* Scientists
interested in translating their postnatal research to fetal applications
* Physicians
curious how MRI may contribute to fetal cardiovascular assessment, and
what the future holds
Normal fetal development
requires a steady supply of oxygenated blood to the fetus. Disruption of this
supply from placental dysfunction, fetal heart disease or fetal anemia can
result in fetal death or injury to critical organs. While early delivery is an
effective treatment, this option must be weighed against the risks of premature
birth such as infection, poor organ development and cognitive delay (1–3). Emerging treatments intended to modify
fetal blood flow in utero include drugs delivered through the maternal
circulation, maternal oxygen supplementation, blood transfusions to the fetus,
and even percutaneous surgical correction of fetal cardiovascular anatomy (4–6). Selecting the appropriate therapy and
monitoring its efficacy requires accurate fetal cardiovascular assessment.
MRI is an appealing
technology for fetal assessment because it can visualize both cardiac and
vascular anatomy (steady-state free precession imaging), it can quantify flow through
the complex fetal circulation (phase contrast imaging), and it is also
sensitive to the oxygen saturation of blood (relaxometry and susceptometry). However,
there are practical limitations to the use of conventional MRI for fetal
assessment including the small size and high heart rate of the human fetus – even at full gestation the largest fetal vessels are only ~8 mm
in diameter and heart rates range between 110-180 bpm (7,8). The fetus is also prone to unpredictable
movements which corrupt MRI data. Finally, conventional cardiac gating (e.g., ECG) is not
available to synchronize data acquisition to the fetal cardiac cycle.
Unfortunately, even the most advanced real-time MRI methods cannot
provide sufficient temporal and spatial resolution for dynamic fetal imaging (9).
Thus, innovative MRI
acquisition and reconstruction methods are required to obtain dynamic cardiovascular
data at high temporal and spatial resolutions while correcting for gross fetal
motion. In this presentation, I will provide an overview of fetal
cardiovascular MRI methods developed in our lab and elsewhere. I will also describe our initial experience using these
methods to study healthy and at-risk pregnancies, to
demonstrate the potential contribution of MRI to fetal assessment.
Development of fetal MRI
dates back over 30 years, with early applications focused on anatomical imaging
of the brain and other static organs (10,11). Conversely, use of MRI to study fetal
cardiovascular function has become possible only recently. The first impediment
to translating conventional cardiac MRI methods to fetal applications was the
lack of a cardiac gating signal. A number of solutions to this problem have
been proposed which I will cover in my presentation including retrospective self-gating,
metric-based gating, and data combination with motion correction, as well as
prospective gating using external hardware to monitor the fetal cardiac cycle (12–17).
MRI of the fetal heart and
blood flow is complemented by the sensitivity of MRI to the oxygen saturation
of blood. Susceptibility differences between oxyhemoglobin and deoxyhemoglobin
manifest as altered transverse relaxation times (T2* and T2) that can be used
to quantify fetal blood oxygenation (18,19). Alternatively, MR susceptometry can be
used to quantify oxygen saturation based on susceptibility differences between
large blood vessels and the surrounding tissue (20). These approaches have been used to study
oxygenation in fetal mice and sheep during maternal hypoxic and hyperoxic ventilation (21–23), and in human pregnancy during maternal normoxic
and hyperoxic ventilation (24–28).
We have developed a
comprehensive MRI examination protocol to assess fetal anatomy, blood flow and
oxygenation (29). In
my presentation, I will describe our experience using these methods to map the
normal fetal circulation (30,31)
and to assess changes in fetal blood flow and oxygenation associated with pathologies
including congenital heart disease and intrauterine growth restriction (27,28,32).
Although
great strides have been made in the development of fetal cardiovascular MRI,
challenges remain. In particular, fetal motion and total examination times
limit many cardiovascular MRI studies. As such, our work has focused on late
gestation when fetal movement is restricted. Accelerated imaging, together with
motion compensation strategies developed for neuroimaging, hold promise for
overcoming these issues to facilitate scanning at earlier gestation (33–35). Meanwhile,
ultrasound remains the dominant imaging modality for obstetrics and recent
advances using plane-wave imaging provide unprecedented temporal resolution for
assessing fetal anatomy and flow. The role of MRI may thus focus on the
comprehensive information is provides regarding flow and oxygen together (total
oxygen delivery and consumption).
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