Biomedical Engineering Reference
In-Depth Information
Fig. 9.4
Cardiac magnetic resonance image views. Display of cardiac magnetic resonance images
is based on the short axis and also on the long axis (with 2-chamber, 3-chamber and 4-chamber
views). Each configuration is selected to show the relevant chambers and cardiovascular struc-
tures with clarity. Different orientations display the chamber of interest at various positions and
bisections
motion by referencing contrasting moving light sources, we state Observations1
and 2 as follows:
•
Observation 1:
The movement of light lines can be perceived as the movement
of shadow lines on the illuminated background. We perceive light and darkness
as opposing signals and this is used to determine motion.
•
Observation 2:
When observing blood movement in MRI scans of the heart, it
is a mixture of observing moving dark shadows of asynchronous proton spins
among the bright imaged blood, and moving bright shadows of synchronous
spins among the contrasting imaged blood.
The computerized motion tracking technique makes use of the moving accumula-
tion of asynchronous spin phase shifts. The incoherent spins cause the blood to
appear dark in steady-state free precession (SSFP) MRI. Apart from passing blood
through heterogeneous magnetic fields, turbulence in blood flow entering into the
scanned slice may consist of different spin shifts from the imaged region of interest.
It is the in-plane tracking of such asynchronous spins that allows the velocity field
of blood to be developed. Since the dark regions represented by the irregular spins
correspond to the general flow path of the blood, the perception of motion devel-
oped from their intensity shifts may be correlated to the actual cardiac flow speed
and direction (Fig.
9.5
). If the blood containing a collection of spins (X) that has co-
herent phases leaves the slice of interest perpendicularly, and a new region of blood
with incoherent spins (Y) with multiple degrees of phase shift enters the same slice
at the same point, the pixel region at that point turns from white to black. Therefore,
the motion estimation algorithm identifies the blood in that region as stationary in-
plane. There is no specific motion in-plane but a velocity component through-plane.
The aim is to track flow patterns that relate to the in-plane orientation. This is
achieved by estimating the pixel displacement presented by the movements of a col-
lective group X within a region filled with Y or vice versa. If we assume a collective
group of spins Y moving in the in-plane direction then this appears in the image as
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