Introduction - Motion Correction in Thoracic Positron Emission Tomography - page 7

Recently, intrinsic data driven gating schemes have been investigated as they
do not require any extra effort at the time of the scan in terms of auxiliary
measurements [ 23 ]. The respiratory signal is estimated on basis of list mode data
itself. To this end, the list mode stream is divided into small time frames (about
50 ms [ 23 ]) with subsequent computation of the axial center of mass along the
scanner axis of the measured counting rates in the respective frames. The respiratory
signal can then be deduced from changes in the center of mass.
The illustration of an amplitude-based respiratory gating scheme with variable
widths and equal widths within each cycle is shown in Fig. 1.3 . This scheme was
found to be superior to other gating schemes [ 35 ].

a b

time number

Respiratory gating signal Histogram

Fig. 1.3 The respiratory signal ( solid black curve in ( a )) is processed into a histogram in ( b ). The
amplitude is divided into sections ( vertical sections in ( b )) with variable widths such that each
section has the same area under the curve of the histogram. The amplitude subdivision remains
constant throughout all cycles. This subdivision is transferred back to the respiratory gating signal
which is then divided into different temporal sections ( horizontal sections in ( a )). All sections
belonging to the same vertical section in ( b ) together represent one of the respiratory gates

An example of respiratory gating applied to patient data can be seen in Fig. 1.4 .
The list mode data set was divided into four gates according to an amplitude-based
respiratory gating scheme with variable widths and equal widths within each cycle,
cf. Fig. 1.3 . More gates, typically in the range of eight to ten gates, are used in
practice [ 36 ].

1.3.2 Cardiac Gating

The objective of cardiac gating is to create a set of images that represent the dif-
ferent motion phases of the cardiac cycle, going from diastole over systole back to
diastole. For capturing the cardiac motion, a triggered ECG file can be recorded
(synchronized) to the PET scan. Usually only the R-wave time points are stored.
Based on this information a phase-based subdivision can be performed with
equidistant or variable intervals.