Bioluminescence imaging of point sources implanted in small animals post mortem: evaluation of a method for estimating source strength and depth

Spatially resolved diffuse imaging for high-speed depth estimation of jet injection

We investigate the use of spatially resolved diffuse imaging to track a fluid jet delivered at high speed into skin tissue. A jet injector with a short needle to deliver drugs beneath the dermis, is modified to incorporate a laser beam into the jet, which is ejected into ex vivo porcine tissue. The diffuse light emitted from the side and top of the tissue sample is recorded using high-speed videography. Similar experiments, using a depth-controlled fiber optic source, generate a reference dataset. The side light distribution is related to source depth for the controlled-source experiments and used to track the effective source depth of the injections. Postinjection X-ray images show agreement between the jet penetration and ultimate light source depth. The surface light intensity profile is parameterized with a single parameter and an exponential function is used to relate this parameter to source depth for the controlled-source data. This empirical model is then used to estimate the effective source depth from the surface profile of the injection experiments. The depth estimates for injections into fat remain close to the side depth estimates, with a root-mean-square error of 1.1 mm, up to a source depth of 8 mm.

Classification of diffuse light emission profiles for distinguishing skin layer penetration of a needle-free jet injection

In this work, a system is developed for tracking the skin layer to which a needle-free jet injection of fluid has penetrated by incorporating a laser beam into the jet, and measuring the diffuse light emitted from skin tissue. Monitoring the injection in this way offers the ability to improve the reliability of drug delivery with this transdermal delivery method. A laser beam, axially aligned with a jet of fluid, created a distribution of diffuse light around the injection site that varied as the injection progressed. High-speed videography was used to capture the diffuse light emission from laser-coupled jet injections into samples of porcine skin, fat, and muscle. The injection produced a distribution of diffuse light around the injection site that varied as the injection descended. A classifier, trained to distinguish whether the light source was located in the fat or muscle from surface intensity profile measurements, correctly identified the injected layer in 97.2 % of the cases when cross-examined against estimates using the light distribution emitted from the side of the sample.

Small animal fluorescence and bioluminescence tomography: a review of approaches, algorithms and technology update

Emerging fluorescence and bioluminescence tomography approaches have several common, yet several distinct features from established emission tomographies of PET and SPECT. Although both nuclear and optical imaging modalities involve counting of photons, nuclear imaging techniques collect the emitted high energy (100-511 keV) photons after radioactive decay of radionuclides while optical techniques count low-energy (1.5-4.1 eV) photons that are scattered and absorbed by tissues requiring models of light transport for quantitative image reconstruction. Fluorescence imaging has been recently translated into clinic demonstrating high sensitivity, modest tissue penetration depth, and fast, millisecond image acquisition times. As a consequence, the promise of quantitative optical tomography as a complement of small animal PET and SPECT remains high. In this review, we summarize the different instrumentation, methodological approaches and schema for inverse image reconstructions for optical tomography, including luminescence and fluorescence modalities, and comment on limitations and key technological advances needed for further discovery research and translation.

A novel animal model of human breast cancer metastasis to the spine: a pilot study using intracardiac injection and luciferase-expressing cells

OBJECT

Metastatic spine disease is prevalent in cancer victims; 10%-30% of the 1.2 million new patients diagnosed with cancer in the US exhibit spinal metastases. Unfortunately, treatments are limited for these patients, as disseminated disease is often refractory to chemotherapy and is difficult to treat with surgical intervention alone. New animal models that accurately recapitulate the human disease process are needed to study the behavior of metastases in real time.

METHODS

In this study the authors report on a cell line that reliably generates bony metastases following intracardiac injection and can be tracked in real time using optical bioluminescence imaging. This line, RBC3, was derived from a metastatic breast adenocarcinoma lesion arising in the osseous spine of a rat following intracardiac injection of MDA-231 human breast cancer cells.

RESULTS

Upon culture and reinjection of RBC3, a statistically significantly increased systemic burden of metastatic tumor was noted. The resultant spine lesions were osteolytic, as demonstrated by small animal CT scanning.

CONCLUSIONS

This cell line generates spinal metastases that can be tracked in real time and may serve as a useful tool in the study of metastatic disease in the spine.

In vivo bioluminescence tomography with a blocking-off finite-difference SP3 method and MRI/CT coregistration

PURPOSE

Bioluminescence imaging is a research tool for studying gene expression levels in small animal models of human disease. Bioluminescence light, however, is strongly scattered in biological tissue and no direct image of the light-emitting reporter probe's location can be obtained. Therefore, the authors have developed a linear image reconstruction method for bioluminescence tomography (BLT) that recovers the three-dimensional spatial bioluminescent source distribution in small animals.

METHODS

The proposed reconstruction method uses third-order simplified spherical harmonics (SP3) solutions to the equation of radiative transfer for modeling the bioluminescence light propagation in optically nonuniform tissue. The SP3 equations and boundary conditions are solved with a finite-difference (FD) technique on a regular grid. The curved geometry of the animal surface was taken into account with a blocking-off region method for regular grids. Coregistered computed tomography (CT) and magnetic resonance (MR) images provide information regarding the geometry of the skin surface and internal organs. The inverse source problem is defined as an algebraic system of linear equations for the unknown source distribution and is iteratively solved given multiview and multispectral boundary measurements. The average tissue absorption parameters, which are used for the image reconstruction process, were calculated with an evolution strategy (ES) from in vivo measurements using an implanted pointlike source of known location and spectrum. Moreover, anatomical information regarding the location of the internal organs and other tissue structures within the animal's body are provided by coregistered MR images.

RESULTS

First, the authors recovered the wavelength-dependent absorption coefficients (average error of 14%) with the ES under ideal conditions by using a numerical mouse model. Next, they reconstructed the average absorption coefficient of a small animal by using an artificial implanted light source and the validated ES. Last, they conducted two in vivo animal experiments and recovered the spatial location of the implanted light source and the spatial distribution of a bioluminescent reporter system located in the kidneys. The source reconstruction results were coregistered to CT and MR images. They further found that accurate bioluminescence image reconstructions could be obtained when segmenting a voidlike cyst with low-scattering and absorption parameters, whereas inaccurate image reconstructions were obtained when assuming a uniform optical parameter distribution instead. The image reconstructions were completed within 23 min on a 3 GHz Intel processor.

CONCLUSIONS

The authors demonstrated on in vivo examples that the combination of anatomical coregistration, accurate optical tissue properties, multispectral acquisition, and a blocking-off FD-SP3 solution of the radiative transfer model significantly improves the accuracy of the BLT reconstructions.

Spectrally resolved bioluminescence tomography using the reciprocity approach

Spectrally resolved bioluminescence optical tomography is an approach to recover images of, for example, Luciferase activity within a volume using multiwavelength emission data from internal bioluminescence sources. The underlying problem of uniqueness associated with nonspectrally resolved intensity-based bioluminescence tomography is demonstrated and it is shown that using a non-negative constraint inverse algorithm, an accurate solution for the source distribution can be calculated from the measured data. Reconstructed images of bioluminescence are presented using both simulated complex and heterogeneous small animal models as well as real multiwavelength data from a tissue-simulating phantom. The location of the internal bioluminescence source using experimental data is obtained with 0.5 mm accuracy and it is shown that small (2.5 mm diameter) sources of up to 12.5 mm deep, within a complex mouse model, can be resolved accurately using a single view data collection strategy. Finally, using the reciprocity approach for image reconstruction, a dramatic improvement in computational time is shown without loss to image accuracy with both experimental and simulated data, potentially reducing computing time from 402 to 3.75 h.

Validation of bioluminescent imaging techniques

Bioluminescence imaging (BLI) is frequently cited for its ease of quantification. This fundamental strength of BLI has led to applications in cancer research, cell transplantation, and monitoring of infectious disease in which bioluminescence intensity is correlated with other metrics. However, bioluminescence measurements can be influenced by a number of factors, among them source location, tissue optical properties, and substrate availability and pharmacokinetics. Accounting for these many factors is crucial for accurate BLI quantification. A number of methods can be employed to ensure correct interpretation of BLI results and validate BLI techniques. This chapter summarizes the use of calibrated light-emitting standards, bioluminescence tomography, and post-mortem validation of luciferase expression for validating quantitative BLI measurements.