Biomedical Engineering Reference
In-Depth Information
Cardiac motion is hitherto rarely considered in methods for motion correction.
It poses new challenges to motion correction techniques due to partial volume effect
(see Sect.
1.4
) induced intensity modulations. This issue is addressed and resolved
in Chap.
2
(Sect.
2.1.4
respectively Sect.
2.2.4
) as one of the major contributions of
this topic.
1.3.3
Dual Gating
In pure respiratory gated PET, each gate still contains cardiac motion. Analogously,
pure cardiac gated PET images still contain respiratory motion. This can be observed
in Figs.
1.4
and
1.6
.
Dual gating
is an attempt to even further reduce the amount of
motion contained in the images [
74
,
76
,
81
,
82
,
92
,
127
]. In dual gating, an
n
m
matrix of
n
cardiac and
m
respiratory images is built, i.e., each cardiac phase
is over again divided into all respiratory phases (or vice versa). Consequently,
time information about cardiac and respiratory motion is required. Options for
the division of the individual components of motion are discussed in the above
Sects.
1.3.1
and
1.3.2
. For a more detailed explanation of dual gating we refer
to [
92
].
The same patient data used for the pure respiratory and pure cardiac gating in
Figs.
1.4
and
1.6
serves as the basis for the 4
×
4 dual gating shown in Fig.
1.7
.
It can be seen that dual gating features an increased reduction of motion artifacts
compared to pure respiratory or pure cardiac gating. This is, of course, accompanied
by increased noise levels. Techniques to overcome the noise problem are provided
in Sect.
3.2
with a simplified pipeline for motion correction.
×
1.4
Partial Volume Effect and Mass-Preservation
The basic principles of the PET image generation process were already introduced
in Sect.
1.2
. In this section we will discuss some further properties of this process,
known as the Partial Volume Effect (PVE), which lead to a loss of spatial resolution.
The PVE can be divided into two distinct components:
1. Tissue fraction (Sect.
1.4.1
) and
2. Spill-over (Sect.
1.4.2
).
Especially in cardiac gated PET (cf. Sect.
1.3.2
), the PVE can lead to varying
intensities at corresponding anatomical points in different gates (cf. Fig.
1.11
),
which impairs motion estimation. The identification of the radioactivity-
preservation principle (respectively mass-preservation) in Sect.
1.4.3
allows to
account for this effect during motion estimation.
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