Image Processing Reference
In-Depth Information
MRI to assess right- and left-ventricular parameters as represented by stroke
volume (SV), EF, LVM [44-49], wall thickening (WT) [50], myocardial motion [51],
and circumferential shortening of myocardial fibers [52]. Data from MRI are
more accurate than those derived from left-ventricular angiocardiography, where
the calculation is based on the assumption that the LV is ellipsoidal in shape.
Volume measurements by MRI are independent of cavity shape, with the area
from contiguous slices integrated over the chamber of interest.
Multislice multiphase imaging enables assessment of regional function in terms
of the local-wall-geometry changes over the cardiac cycle. However, the complex
twisting motion during cardiac contraction cannot be imaged with ordinary multislice
multiphase acquisitions, and therefore the gold standard for accurate regional con-
tractility analysis is cardiac MR tagging [53,54]. By locally changing the magnetiza-
tion of the tissues, a stripe grid can be applied to the myocardium, defining a material
coordinate frame. As the tagging stripes are deformed during cardiac contraction, the
material coordinates inside the myocardium can be tracked over the cardiac cycle.
Myocardial deformation can thus be tracked, allowing for stress and strain measure-
ments, which are assumed to be early indicators of myocardial dysfunction. Typically,
such measures are derived using an intermediary finite element (FE) continuum
model, which is coupled to the tagging intersection locations. From the deformations
of the continuum model, estimates for stress and strain are computed.
Apart from global function and regional myocardial contractility and motion,
the perfusion of the myocardium provides important diagnostic information on
coronary function [55]. The primary means to image perfusion with MR is first-
pass perfusion imaging. These images monitor the arrival and subsequent distribu-
tion within the myocardium of a contrast bolus. The rate and extent of perfusion
can be quantified by following the intensity profile of myocardial pixels over time.
Following first-pass perfusion, delayed enhancement images are commonly
acquired 15-20 min after contrast medium injection. Delayed enhancement imaging
[56,57] exploits the fact that the contrast medium tends to accumulate in necrotic
tissue, greatly enhancing the signal from infarcted regions, with an image resolution
much higher than that seen in common nuclear scans. Quantification can be per-
formed by measuring enhanced signal intensity within the myocardium and com-
paring it to nonenhanced myocardium, enabling an assessment of the extent and
location of necrotic tissue. In addition, an indicator for myocardial viability can be
derived from this analysis: infarct transmurality. As myocardial infarctions tend to
originate from the endocardial surface, the penetration of the infarction in the myo-
cardium toward the epicardial wall is regarded as a measure of infarct severity.
Myocardial viability can be inferred from this infarct transmurality, where a higher
transmurality typically signifies a decreased viability and, thus, a reduced chance of
myocardial recovery after intervention.
Alternatively, global and regional function can also be quantified from phase-
contrast MRI. These images depict the velocity of a material point in the scanner,
where the gray values represent the velocity. By acquiring phase-contrast images
perpendicular to the aorta slightly distal to the aortic root (aortic-flow images),
global parameters such as SV and cardiac output (CO) can be quantified with
Search WWH ::
Custom Search