DISCUSSION
In patients with ARVC/D, it has been demonstrated that the RVOT is the
most commonly dilated anatomical structure.[4] It has therefore been
proposed, when associated with regional RV akinesia, dyskinesia, or
aneurysm, as a major diagnostic criterion in the 2010 revised TFC of
ARVC/D.[1] Since 2D TTE is the most readily available imaging
modality to evaluate cardiac anatomy, RVOT dilatation is defined in the
revised TFC as an echocardiographic diameter ≥ 21
mm/m2 in the PSAX view or ≥ 19 mm/m2in the PLAX view. However, accurate measurements of the RVOT diameter by
2D TTE may be difficult to obtain in patients with poor
echocardiographic windows and there may be significant variation between
operators.[5] CMR may overcome these echocardiographic limitations
and provide very accurate and reliable measurements.[6] Since static
bSSFP images in the three orthogonal planes are routinely obtained with
all CMR protocols in our institution, we sought to provide reference
ranges for the RVOT diameter in children and adolescents in the strict
transverse and sagittal views and compare the measurements with the
echocardiographic PSAX and PLAX views, respectively.
We obtained high SDs and consequently very wide reference ranges for the
RVOT diameter by CMR. This could be explained by possible imprecision of
the measurements due to lower spatial and temporal resolution of CMR
compared to 2D TTE. However, we observed even higher SDs for the RVOT
diameter by 2D TTE, with a large proportion of the measurements being
greater than the cut-off values for RVOT dilatation according to the
2010 revised TFC. In fact, 22% of the measurements in the PSAX view
were ≥ 21 mm/m2 and 30% of the measurements in the
PLAX view were ≥ 19 mm/m2. These wide variations may
be attributed to the fact that 2D TTE was performed by many different
operators, including less experienced ones, and the RVOT may have been
imaged in many cases in an oblique view. Alternatively, pediatric data
on normal values of echocardiographic RVOT diameter measurements is
rare, and the RVOT diameter may normally exhibit wider variations in
children compared to adults.[7,8] Although to a lesser degree than
in our study, Koestenberger et al. also found wide reference ranges for
the RVOT diameter in older children and adolescents.[9] Finally, the
echocardiographic measurements of the RVOT diameter as recommended in
the 2010 revised TFC may not be optimal for ARVC/D diagnosis, as they
have been shown to have low sensitivity.[10]
Despite their wide reference ranges, the RVOT diameter measurements by
CMR showed a strong correlation with the echocardiographic measurements,
suggesting that the static bSSFP images triggered to end-diastole in the
strict transverse and sagittal views may provide accurate measurements
of the RVOT diameter. However, there was poor agreement between the CMR
and 2D TTE measurements, with CMR on average significantly
underestimating the RVOT diameter compared to 2D TTE. This may not be
surprising as the strict transverse and sagittal views by CMR are
oriented more perpendicular to the RVOT than the PSAX and PLAX views by
2D TTE, which image the RVOT obliquely. Gotschy et al. demonstrated much
better agreement between RVOT diameter measurements by CMR and 2D TTE
when equivalent views were compared [10]. This was not possible in
our study as the CMR views equivalent to the 2D TTE PSAX and PLAX views
are not routinely obtained with all CMR protocols in our institution,
and therefore too few measurements would have been compared.
Limitations
Limitations of our study include a sample size that is too small to
provide normal values for the RVOT diameter by CMR, and the fact that we
did not assess intraobserver and interobserver reliability of the
measurements.