Phase navigator for respiratory triggering

Introduction: Respiratory triggering is a technique that reduces motion artifacts by synchronizing the measurement with the breathing cycle of the patient and by placing the data acquisition period into the relatively quiet end expiration phase [1]. The physiological signal needed by the trigger algorithm can be derived from the motion of the abdominal wall with a pneumatic sensor or by measuring the diaphragm position with a navigator [2]. Novel short bore MR systems increase patient comfort and help to reduce claustrophobia significantly. However, in large patients it can be difficult to place the imaging volume and the navigator into the homogeneity volume of such a scanner. In FMRI navigator echoes are used to correct physiologically related phase errors [3]. In this work the signal for respiratory triggering is derived with a navigator that measures respiratory induced off-resonance effects. Methods: In gradient echo imaging an off-resonance spin accumulates an extra phase that is proportional to the off-resonance and increases linearly in time. A respiratory induced global off-resonance can hence be derived from the phase difference of two successive echoes. The navigator pulse sequence used to measure the phase difference in this work is shown in Fig.1. The excitation volume of this sequence is an axial, 5 mm slice through the patient’s abdomen. The readout direction was chosen along the left-right direction. Fig. 2 shows the processing of the data. The data of each individual echo is first Fourier transformed along the readout direction to yield two projections. Next a pixel-wise complex multiplication of the second projection times the conjugate of the first projection is performed. The phase of the resulting data set is correlated with the patient’s breathing cycle. The combination of the different receiver channels, in the next step, can be performed by a simple complex addition since the phase contribution of an individual coil element was removed by the complex conjugate operation. The result of the channel combination is a one dimensional complex vector ) q ( pn , where n is the navigator index and q the pixel index, which is associated with the spatial coordinate along the readout direction. The signal output is set equal to the weighted mean phase of a window, which is crudely located in the liver of the patient: