Biomedical Engineering Reference
In-Depth Information
the entire volume. As indicated by the recent work of Sundareswaran et al. ( 2012 ),
these methods of 4D in vivo flow visualization and quantification have significant
clinical potential. When acquired and analyzed over a larger series of patients, the
results can be used to provide insights on a more generalizable level, such as the
hemodynamic implications of different approaches to Fontan surgery (extra- vs.
intra-cardiac) or the accuracy of computational fluid solvers for analyzing the same
flow conditions. The increased availability of these 4D flow imaging sequences, and
the development of volumetric phase contrast techniques (Markl et al., 2011 ), will
make such analyses possible on a much broader scale in the near future.
Furthermore, for failing patients referred for surgical planning, these imaging
techniques can be an invaluable means to directly identify adverse hemodynamic
characteristics, as demonstrated in the following case study.
16.2.1.1 Patient-Specific Case Study
This was an eleven year old patient with bilateral superior vena cavae (SVC), in-
terrupted IVC and azygos vein continuation, and a previous Fontan connection. In
this rare and complex anatomy, the majority of inferior venous blood flow is carried
by an enlarged azygos vein, which connects to one of the two SVCs. The Fontan
baffle therefore carries only hepatic blood from the liver to the pulmonary arteries.
Hemodynamic analysis was necessitated by the presence of PAVM in the right lung:
arterial to venous shunts within the pulmonary vasculature that bypass the oxygenat-
ing alveolar beds, leading to progressive hypoxia. Studies suggest that this condi-
tion can develop because of a lack of hepatic blood flow, and a so-called 'hepatic
factor', to either or both lungs (Duncan and Desai, 2003 ). This strong and well-
characterized relationship between PAVM and the local TCPC hemodynamics has
made it the primary indication in the preliminary series ( n
15) of patient-specific
surgical planning; identifying connection designs that achieve a balanced hepatic
flow distribution (HFD) has been shown to be effective in alleviating the disease
state. Therefore, the objective of the analysis was to (i) confirm the unilateral dis-
tribution of hepatic flow from the liver, which is consistent with PAVM in the right
lung; (ii) determine the factors mediating such undesirable hemodynamics.
Figure 16.2 (left panel) provides a single slice example of the coronal 4D MRI
data acquired: the magnitude image is shown on top and the three orthogonal vector
components (anatomically: anterior-posterior, foot-head, left-right) are shown at the
bottom. The results from the volumetric interpolation are also shown (right panel)
as instantaneous velocity streamlines, shaded based on the vessel of origin (LSVC
flow omitted). The overlaid numbers report the time-averaged contribution of each
vessel to the total flow, as determined from analysis of the through-plane phase
contrast data.
From the reconstructed velocity field, it was appreciated that flow through the
left-sided hepatic venous baffle (HepV; blue streamlines) was both very low in mag-
nitude and heavily influenced by the momentum of the RSVC and Azygos (Azy)
=
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