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INTRODUCTION The ability to determine the microscopic distribution of glass microspheres in 90 Y radioembolization has important applications in post-treatment microdosimetry and cluster analysis. Current methods are time-intensive and labor-intensive and thus are typically only applied to small samples. MATERIALS AND METHODS A high-resolution micro-CT image with a voxel size of 8.74 µm was acquired of phantoms containing ~25 µm-diameter glass microspheres embedded in tissue-equivalent materials that were optically transparent, which allowed true microsphere locations to be determined using transmission light microscopy. A 3-stage algorithm was developed to estimate the number and locations of microspheres in tissue regions. The stages are thresholding the CT image and discarding regions with insufficient voxels, estimating the number of microspheres in each region using the values of the detected and neighboring region voxels and estimating locations for each microsphere using the outputs of the previous two stages. Two different methods for estimating the number of microspheres in each region were derived, as were five methods for localizing microspheres. Metrics for each stage were computed, and the mean absolute error (MAE) between the dose to 72 µm voxels of the true and estimated dose maps created from the microsphere locations was used as the figure of merit for overall algorithm performance. Microsphere locations identified in the optical micrograph were used as the gold standard for the metrics of all stages. The method's utility was then demonstrated using a specimen from a human neuroendocrine tumor (NET) treated with glass 90 Y microspheres. RESULTS The stage detecting regions containing microspheres found 100% of microspheres inside regions. The number of incorrectly detected regions without microspheres was 1.5% of the total number of regions. In stage 2, with these parameters, nearly 94% of the actual number of spheres in each region was correctly counted, and only 5% of the estimated sphere quantities in each region were false positives. The MAE between the true dose maps and dose maps estimated using the full algorithm with optimal parameter and method choices was 4.2%. A total of 5,713 glass microspheres were identified as being distributed heterogeneously in the NET specimen with a maximum tumor dose of >2500 Gy and 46% of the specimen receiving

Transmission electron microscopy (TEM) provides sub-nanometer-scale details in volumetric samples. Samples such as pathology tissue specimens are often stained with a metal element to enhance contrast, which makes them opaque to optical microscopes. As a result, it can be a lengthy procedure to find the region of interest inside a sample through sectioning. We describe micro-CT scouting for TEM that allows non-invasive identification of regions of interest within a block sample to guide the sectioning step. In a tissue pathology study, a bench-top micro-CT scanner with 10 μm resolution was used to determine the location of patches of the mucous membrane in osmium-stained human nasal scraping samples. Once the regions of interest were located, the sample block was sectioned to expose that location, followed by ultra-thin sectioning and TEM to inspect the internal structure of the cilia of the membrane epithelial cells with nanometer resolution. This method substantially reduced the time and labor of the search process from typically 20 sections for light microscopy (LM) to 3 sections with no added sample preparation.

The high spatial resolution of micro-computed tomography (micro-CT) is ideal for 3D imaging of coronary arteries in intact mouse heart specimens. Previously, micro-CT of mouse heart specimens utilized intravascular contrast agents that hardened within the vessel lumen and allowed a vascular cast to be made. However, for mouse coronary artery disease models, it is highly desirable to image coronary artery walls and highlight plaques. For this purpose, we describe an ex vivo contrast-enhanced micro-CT imaging technique based on tissue staining with osmium tetroxide (OsO(4) ) solution. As a tissue-staining contrast agent, OsO(4) is retained in the vessel wall and surrounding tissue during the fixation process and cleared from the vessel lumens. Its high X-ray attenuation makes the artery wall visible in CT. Additionally, since OsO(4) preferentially binds to lipids, it highlights lipid deposition in the artery wall. We performed micro-CT of heart specimens of 5- to 25-week-old C57BL/6 wild-type mice and 5- to 13-week-old apolipoprotein E knockout (apoE(-/-) ) mice at 10 μm resolution. The results show that walls of coronary arteries as small as 45 μm in diameter are visible using a table-top micro-CT scanner. Similar image clarity was achieved with 1/2000th the scan time using a synchrotron CT scanner. In 13-week-old apoE mice, lipid-rich plaques are visible in the aorta. Our study shows that the combination of OsO(4) and micro-CT permits the visualization of the coronary artery wall in intact mouse hearts.