Introduction
The antenatal work-up of fetal thoracic anomalies increasingly includes
fetal MRI in recent years. Fetal MRI aids in cases of diagnostic
uncertainty by detailed delineation of soft tissues within the fetal
chest, differentiating lung, bowel and liver more easily than ultrasound1, helping in the assessment of congenital
diaphragmatic hernia (CDH) and congenital lung lesions (CLL, including
congenital pulmonary airway malformation [CPAM] and bronchopulmonary
sequestration [BPS]). The currently standardised prognostic markers
for CDH and CPAM are the observed/expected Lung Head Ratio (O/E LHR,2) and the CPAM volume ratio (CVR,3), both measured by ultrasound. Both of these metrics
utilise single plane measurements of aspects of the fetal chest to
extrapolate volumes. Although these metrics are valid surrogates for
total lung volume (TLV, 4), generation of such values
may have inherent intra- and inter-operator
variability5. Discrepancies between 2D ultrasound and
MRI-derived TLV measurements have been shown to be principally related
to the failure to account for the contribution of the ipsilateral
lung6. Furthermore, ultrasound-based imaging struggles
with maternal habitus and fetal positioning, as well as extremes of
liquor volume. 3D ultrasound lung volume measurements have been shown to
be more difficult than by MRI, mainly because the most hypoplastic lung
cannot be properly visualized7. High volume fetal
medicine centres have suggested that MRI-calculated lung volumes may be
a more accurate predictor of survival but that has not been formally
proven8, however there is no standardised methodology
for the use of MRI in these cases which limits the compilation of large
datasets necessary for further study9.
Modern clinical fetal MRI protocols (primarily based on single shot
turbo spin echo (ssTSE) sequence) allow fast acquisition of individual
2D slices that “freeze” fetal position in time. These slices have
sufficiently high image and contrast resolution for diagnostic purposes
in cases where fetal motion may previously have limited the information
available. However, the misalignment between individual slices leads to
corruption of volumetric information and loss of structural continuity
within a slice stack. Therefore, output MRI stacks are termed “motion
corrupted”. In general, to achieve a sufficient coverage of fetal
visceral organs, clinical examinations require 3-6 MRI stacks acquired
under different orientations with respect to the fetal body. The degree
of motion corruption (misalignment of slices) can vary between the
stacks and normally depends on the gestational age, amount of amniotic
fluid, as well as fetal lie and mobility. It directly influences the
accuracy of the fetal lung volume assessment, which is based on 2D
slice-wise segmentation of motion-corrupted stacks followed by 3D
interpolation. Therefore, calculated TLV values might vary for different
stacks (from the same acquisition / the same subject) depending on the
amount of motion corruption and acquisition plane. This is considered
one of the limiting factors for MRI-based TLV assessment.
The recently proposed deformable slice-to-volume registration (DSVR)
method 10 is used for reconstruction of
high-resolution (e.g., 0.8x0.8x0.8mm) isotropic 3D images of fetal body
from multiple low-resolution (e.g., 1.25x1.25x1.25mm) motion-corrupted
stacks. In DSVR method, one of the low-resolution stacks is selected as
an initial target space and it is then registered to each of the slices
using nonlinear free form deformation registration. This is followed by
super-resolution reconstruction (SR) of the 3D image from the registered
slices. The full pipeline includes 3 interleaved SR and SVR steps. The
resulting reconstructed images provide detailed 3D volumetric
information and can be reoriented in any plane. This facilitates
accurate 3D segmentations of fetal lungs and other organs allowing
volumetric analysis.
In this work, we sought to explore how DSVR-derived lung volumes would
compare to those calculated by conventional manual 2D segmentation in
cases of fetal thoracic anomalies, with a comparison to normal control
cases.