μCT-FMT.

Evaluation of surgical procedures of mouse urethra by visualization and the formation of fistula

Visualization of the surgically operated tissues is vital to improve surgical model animals including mouse. Urological surgeries for urethra include series of fine manipulations to treat the increasing number of birth defects such as hypospadias. Hence visualization of the urethral status is vital. Inappropriate urethral surgical procedure often leads to the incomplete wound healing and subsequent formation of urethro-cutaneous fistula or urethral stricture. Application of indocyanine green mediated visualization of the urethra was first performed in the current study. Indocyanine green revealed the bladder but not the urethral status in mouse. Antegrade injection of contrast agent into the bladder enabled to detect the urethral status in vivo. The visualization of the leakage of contrast agent from the operated region was shown as the state of urethral fistula in the current hypospadias mouse model and urethral stricture was also revealed. A second trial for contrast agent was performed after the initial operation and a tendency of accelerated urethral stricture was observed. Thus, assessment of post-surgical conditions of urogenital tissues can be improved by the current analyses on the urethral status.

Evaluation of skeletal muscle perfusion in canine hind limb ischemia model using color-coded digital subtraction angiography

OBJECTIVE

To evaluate perfusion alterations in skeletal muscle in a canine hind limb ischemia model using color-coded digital subtraction angiography (CC-DSA).

METHODS

Twelve beagles underwent embolization at the branch of their left deep femoral artery. Right hind limbs were used as the control group. Angiography was performed before and immediately after embolization. Upon CC-DSA analysis, time to peak (TTP) was measured before embolization in both sides of the beagles' hind limbs at the middle iliac artery, and the distant, middle and proximal femoral artery. Regions of interest (ROI) peak and ROI peak time were symmetrically computed in proximal and distal thigh muscles before and immediately after embolization. The data were analyzed and compared using the Wilcoxon signed rank test.

RESULTS

Before embolization, ROI peak in the proximal thigh was lower than in the ipsilateral distal thigh, whereas ROI peak time in the proximal thigh was longer than in the distal thigh. In the iliac femoral artery, there was no significant difference in ROI peak, ROI peak time, or TTP between right and left sides. After embolization, ROI peaks in proximal and distal skeletal muscles of the left hind limb were significantly lower than on the contralateral side. ROI peak time was significantly longer in the left proximal and left distal thigh compared to the contralateral side. There were no significant changes in ROI peak or ROI peak time in the right proximal and right distal thigh compared to pre-embolization values. Changes in ROI peak and ROI peak time were larger in the left proximal than in the left distal thigh.

CONCLUSION

CC-DSA provided real-time measurement of changes in vascular hemodynamics and skeletal muscle perfusion without increasing X-ray usage or contrast agent dose.

Enhancing in vivo renal ischemia assessment by high-dynamic-range fluorescence molecular imaging

Fluorescence imaging has been used to evaluate the physiological features of renal ischemia in animal model. However, the fluorophore distribution details of the ischemia model could not be fully represented due to the limited dynamic range of the charged-couple device. A high-dynamic-range (HDR) strategy was adopted in renal ischemia fluorescence imaging, both ex vivo and in vivo. The HDR strategy successfully combined ischemia relevant biological features that could only be captured with different exposure times, and then presented these features in the HDR results. The HDR results effectively highlighted the renal ischemic areas with relatively better perfusion and diminished the saturation that resulted from long exposure time. The relative fluorescence intensities of the ischemic kidneys and the image entropy values were significantly higher in the HDR images than in the original images, therefore enhancing the visualization of the renal ischemia model. The results suggest that HDR could serve as a postprocessing strategy to enhance the assessment of in vivo renal ischemia, and HDR fluorescence molecular imaging could be a valuable imaging tool for future studies of clinical ischemia detection and evaluation.

Quantitative evaluation of graded hindlimb ischemia based on pharmacokinetic modelling and hemodynamic analysis of indocyanine green

OBJECTIVE

Accurate evaluation of the degree of hindlimb ischemia is challenging but essential for the diagnosis and treatment of peripheral vascular insufficiency. The aim of the study is to apply a multiparametric method for the quantitative estimation of mouse models with different degrees of hindlimb ischemia based on a dynamic fluorescence imaging-based strategy.

APPROACH

An adjustable hydraulic occluder was placed around the thigh root of one hindlimb to induce six different degrees of hindlimb ischemia. Five parameters were extracted to quantitatively evaluate the degree of ischemia, including perfusion rate (PR) and perfusion vascular density (PVD) from a mathematical model of indocyanine green (ICG) pharmacokinetics, rising time (T _rise), blood flow index (BFI) and mean fluorescence intensity (MFI) from time-series analysis of ICG hemodynamics.

MAIN RESULTS

The results showed that the normalized PR and BFI decreased while the normalized T _rise increased progressively with the degree of ischemia. The normalized PVD and MFI first increased and then decreased with the degree of ischemia. High correlation was observed between the degree of ischemia and the arterial oxygen saturation which was measured by an oximeter.

SIGNIFICANCE

The results of this work demonstrated that PR, BFI and T _rise can be used for the quantitative and comprehensive evaluation of graded hindlimb ischemia.

Image Restoration for Fluorescence Planar Imaging with Diffusion Model

Fluorescence planar imaging (FPI) is failure to capture high resolution images of deep fluorochromes due to photon diffusion. This paper presents an image restoration method to deal with this kind of blurring. The scheme of this method is conceived based on a reconstruction method in fluorescence molecular tomography (FMT) with diffusion model. A new unknown parameter is defined through introducing the first mean value theorem for definite integrals. System matrix converting this unknown parameter to the blurry image is constructed with the elements of depth conversion matrices related to a chosen plane named focal plane. Results of phantom and mouse experiments show that the proposed method is capable of reducing the blurring of FPI image caused by photon diffusion when the depth of focal plane is chosen within a proper interval around the true depth of fluorochrome. This method will be helpful to the estimation of the size of deep fluorochrome.

Biophotonics in China

Biophotonics is a highly interdisciplinary field where physicists, chemists, biologists, physicians and engineers work together to solve the problems appearing in biology and medicine. In China, the Biophotonics discipline is often referred to as Biomedical Photonics, under the first-level disciplines Biomedical Engineering or Optical Engineering, and was initiated in the late 1990s. Over the past 20 years, biophotonics research in China expanded extraordinarily and has reached the frontiers of the world-level sciences. This white paper introduces the research groups in the biophotonics field in China, and their representative contributions.

Facilitating in vivo tumor localization by principal component analysis based on dynamic fluorescence molecular imaging

Fluorescence molecular imaging has been used to target tumors in mice with xenograft tumors. However, tumor imaging is largely distorted by the aggregation of fluorescent probes in the liver. A principal component analysis (PCA)-based strategy was applied on the in vivo dynamic fluorescence imaging results of three mice with xenograft tumors to facilitate tumor imaging, with the help of a tumor-specific fluorescent probe. Tumor-relevant features were extracted from the original images by PCA and represented by the principal component (PC) maps. The second principal component (PC2) map represented the tumor-related features, and the first principal component (PC1) map retained the original pharmacokinetic profiles, especially of the liver. The distribution patterns of the PC2 map of the tumor-bearing mice were in good agreement with the actual tumor location. The tumor-to-liver ratio and contrast-to-noise ratio were significantly higher on the PC2 map than on the original images, thus distinguishing the tumor from its nearby fluorescence noise of liver. The results suggest that the PC2 map could serve as a bioimaging marker to facilitate in vivo tumor localization, and dynamic fluorescence molecular imaging with PCA could be a valuable tool for future studies of in vivo tumor metabolism and progression.