Knowledge-based vascular segmentation methods typically rely on a pre-built training set of segmented images,
which is used to estimate the probability of each voxel to belong to a particular tissue. In 3D Rotational Angiography
(3DRA) the same tissue can correspond to different intensity ranges depending on the imaging device,
settings and contrast injection protocol. As a result, pre-built training sets do not apply to all images and
the best segmentation results are often obtained when the training set is built specifically for each individual
image. We present an Image Intensity Standardization (IIS) method designed to ensure a correspondence between
specific tissues and intensity ranges common to every image that undergoes the standardization process.
The method applies a piecewise linear transformation to the image that aligns the intensity histogram to the
histogram taken as reference. The reference histogram has been selected from a high quality image not containing
artificial objects such as coils or stents. This is a pre-processing step that allows employing a training set
built on a limited number of standardized images for the segmentation of standardized images which were not part of the training set. The effectiveness of the presented IIS technique in combination with a well-validated knowledge-based vasculature segmentation method is quantified on a variety of 3DRA images depicting cerebral arteries and intracranial aneurysms. The proposed IIS method offers a solution to the standardization of tissue classes in routine medical images and effectively improves automation and usability of knowledge-based vascular segmentation algorithms.
In this study, the relationship between hemodynamics patterns and aneurysmal rupture was investigated in cerebral aneurysms of a single morphological type (terminal aneurysms) regardless of their location. Hemodynamics information (intra-aneurysmal velocity and pressure fields and wall shear stress distributions) was derived from image-based computational fluid dynamics models with realistic patient specific anatomies. A total of 41 patient-specific models constructed from 3D rotational angiography images were analyzed. The results suggest that high wall shear stress may be associated with aneurysm rupture and that in turn different flow splitting patterns from the parent artery to the daughter branches and the aneurysm produce different levels of wall shear stress.
We investigate whether blebs in cerebral aneurysms form in regions of low or high wall shear stress (WSS), and how
the intraaneurysmal hemodynamic pattern changes after bleb formation. Seven intracranial aneurysms harboring well
defined blebs were selected from our database and subject-specific computational models were constructed from 3D
rotational angiography. For each patient, a second anatomical model representing the aneurysm before bleb formation
was constructed by smoothing out the bleb. Computational fluid dynamics simulations were performed under pulsatile
flow conditions for both models of each aneurysm. In six of the seven aneurysms, the blebs formed in a region of
elevated WSS associated to the inflow jet impaction zone. In one, the bleb formed in a region of low WSS associated to
the outflow zone. In this case, the inflow jet maintained a fairly concentrated structure all the way to the outflow zone,
while in the other six aneurysms it dispersed after impacting the aneurysm wall. In all aneurysms, once the blebs
formed, new flow recirculation regions were formed inside the blebs and the blebs progressed to a state of low WSS.
Assuming that blebs form due to a focally damaged arterial wall, these results seem to indicate that the localized injury
of the vessel wall may be caused by elevated WSS associated with the inflow jet. However, the final shape of the
aneurysm is probably also influenced by the peri-aneurysmal environment that can provide extra structural support via
contact with structures such as bone or dura matter.
Although the incidence of ruptured cerebral aneurysms is relatively small, when rupture occurs, morbidity and mortality
are exceptionally high. The understanding of the pathological and physiological forces driving aneurysmal pathogenesis
and progression is crucial. In this paper we analyze the occurrence of mirror cerebral aneurysms in 8 patients and
speculate on the effect of haemodynamics on the localization and course of the disease. By mirror cerebral aneurysms
we indicate two aneurysms in the same patient and at the same location in the cerebral vasculature but symmetrically
with respect to a sagittal plane. In particular we focus on cases of mirror cerebral aneurysms where only one of the two
aneurysms presented subarachnoid hemorrhage (SAH). Anatomical information is extracted from 3D rotational
angiography (3DRA) images and haemodynamic information is obtained through blood flow simulation in patientspecific
anatomical models. The distribution of Wall Shear Stress (WSS) and the flow patterns through the vessels and
inside the aneurysms are reported. By combining clinical observations on asymmetry of the cerebral vasculature and
aneurysmal shape and size with computed information on blood flow patterns we explore the causes behind a specific
localization and a different outcome of disease progression.
KEYWORDS: Angiography, 3D modeling, Volume rendering, Cerebral aneurysms, Hemodynamics, Visualization, Arteries, 3D image processing, Chemical elements, Systems modeling
In this study we qualitatively compare the flow structures observed in cerebral aneurysms using conventional
angiography and virtual angiograms produced from patient-specific computational fluid dynamics (CFD) models. For
this purpose, high frame rate biplane angiograms were obtained during a rapid injection of contrast agent in three
patients with intracranial aneurysms. Patient-specific CFD models were then constructed from 3D rotational
angiography images of each aneurysm. Time dependent flow fields were obtained from the numerical solution of the
incompressible Navier-Stokes equations under pulsatile flow conditions derived from phase-contrast magnetic
resonance measurements performed on normal subjects. These flow fields were subsequently used to simulate the
transport of a contrast agent by solving the advection-diffusion equation. Both the fluid and transport equations were
solved with an implicit finite element formulation on unstructured grids. Virtual angiograms were then constructed by
volume rendering of the simulated dye concentration field. The flow structures observed in the conventional and virtual
angiograms were then qualitatively compared. It was found that the finite element models showed distinct flow types
for each aneurysm, ranging from simple to complex. The virtual angiograms showed good agreement with the images
from the conventional angiograms for all three aneurysms. Analogous size and orientation of the inflow jet, regions of
flow impaction, major intraaneurysmal vortices and regions of outflow were observed in both the conventional and
virtual angiograms. In conclusion, patient-specific image-based computational models of intracranial aneurysms can
realistically reproduce the major intraaneurysmal flow structures observed with conventional angiography.
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