MRI Measurement of Heart Wall Motion - Advance Image Processing in Magnetic Resonance Imaging - page 338

information on myocardial tissue kinematics is required to evaluate mathematical
models of cardiac mechanics, which can be used to gain an understanding of
how tissue properties such as myocyte contraction, electrical activation, and
extracellular coupling combine to effect whole-organ function [2,3].
In order to measure tissue motion and strain, researchers have used implanted
radiopaque beads [4], ultrasonic markers [5], or natural landmarks such as the
coronary arteries [6]. These provide useful information, but are too invasive or
too sparse for clinical use. Recently, there has been a lot of clinical interest in
echocardiographic tissue Doppler imaging (TDI), which allows measurement of
tissue velocity along the direction of the ultrasound beam [7]. Spatial derivatives
of the velocity field give rise to strain rates, which can then be integrated through
time to give myocardial strain [8]. Although limited in signal-to-noise ratio (SNR),
regions that can be imaged, and components of the deformation available, this
technique is proving useful in the clinical evaluation of tissue function.
MRI also provides quantitative information on motion and deformation in the
heart, and offers the potential of precise, noninvasive assessment of all components
of deformation, in all regions of the heart, throughout the cardiac cycle, at acceptable
spatial and temporal resolution. Many of these techniques have been available for
over a decade for research purposes, but the translation into the clinical domain has
been hindered by the complex and time-consuming nature of the image postpro-
cessing. Recently, however, there have been rapid developments in the field, includ-
ing harmonic phase (HARP) and displacement encoding with stimulated echoes
(DENSE) acquisition techniques. These have the potential for higher spatial and
temporal resolution, and faster evaluation procedures. This chapter will review the
current state of the art, with a view to demonstrating the common aspects of the
different techniques. This approach will highlight a convergence of techniques for
magnetic resonance imaging (MRI) assessment of cardiac tissue kinematics. An
overview of image analysis and methods for reconstruction of 2-D and 3-D motion
and strain is also given, with some discussion on how these methods can be applied
to the different imaging procedures. For a more comprehensive review of MRI

cardiac deformation analysis techniques, see Reference 9 .

10.2 TAGGING

10.2.1 S T

ELECTIVE AGGING

Selective saturation pulses have been employed for many years to label tissue
and blood [10] and thereby obtain measures of motion and blood flow [11,12].
The basic spin preparation pulse sequence involves a selective (soft) radio fre-
quency (RF) 90 pulse combined with a slice-select gradient (typically oriented

orthogonal to the imaging plane), followed by a spoiler gradient designed to
dephase the transverse magnetization in the tag plane. This gives rise to a signal
void in the tagged slice, which can be subsequently tracked or used to label
blood flow by imaging the remaining longitudinal magnetization. Although this
technique can place magnetic tag planes at any position and orientation in the

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