  Purpose: To develop a fully automatic method for extraction and
directionality determination of respiratory signal in free-breathing, real-time
(RT) cardiac MRI.
  Methods: The respiratory signal is extracted by a principal component
analysis method from RT cine images. Then, a two-step procedure is used to
determine the directionality (sign) of the respiratory signal. First, the
signal polarity of all slices is made consistent with a reference slice.
Second, a global sign correction is performed by maximizing the correlation of
the respiratory signal with the zeroth-moment center curve. The proposed method
is evaluated in multi-slice RT cine from eleven volunteers and two patients.
The motion in a manually selected region-of-interest is used as reference.
  Results: The extracted respiratory signal using the proposed method exhibits
high, positive correlation with the reference in all cases and is more robust
compared to a recently proposed method. To demonstrate its clinical utility,
the method is used to identify heartbeats from inspiratory and expiratory
phases and, in turn, to account for respiration-induced changes in the cardiac
output.
  Conclusions: The proposed method enables fully automatic extraction and
directionality determinations of respiratory signal from RT cardiac cine
images, allowing accurate cardiac function quantification.
