Corrections for Physical Factors - Correction Techniques in Emission Tomography - page 89

FIGURE 5.19: Non-gated (top) and gated (bottom) FDG PET images of
tumors located in the lower lung, coronal view. The arrow denotes the line
along which the profile (right) was determined. Respiratory gating reduces
motion-induced image blur and enhances tracer quantification.
One of the key elements of respiratory gating is the acquisition of a valid
respiratory signal during the scan. For this task, different implementations
have been introduced. Commonly used are pressure-sensitive devices fixed at
the patient's abdomen during the scan that record changes in pressure due
to respiration [46] [51], IR or video camera techniques monitoring markers
placed on the patient [25] [69] or devices measuring changes in temperature of
the exhaled air [13]. Generally, such a setup should be able to give absolutely
quantifiable motion information instead of just giving time information of in-
spiratory and expiratory phases. This is due to the fact that patients usually do
not breathe regularly, neither in terms of frequency nor in terms of amplitude,
resulting in a loss of motion resolution when using only time-/phase-based
gating. Instead, amplitude-based should be used whenever possible, leading
to superior motion capturing [25].
Recently, the idea of using PET raw data (list mode data) itself to ob-
tain respiratory motion information has been discussed, rendering additional
hardware unnecessary [16] [80]. This may also have the additional advantage
of directly measuring internal organ/lesion motion instead of having only ex-
ternal motion information.
One drawback of respiratory gating is the apparent loss of image statis-
tics, as a significant part of raw data is discarded and not taken into account
for reconstruction. However, gating may just be a first step in more general-
ized motion correction algorithms that aim to reconstruct motion-free images
without loss of statistics applying a respiratory motion model during recon-