Optical Imaging

Facilities and Equipment

A G.E. eXplore Optix [1] is located in the IVISR of the Moores Cancer Center. It consists of a light-tight box that houses the excitation lasers, detector (photomultiplier tube with 250ps temporal resolution), filters, and appropriate optics. Time-correlated single photon counting (TCSPC) is used to measure the temporal dispersion of fluorescent photons following excitation of a fluorophore by a picosecond laser pulse operating at 80 MHz. This temporal dispersion curve, known as the fluorescent temporal point spread function (TPSF), can be analyzed to derive the in vivo fluorophore depth, concentration, and lifetime or simply temporally integrated to provide the continuous wave (CW) fluorescent intensity. The imaging point (1mm in size) is raster-scanned over the entire region-of-interest. Laser power, count time per raster point (0.1-1s), translation increment (0.5-3mm) and region-of-interest (up to 8.4x20cm) are selected by the operator to achieve optimal temporal and spatial resolution and SNR over the desired region. 

      Service

Optical imaging offers higher sensitivity and temporal resolution than PET at the same spatial resolution in small animals, but unlike PET is limited to a few centimeters of tissue depth. The GE eXplore Optix is a time domain (TD) imaging system. Most instruments use CW methods that measure the total light intensity emitted from an excited fluorophore or bioluminescent reporter [2, 3]. Probe concentration is difficult to quantify with CW because high concentrations deep in tissues can emit the same total light intensity as lower concentrations near the surface. In contrast, TD measures the light intensity as a function of arrival time in nanoseconds, where the signal from deeper tissues arrives later allowing the estimation of relative concentration difference [4]. Note that the CW signal is equal to the integral under the TPSF TD curve [5]. Equally important is that TD also permits the measurement of fluorescence lifetime that is independent from intensity [6], however, in vivo applications add the challenge of scattering and diffusion. The benefits of the fluorescence life-time imaging that is only possible in TD systems are: 1) it allows the potential to distinguish two fluorophores that emit at similar wavelengths; 2) it allows the recognition that the same fluorophore is in a different environment such as higher or lower pH; and 3) it allows the indirect measure of the spatial proximity of a donor and acceptor [7] to explore in-vivo binding or dissociation. Specific services include:

• Imaging of receptor-targeted fluorophore agents
• Depth resolved imaging that uses time delay (nanoseconds) to determine the location of the source
• Measurement of tumor biochemistry and/or physiology via fluorescence decay time and kinetic modeling.

Imaging of receptor-targeted fluorophore agents

This is a standard imaging procedure on the eXplore optix instrument that will be operated by technicians according to prescribed protocol. The instrument acquires a TD TPSF curve at each raster point and evaluates its integral to generate a CW-equivalent image (Figure 11). The 2D image is the classic optical image that will be used by investigators interested in localizing their fluorophores in the mouse, when concentration and kinetic modeling are not needed.

Depth resolved imaging that uses time delay to determine the location of the source

Since we believe that kinetic modeling is critical to assess targeting, calculating relative probe concentration as a function of time is important. The TPSF measured by the eXplore Optix is available for processing by a variety of algorithms to yield not only the fluorescent intensity measured by simpler CW systems, but also the fluorophore depth, relative concentration, and lifetime. A simple algorithm is available as a MATLAB program to process the TPSF data and provide the fluorophore depth and relative concentration. The field of near infrared optical image reconstruction in vivo is dominated by the diffusion approximation, the first order approximation to the radiative transfer equation, which describes the propagation of light through highly scattering media such as biological tissue.

There are a plethora of algorithm implementations [8] ranging from analytic models [9], perturbation theory [10], and non-linear finite element model inversions [11]. For TD optical imaging of fluorophores, the parameters of interest are the optical absorption and scattering coefficients of the tissue at both the excitation and emission wavelengths, and the concentration and lifetime on the fluorophore [12]. These parameters can be reconstructed in 3D and yield the distribution of the optical probe in vivo. An example of a more advanced image reconstruction method used to reconstruct a 3D distribution of relative fluorophore concentration from the TPSF is shown in Figure 12.