Analysis of 3D images of vascular trees presents a major logistic and multi-scale imaging challenge. One approach
that greatly reduces the image analysis difficulty is to apply an 'erode/dilate' approach to a binarized, segmented,
image so as to progressively eliminate branches of increasing diameter. Although this provides useful data for
detecting some changes in branching geometry, it eliminates information about the hierarchical structure of
the vascular tree. To quantify the impact of this loss of branching hierarchy information we analyzed 3D
micro-CT images (4μm and 20μm isotropic voxels) of porcine myocardial "biopsies" obtained in control animals
and in animals after 100μm diameter microspheres were injected into the coronary artery perfusing the site
of subsequent biopsy. After the in vivo embolization, the vascular tree was injected with radiopaque Microfil
and "biopsies" of the myocardium harvested. The analysis of the micro-CT images of the biopsies involved
erode/dilate analysis of the opacified vessels in the entire biopsy and also of isolated vascular trees (isolated
via a 'connect' function) within the biopsy. The isolated trees were also analyzed by dimensional measurement
of the individual interbranch segment lengths and volumes, results that were then put into the same form as
those produced by the erode/dilate method. In the embolized specimens the volume-loss of vessels below 60μm
diameter closely matched for (i) erode/dilate of entire biopsy, (ii) erode/dilate of isolated tree, and (iii) direct
measurement of isolated tree. The erode/dilate method quantifies the effects of a microsphere embolization,
indicating what diameter interbranch segments trap a microsphere of a given size.
A BFU is an organ's smallest assembly of diverse cells that functions like the organ, such as the liver's hepatic lobules. There are approximately 10<sup>7</sup> BFUs in a human organ. These 100-200
μm structures are perfused by capillaries fed by a terminal arteriole (15μm diameter). BFU sizes, function and number per organ vary with disease, either by loss of BFUs and/or their decrease in function. The BFU is the upper limit of a spherical assembly of cells, immersed in a suitably nutrient medium, which can survive without its own blood supply. However, each BFU has its own blood supply to support the extra energy and/or solutes needed for providing its physiological function (e.g., contraction or secretion).
A BFU function is best evaluated by its micro-perfusion, which can be readily evaluated with whole-body CT. Resolution of individual BFUs within in-situ organs, using clinical imaging devices, would require high radiation doses and/or the intolerably long scan-durations needed for suitable signal-to-noise image-data. However, it is possible to obtain a statistical description of the BFU number, size and function from wholebody CT by way of a model. In this study we demonstrate this capability by using the distribution of
myocardial terminal arteriolar perfusion territories by way of a nested, multiple, regions-of-interest analysis of the heart wall imaged during transient opacification of its blood supply.
Fast CT has shown that myocardial perfusion (F) is related to myocardial intramuscular blood volume (Bv) as
Bv=A*F+B*F<sup>1/2</sup> where A,B are constant coefficients. The goal of this study was to estimate the range of
diameters of the vessels that are represented by the A*F term. Pigs were placed in an Electron Beam CT
(EBCT) scanner for a perfusion CT scan sequence over 40 seconds after an IV contrast agent injection.
Intramyocardial blood volume (Bv) and flow (F) were calculated in a region of the myocardium perfused by
the LAD. Coefficients A and B were estimated over the range of F=1-5ml/g/min. After the CT scan, the
LAD was injected with Microfil<sup>(R)</sup> contrast agent following which the myocardium was scanned by micro-CT
at 20μm, 4μm and 2.5 μm cubic voxel resolutions. The Bv of the intramyocardial vessels was calculated for
diameter ranges d=0-5, 5-10, 10-15, 15-20μm, etc. EBCT-derived data were presented so that it could be
directly compared the micro-CT data. The results indicated that the blood in vessels less than 10μm in lumen
diameter occupied 0.27-0.42 of total intravascular blood volume, which is in good agreement with EBCT-based
values 0.28-0.48 (R<sup>2</sup> =0.96). We conclude that whole-body CT image data obtained during the passage
of a bolus of IV contrast agent can provide a measure of the intramyocardial intracapillary blood volume.
Myocardial microcirculation disturbances often precede angiographically visible of narrowing large epicardial coronary arteries and associated symptoms. Clinical tomographic imaging cannot resolve the microcirculation, hence an indirect method of quantitating microvascular disturbances in those images must be developed. We propose that such an indirect method can be based on the characterization of the spatial heterogeneity of myocardial intravascular blood volume. We evaluated the relationship of multi-resolution, nested multi Region-of-Interest (ROI) analysis of EBCT images to the actual intravascular volume of microvascular branches as measured directly with micro-CT images in the same myocardial regions. We selectively altered the intravascular volume of vessels by injecting 30, 100, 200 or 300μm diameter microspheres into anesthetized pigs’ LAD coronary arteries prior to EBCT scanning during contrast injection. The heart was then harvested and the LAD coronary artery was infused with Microfil polymer. An approximately 2cm<sup>3</sup> transmural “biopsy” of the same ROI within the myocardium analyzed in the EBCT images was scanned by micro-CT resulting in a 3D image of 20μm cubic voxels. Myocardial opacification was measured in both the EBCT and micro-CT images. The EBCT and micro-CT images were analyzed with the nested multi ROI method which provides an index of spatial heterogeneity of intramyocardial blood volume in terms of the linear relationship between the logarithms of the coefficient of variation within the data obtained at any one size of the ROI, and the logarithm of the volume of that selected ROI. The minimum ROI volume in the EBCT analysis was 8.96 mm<sup>3</sup> and for the micro-CT it was 0.07 mm<sup>3</sup>. There is linear correlation when EBCT and micro-CT image CT gray-scale numbers are plotted as Log (standard deviation/mean) against Log (Volume of ROI). The results show that the slopes and offsets of the EBCT-based and micro-CT-based regression lines were indistinguishable. Moreover, when a fraction of microvessels of selected diameter was embolized, the change in the resulting regression line was characteristic for that diameter. In summary, the EBCT-based analysis spatial heterogeneity of myocardial blood volume can be extrapolated to describe the spatial distribution of the microcirculatory branching geometry in terms of intra segmental lumen volume.