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When performing Computed Tomographic (CT) image reconstruction on digital subtraction angiography (DSA)
projections, loss of vessel contrast has been observed behind highly attenuating anatomy, such as dental implants
and large contrast filled aneurysms. Because this typically occurs only in a limited range of projection angles, the
observed contrast time course can potentially be altered. In this work, we have developed a model for acquiring
DSA projections that models both the polychromatic nature of the x-ray spectrum and the x-ray scattering
interactions to investigate this problem. In our simulation framework, scatter and beam hardening contributions to
vessel dropout can be analyzed separately. We constructed digital phantoms with large clearly defined regions
containing iodine contrast, bone, soft issue, titanium (dental implants) or combinations of these materials. As the
regions containing the materials were large and rectangular, when the phantoms were forward projected, the
projections contained uniform regions of interest (ROI) and enabled accurate vessel dropout analysis. Two phantom
models were used, one to model the case of a vessel behind a large contrast filled aneurysm and the other to model a
vessel behind a dental implant. Cases in which both beam hardening and scatter were turned off, only scatter was
turned on, only beam hardening was turned on, and both scatter and beam hardening were turned on, were simulated
for both phantom models. The analysis of this data showed that the contrast degradation is primarily due to scatter.
When analyzing the aneurysm case, 90.25% of the vessel contrast was lost in the polychromatic scatter image,
however only 50.5% of the vessel contrast was lost in the beam hardening only image. When analyzing the teeth
case, 44.2% of the vessel contrast was lost in the polychromatic scatter image and only 26.2% of the vessel contrast
was lost in the beam hardening only image.
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