Synchrotron-based micro-CT of in situ biological basic functional units and their integration

Use of synchrotron generated x-ray for micro-CT is particularly powerful for several reasons. These include the high x-ray flux which permits short duration exposures and hence scan durations, the narrow bandwidth of the xray energy which permits quantitative CT imaging with high accuracy of the measured attenuation coefficients and the fact that the x-ray photon energy can be adjusted allows element selective imaging. Another advantage is that the radiation is close to parallel so that the tomographic image reconstruction process is facilitated. On the other hand, synchrotron-based micro-CT imaging does have the limitation of a rather small field of view being illuminated. This means that specimens larger than the field of view (but the volume of interest within the specimen is fully illuminated by the x-ray) also create problems for the conventional ''Global'' tomographic image reconstruction algorithms. Fortunately, recently developed ''Local'' reconstruction algorithms can, in large measure, overcome this limitation of the synchrotron generated x-ray field. We have used the EXXON Company micro-CT scanner on the X2B beam line of the Brookhaven National Laboratories' (BNL) National Synchrotron Light Source (NSLS) to scan a variety of intact rat and mouse organs. This has enabled us to study organs' in situ Basic Functional Units. Here the key issue is not increased spatial resolution so much as reasonable spatial resolution throughout a large volume (relative to the resolution unit). Specifically, we give examples of resolution test phantom, heart, bone and kidney images to illustrate the spatial resolution, ability to image an intact rodent organ and the associated capability to quantitatively analyze the 3D image data in a logistically convenient manner. The resolution of the 3D images (typically 512(3) cubic voxels, each 2 to 10 mu m on a side) was evaluated by estimation of the Modulation Transfer Function obtained from the imaged edge of a gold foil held in the specimen space and with a rest phantom which consisted of a gelatin cylinder, 1 cm in diameter, in which glass microspheres, 10, 30, 100 and 300 mu m diameter, were suspended. That the imaged 3D structure could be quantitatively related to conventional histological characteristics was illustrated by close correspondence between a slice computed from the scanned volume to match a histological section, obtained after the intact specimen was scanned. These preliminary studies demonstrated the ability to visualize Basic Functional Unit structures within intact organs - such as the renal glomeruli and their post gIomerular vessels, the Haversian canals in bone (as well as the detailed microtrabecular architecture), cardiac coronary branching architecture and myocardial muscle fiber architecture.