Navigator echoes?

What are navigator echoes and how do they reduce motion artifacts?  

Navigators are additional RF-pulses used to dynamically track anatomic motion, especially superior-inferior position of the diaphragm. Navigator pulses may be either spin echo (SE) or gradient echo (GRE). 

The simplest form of navigator is graphically placed using a scout image over the dome of the liver. The navigator may be prescribed from a single plane (left), or by using two intersecting bands (right) to make a more accurate "pencil beam" or cylindrical column. The navigator band is typically 1-2 cm wide with about 1-mm spatial resolution along the beam.

Above: Navigator prescribed from single coronal scout image.
Right: Navigator prescribed by two intersecting bands on axial scout image.

M-mode display of navigator data showing motion of liver (lower gray bands) with respiration. Dark area is lung.

The echo signal returned by the navigator is reconstructed in the readout (motion) direction and displayed as lines of data along the beam direction. This technique is a form of line-scan imaging. It is also known as M-mode display since is equivalent to that used for viewing valve motion in M-mode echocardiography. 

Automated software detects the diaphragmatic peaks and troughs of motion. The MR technologist can further define at which positions data acquisition is permitted of excluded, usually within a certain tolerance, typically 3-6 mm. 

Information from navigator echo to trigger data acquisition during a specified portion of the respiratory cycle; can also be used to adjust the location of a group of slices to follow the organ displacement from respiration. The same triggering and phase reordering algorithms used with respiratory bellows can be used with navigators.

Although navigators are used primarily to track diaphragmatic motion, they can be placed through any moving object, such as the heart. Navigator echoes are frequently incorporated into multi-shot diffusion-weighted pulse sequences to self-correct for motion that occurs during image acquisition. They are commonly used to monitor and correct for subtle head movement in functional MRI studies. Objects like the entire head may be tracked in three dimensions using spherical navigators.

Advanced Discussion (show/hide)»

Additional Comments about Navigators:

One parallel imaging coil can be assigned to detect the navigator, or the entire body coil can be used.

Navigators may be leading, trailing, or both. The trailing navigator can be used to determine whether the diaphragm has returned to the same position before and after data acquisition. If the difference in position for the leading and trailing navigators exceeds a predefined threshold, the corresponding data can be discarded and remeasured.

Slice Tracking (also known as slice following) moves the scan volume to lie at approximately the same location with respect to patient anatomy for each acquisition. With tracking the position of the imaging stack is dynamically adjusted to the actual position of the anatomy of interest. A "leading navigator" is required. Both gating and tracking can be performed simultaneously.

Navigator gating levels may drift during acquisition. Automatic detection schemes are available on most scanners to correct for this by continuously monitor and update the expiration level during the scan.

Multiple navigator beams are possible on some scanners.

Two-dimensional navigator methods (e.g., Siemens' 2D PACE and Philips' MotionTrak) offer some advantages over line-scan methods. 2D methods excite a circular column of spins using excitation with a spiral k-space trajectory. This creates less extensive saturation artifacts, especially with small flip angles.

2D spiral RF-navigators may be modified by changing flip angle and number of cycles. Fewer cycles leads to a shorter pulse duration and fewer off-resonance problems but with more aliasing ring artifact near the beam. The shape of the 2D-RF pulse is typically either block or jinc, the latter having a superior excitation profile.

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References
     Ehman RL, Felmlee JP. Adaptive technique for high-definition MR imaging of moving structures. Radiology 1989; 173:255-263.
     Welch EB, Manduc A, Grimm RC et al. Spherical navigator echoes for full 3D rigid body motion measurements in MRI. Magn Reson Med 2002; 47:32-41.

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Related Questions
     How can navigators track heart position if placed on the diaphragm?  

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