Introduction - Motion Correction in Thoracic Positron Emission Tomography - page 2

attenuation correction, the CT images themselves hardly suffer from motion since
they are usually acquired during breath holding and can be corrected for cardiac
motion by prospective electrocardiogram (ECG) triggering. In the presence of
severe motion, however, part of the PET data may not be in spatial correspondence
with the CT data and will be wrongly corrected for attenuation, e.g., activity from
the heart may be corrected with lung density [ 11 , 34 , 56 , 105 , 106 ], cf. Fig. 2.13 .
Another unwanted effect of motion is image blur. Motion at the source of
radioactive emission results in a spatial blurring in the reconstructed PET images
proportional to the magnitude of motion and thus loss of contrast. Generally, the
inherent motion during PET image acquisition has a number of negative conse-
quences like wrong attenuation correction (see above), misstaging of tumors [ 42 ],
inaccurate localization of lesions [ 105 ], and wrong calculation of standard uptake
values [ 96 ]. Therefore, there is a strong need of reducing motion artifacts for
advanced PET imaging.
Gating-based techniques were found applicable for this purpose [ 87 ]. Gating is
the decomposition of the whole data set into units that represent different breathing
and/or cardiac phases [ 23 ], cf. Sect. 1.3 . After gating, each single gate shows little
motion only, but suffers from a relatively low Signal-to-Noise-Ratio (SNR) and a
reduced contrast as only a small portion of all available events is used [ 133 ]. The
fact that images contain both cardiac and respiratory motion motivates the reduction
of both types by means of dual gating introduced in Sect. 1.3.3 . Most approaches in
the literature deal with respiratory motion only. However, as maximal displacements
for cardiac motion of 42 mm [ 134 ] are in the same range as maximal respiratory
displacements of 23 mm [ 122 ], the cardiac motion component should be treated
as well.
The effect of motion and gating is demonstrated in Fig. 1.1 with cardiac planes
of a human heart (20 min 18 F-FDG PET scan without attenuation correction).
An introduction to the used visualizations is given in Sect. 1.6 . A reconstruction
of the whole data set without gating can be seen in Fig. 1.1 a. Respiratory and
cardiac motion causes an obvious blurring of the heart contour. In contrast, a single
phase of the respiratory and cardiac cycle (dual gating with five respiratory and
five cardiac gates) is shown in Fig. 1.1 b. The blurring is clearly reduced. However,
simultaneously the amount of noise is increased due to the reduced number of events
used for the reconstruction. Furthermore, motion leads to an apparently higher
blood pool activity in the image without gating compared to the other images. This
phenomenon is illustrated with line profiles in Fig. 1.1 d. The left plot shows the line
profiles of the first column from left to right (respectively the second column from
left to right). The middle plot shows the line profiles of the first column from top to
bottom (respectively the third column from top to bottom). The right plot shows the
line profiles of the second column from bottom to top (respectively the third column
from left to right). The maximum peaks of the dotted profile (no gating) are clearly
lower in the central plot compared to the dashed profile (single gate). The overall
aim of gating-based motion correction is to combine the reduced blurring achieved
by gating in Fig. 1.1 b with the full statistics of the whole measurement in Fig. 1.1 a
towards enhanced motion compensated PET images. A preview of the result after
applying our proposed methods is finally given in Fig. 1.1 c.