Discussion
This study demonstrates that MRI of the fetal thorax can be processed by
3D DSVR reconstruction, and subsequent lung volumetry can be performed
with similar volumes generated to the 2D-manual segmentation derived
volumes currently used in clinical practice for O/E TLV assessment.
Furthermore, 3D lung segmentations potentially provide more accurate TLV
estimation since they minimise the errors on the conventional 2D-based
approach affected by the combination of misalignment of slices and
interpolation. The analysis of the quantitative comparison results as
well as the general visual inspection of 2D vs. 3D segmentations suggest
that using 3D DSVR-based TLV assessment may potentially resolve one of
the current limitations to the validity of MRI derived volumes in
predicting outcomes as reliably as LHR13. This series
demonstrates that DSVR method provides the means for accurate
calculation of lung volumes for both normal and abnormal cases. This is
especially useful for CDH cases for more accurate evaluation of the
observed to expected lung volume ratios. The recent advances in image
processing methods reduce variability in
segmentation14, further improving the reliability of
the volumetry results. We anticipate that MR based lung volumetry may
soon be validated as a reliable and more accurate prognostic indicator
than the current use of the LHR; since it would invariably more closely
predict true TLV. Furthermore, the ability to outline lesion volumes in
cases of CLL will likely produce a prognostic marker more faithful to
true lesion size than the currently utilised CVR where cross sectional
areas are utilised to estimate lesion volume relative to fetal size. The
consistency of such a method will certainly deliver on the recent plea
for consistency in prognostication of these cases9. It
should be noted that although we have provided lesion volume
proportionate to TLV, it would be possible to segment any aspect of the
fetus and derive a corresponding volume ratio proportional to this as
well as the relative position of the organs (e.g., the degree of liver
herniation or position of the stomach in CDH cases)
Recently, it has also been demonstrated that DSVR reconstructed MRI
images may provide additional diagnostic information for complex fetal
anomalies1. The illustrative cases included within
this dataset also show that DSVR will produce datasets that are more
accessible to the clinician by resolving motion corruption with a
resolution superior to that of the original input stacks. This will
certainly aid in prenatal counselling and planning of the operative
approach in neonatal thoracic surgery; the reliable depiction of
systemic feeding vessels in BPS, combined with US Doppler assessment may
reduce the reliance on early postnatal CT imaging 15.
Regarding longitudinal assessment of CDH or CLL, it should be recognised
that the repeated use of MRI is considerably more expensive, time
consuming and lengthy compared to an expert-performed ultrasound.
Therefore, we anticipate that there will continue to be a role for
ultrasound in the monitoring of such conditions to delivery. Indeed,
serial CVR assessment has recently been shown to be effective in
predicting a need for perinatal intervention in cases of
CLL16.