Images - Department of Radiology & Biomedical Imaging ...

6
clinical and research news
Figure 3 Flow visualization at peak systole in the ascending
aorta of a 90-year-old male with severe aortic stenosis. The
solid line indicates the location of the aortic valve. Sparse
streamlines (blue) outline the velocity field and volumerendered
turbulence intensity maps (red/yellow) show regions
and degrees of flow turbulence. Elevated turbulence intensity is
seen in the proximal ascending aorta (solid arrow) where the
post-stenotic flow jet becomes incoherent. Turbulence intensity
is elevated at the outer wall of the distal ascending aorta
(hollow arrow).
Not only can collateral flow be reliably calculated and aortic
flow profiles readily assessed, abnormal 3D flow patterns
can be identified that correlate with post-repair complications,
including aneurysm and rupture.
Aneurysms of the thoracic aorta are associated with
complex abnormal flow patterns, mostly helical in nature.
The significance of these patterns has been debated. Are
they simply the consequence of a dilated aorta, or do they
play an active role in the progression of disease? In a subset
of cases with aortic valve disease, our recent work suggests
that flow may play an active role. Flow similar to that seen
within aortic aneurysms has been demonstrated in aortas
that are not (yet) dilated (Figure 4).
Valve-Related Aortic Disease
Evaluating valve-related disease of the ascending aorta with
4D Flow is a promising clinical application. Many studies
have assessed flow alterations in patients who have undergone
aortic valve and/or ascending aortic replacement. Presurgically,
4D Flow may prove useful by risk-stratifying
patients and guiding the timing of intervention. Aortic
valve disease is relatively common, especially in the elderly,
and is associated with the long-observed phenomenon of
post-stenotic dilation. The mechanism is presumed to be
flow-related. Without 4D Flow, however, the altered hemodynamics
have not been well characterized. The detailed
assessment of 4D Flow reveals altered systolic flow resulting
from eccentric flow jets with stenotic and deformed aortic
valves (Figure 4). The degree of flow eccentricity can be
quantified, and has been shown to correlate with focally
elevated WSS and aortic dilation.
We have focused our efforts on patients with bicuspid
aortic valve (BAV), a defect found in 1-2% of the population
that frequently results in significant aortic pathology,
including aneurysm and dissection. We hypothesize that
eccentric systolic flow jets with BAV, through the mechanism
of elevated WSS, promote aortic dilation. The clinical
relevance of proving this mechanism would be considerable,
as it would allow a non-invasive means of risk-stratifying the
sizable population of patients with BAV (up to four million
in the U.S. alone). For example, 4D Flow assessment could
be performed for patients with BAV before valve or aortic
disease manifests. If normal systolic flow were identified,
patients would need only occasional follow-up. If eccentric
flow were identified, 1) closer follow-up with MRI would
be indicated, as follow-up echocardiography may be inadequate
to assess the entire ascending aorta; 2) medication
with beta-blockers, which lower WSS, could be initiated;
and 3) depending on interval growth rates and the degree
of eccentricity, earlier intervention may be warranted (e.g.,
at 4.5 cm for high-risk patients).
With the help of our collaborators in Cardiology and
Cardiothoracic Surgery, we are collecting data on whether
increased aortic growth rates are seen as a consequence of
the elevated hemodynamic burden experienced by the aortic
wall because of eccentric systolic flow. We are developing
a time-resolved MRA sequence that will enable follow-up
studies to be co-registered spatially and temporally, so aortic
dimensions can be evaluated at identical locations and
orientations. We also are investigating animal models of