Optical computed tomography (optical-CT) and optical emission computed tomography (optical-ECT) are new techniques that enable unprecedented high-resolution 3-D multimodal imaging of tissue structure and function. Applications include imaging macroscopic gene expression and microvasculature structure in unsectioned biological specimens up to 8 cm3. A key requisite for these imaging techniques is effective sample preparation including optical clearing, which enables light transport through the sample while preserving the signal (either light absorbing stain or fluorescent proteins) in representative form. We review recent developments in optical-CT and optical-ECT, and compatible “fluorescence-friendly” optical clearing protocols.
We explore the potential of optical computed tomography (optical-CT) and optical emission computed tomography (optical-ECT) in a new area—whole organ imaging. The techniques are implemented on an in-house prototype benchtop system with improved image quality and the capacity to image larger samples (up to 3 cm) than previous systems based on stereo microscopes. Imaging performance tests confirm high geometrical accuracy, accurate relative measurement of linear attenuation coefficients, and the ability to image features at the 50-µm level. Optical labeling of organ microvasculature was achieved using two stains deposited via natural in vivo circulatory processes: a passive absorbing ink-based stain and an active fluorescin FITC-lectin conjugate. The lectin protein binds to the endothelial lining, and FITC fluorescense enables optical-ECT imaging. Three-dimensional (3-D) optical-CT images have been acquired of a normal rat heart and left lung and a mouse right lung showing exquisite detail of the functional vasculature and relative perfusion distribution. Coregistered optical-ECT images were also acquired of the mouse lung and kidney. Histological sections confirmed effective labeling of microvasculature throughout the organs. The advantages of optical-CT and optical-ECT include the potential for a unique combination of high resolution and high contrast and compatibility with a wide variety of optical probes, including gene expression labeling fluorescent reporter proteins.
We present preliminary investigations that examine the feasibility of incorporating digital tomosynthesis into radiation oncology practice with the use of kilovoltage on-board imagers (OBI). Modern radiation oncology linear accelerators now include hardware options for the addition of OBI for on-line patient setup verification. These systems include an x-ray tube and detector mounted directly on the accelerator gantry that rotate with the same isocenter. Applications include cone beam computed tomography (CBCT), fluoroscopy, and radiographs to examine daily patient positioning to determine if the patient is in the same location as the treatment plan. While CBCT provides the greatest anatomical detail, this approach is limited by long acquisition and reconstruction times and higher patient dose. We propose to examine the use of tomosynthesis reconstructed volumetric data from limited angle projection images for short imaging time and reduced patient dose. Initial data uses 61 projection images acquired over an isocentric arc of twenty degrees with the detector approximately fifty-four centimeters from isocenter. A modified filtered back projection technique, which included a mathematical correction for isocentric motion, was used to reconstruct volume images. These images will be visually and mathematically compared to volumetric computed tomography images to determine efficacy of this system for daily patient positioning verification. Initial images using the tomosynthesis reconstruction technique show much promise and bode well for effective daily patient positioning verification with reduced patient dose and imaging time. Additionally, the fast image acquisition may allow for a single breath hold imaging sequence, which will have no breath motion.
The limitations of conventional dosimeters restrict the comprehensiveness of verification that can be performed for advanced radiation treatments presenting an immediate and substantial problem for clinics attempting to implement these techniques. In essence, the rapid advances in the technology of radiation delivery have not been paralleled by corresponding advances in the ability to verify these treatments. Optical-CT gel-dosimetry is a relatively new technique with potential to address this imbalance by providing high resolution 3D dose maps in polymer and radiochromic gel dosimeters. We have constructed a 1<sup>st</sup> generation optical-CT scanner capable of high resolution 3D dosimetry and applied it to a number of simple and increasingly complex dose distributions including intensity-modulated-radiation-therapy (IMRT). Prior to application to IMRT, the robustness of optical-CT gel dosimetry was investigated on geometry and variable attenuation phantoms. Physical techniques and image processing methods were developed to minimize deleterious effects of refraction, reflection, and scattered laser light. Here we present results of investigations into achieving accurate high-resolution 3D dosimetry with optical-CT, and show clinical examples of 3D IMRT dosimetry verification. In conclusion, optical-CT gel dosimetry can provide high resolution 3D dose maps that greatly facilitate comprehensive verification of complex 3D radiation treatments. Good agreement was observed at high dose levels (>50%) between planned and measured dose distributions. Some systematic discrepancies were observed however (rms discrepancy 3% at high dose levels) indicating further work is required to eliminate confounding factors presently compromising the accuracy of optical-CT 3D gel-dosimetry.