Siemens EMOTION 16-Jun - Manual (Page 494)

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Respiratory Gating
Prospective respiratory triggering ver-
sus retrospective respiratory gating
With prospective respiratory triggering, the lung vol-
ume for example is covered in a "step-and-shoot" tech-
nique. The patient's respiratory signal is used to start
sequential scans at a predefined respiratory level of
the patient' s respiratory curve. With retrospective res-
piratory gating, the lung volume is covered continu-
ously by a spiral scan. The patient's respiratory signal is
recorded simultaneously to allow a retrospective selec-
tion of the respiratory level used for image reconstruc-
tion. Prospective respiratory triggering has the benefit
of smaller patient dose than respiratory-gated spiral
scanning, since scan data is acquired at the previously
selected respiratory level only. It does not, however
provide continuous volume coverage with overlapping
slices and misregistration of anatomical details may
occur. Furthermore, reconstruction of images in differ-
ent levels of the respiratory cycle for functional evalu-
ation needs repeated CT examination of each of the
desired respiration levels along the same volume in z-
direction using Prospective Triggering technique. Since
respiratory triggered sequential scanning depends on
a reliable prediction of the patient's next Inspiration
maximum and expiration minimum, the method
should not be used for patients with arrhythmic
breathing and irregular respiratory rates and the affin-
ity to cough and to sigh.