Synopsis
Assessment
of LV size and global systolic function is a key element of every CMR
examination. Accurate and reproducible quantification is highly important and requires an appropriate MR acquisition protocol as well as careful use of image analysis methods.Ventricular size and global function
Assessment
of LV size and global systolic function is a key element of every CMR
examination. Assessment of global ventricular function requires quantification
of the LV cavity volume at different points in the cardiac cycle. For accurate
quantification of the sometimes complex shape of the LV, 3D imaging is required
in order to obtain accurate quantification results. A typical imaging protocol
includes cine imaging in multiple long-axis views and a parallel stack of
short-axis slices covering the LV from base to apex. For RV size assessment the
use of trans-axial slices has shown to be more accurate and reproducible. The
imaging slices are acquired during repeated patient breath holds using steady
state free precession (SSFP) at a temporal resolution of approximate 40-50 ms.
Automated or manual contour detection is required to segment the LV blood pool
in the end-diastolic and end-systolic phase to derived global function
parameters such as stroke volume an ejection fraction. Various factors will
influence the accuracy and precision of the measurements. Obviously, the accuracy
and precision the global function parameters are influenced by the image
segmentation. Of especial importance is the precise and consistent definition
of the blood pool in the most basal slice. Since the cross-sectional area at
the basal level is relatively large, incorrect inclusion or exclusion of the
most basal slice will have large influence on the quantification.
Cross-referencing the short-axis images with the additionally available
long-axis views may help to make the proper judgement. However, inconsistencies
in the patient breath-hold position during acquisition of the individual slices
may be misleading. The use of novel 3D cine imaging techniques with patient
motion correction techniques may be a solution to overcome this limitation. While
3D cine imaging may prolong the total scanning time, it has the advantage that
it requires less expertise from the scanner operator in planning the required
cardiac views. Retrospectively, every desired view can be generated from the
acquired 3D volume and image analysis will not be hampered by mis-registration
of the acquired views.
Regional function
Several
approaches can be employed to study the regional contractile performance of the
LV. Visual assessment of wall motion abnormalities in short-axis and long-axis
views is clinically the most common approach. With endocardial and epicardial
contours defined, the Centerline method can be applied to quantify regional
wall motion or wall thickening. The Centerline method allows to either quantify
the regional displacement of the endocardial wall, or to quantify the local
thickening of the LV myocardium by quantifying the local change in distance
between the endocardial and epicardial contours. For quantitative assessment MR
tagging is considered the gold standard approach. MR tagging allows
quantitative assessment of intra-myocardial deformation during systole, which
can be used to accurately quantify regional LV systolic strain in longitudinal
and circumferential direction. Other MR techniques, such as DENSE (Displacement
Encoding with Stimulated Echos), SENC (Strain-ENCoded MRI, or phase-contract
tissue velocity mapping are alternative imaging method used for assessment of
regional contractile function. In recent years, the use of feature tracking
(FT) has gained a lot of attention as an alternative method to quantify LV
strain from standard cine MR imaging. FT is a technique that has been known
from cardiac ultrasound as speckle tracking that is used to track the motion of
landmarks (speckles) within the LV myocardium throughout the cardiac cycle,
which can subsequently be used to quantify LV strain. This technique, with
appropriate modifications, has been shown to also to be applicable to standard
cine MRI, which is attractive as it obviates the need for a separate tagging
acquisition. Current literature on this technique suggests FT can be used for
reproducible assessment of global circumferential and longitudinal strain.
Regional strain assessment using FT however, suffers from relatively poor
reproducibility.
Diastolic function
Diastolic
LV function can be assessed by quantifying the mitral inflow using MR flow
velocity mapping. Due to the motion of the mitral valve annulus over the
cardiac cycle, a single fixed imaging with through-plane velocity encoding
cannot be used to obtain reliable measurements of transvalvular flow. Instead,
the use of 4D Flow, with retrospective tracking of the mitral valve position
has shown to allow accurate quantification of mitral flow. Using this approach,
a 3D volume needs to be acquired covering the region of the mitral valve over
the complete cardiac cycle. At each acquired phase the appropriate measurement
plane for transvalvular flow can be defined to obtain accurate measurements of
mitral inflow and mitral regurgitation.
Acknowledgements
No acknowledgement found.References
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