Object ID: 00000018WIA30E4B770GYZ
Topic ID: id_40026532 Version: 1.4
Date: Mar 2, 2022 4:46:27 PM

Navigator: considerations and troubleshooting tips

Use the Navigator option to obtain a navigated, free-breathing, coronary artery or body acquisition. There are two Navigator modes:

Navigator Gating
Navigator Triggering

CAUTION

Motion compensation accuracy may be affected by the presence of contrast agents, resulting in decreased image quality.

Navigator gating

A Navigator gating scan time is almost triple the scan time of a breath-hold scan.

Navigator gating is an alternate acquisition method for the navigator and imaging signal. Note that the image signal is only stored when the navigator signal is within a predefined acceptable range. Navigator gating is compatible with the following PSDs:

Figure 1. Coronary artery scans


Mode	PSD family	Pulse	Imaging options
3D	Gradient Echo	Fast SPGR	Navigator, Cardiac Gating/Triggering
3D	Gradient Echo	Fast GRE	Navigator, Cardiac Gating/Triggering
3D	Gradient Echo	FIESTA	Navigator, Cardiac Gating/Triggering

Figure 2. Body scans


Mode	PSD family	Pulse	Imaging options
3D	Gradient Echo	LAVA	Navigator
3D	Gradient Echo	Fast GRE or Fast SPGR	Navigator Imaging Option and Dual Echo (# of TEs per Scan = 2)

Figure 3. Body scan one-click application


Mode	One-click Application	Imaging options
3D	LAVA-Flex	Navigator
3D	DISCO	Navigator

Navigator triggering

Navigator triggering is a continuous acquisition of the navigator and imaging signal only when the detected position of the navigator signal is within a predefined acceptable range. Navigator triggering is compatible with the following PSDs:

Figure 4. Body scans


Mode	PSD family	Pulse	Imaging option/notes
2D	Echo Planar Imaging	DW EPI	Navigator
2D	Fast Spin Echo	FRFSE, FSE-IR, FSE, SSFSE, SSFSE-IR	Navigator
3D	Fast Spin Echo	FRFSE-XL	Navigator, MRCP
3D	Fast Spin Echo	Cube	Navigator
3D	Vascular	Inhance IFIR, only available on 1.5T systems	Navigator

Navigator troubleshooting tips

Navigator signal enhancement may reduce T1 contrast if a low flip angle (approximately 10 degrees) is used.

Figure 5. Troubleshooting tips for examples of Navigator patterns

Navigator waveform pattern	Cause	Action
Drift down	Patient specific	Wait 3-4 cycles and move the threshold up slightly, if needed.
Drift up	Patient specific	Move threshold up slightly at once (avoids accepting mid respiratory points).
Apnea	Patient specific	No action.
Abnormally small excursion	Patient specific	Reduce Navigator acceptance window.
Acceptable poor signal	The low navigator signal is due to inhomogeneity effects at the dome of the liver.	There is no need to change any Navigator parameters since the green waveform correctly reflects the respiratory status.
Low SNR	Saturation effect by imaging excitation.	There are three options to correct the saturation effect: From the Graphic Rx toolbar, increase the tracker thickness. For LAVA, LAVA-FLEX, 3D Dual Echo, there are two additional options: From the Navigator tab, click Navigator Signal Enhancement option. Reduce the flip angle.
Diaphragm is far from center of grayscale	Tracker is located too far away from diaphragm.	From the Graphic Rx viewport, center the tracker line between the diaphragm and the lung. For details, see: Navigator: acquire a body scan Navigator: acquire a coronary artery scan
Undesirable signal	Interference from any of the following signals: Heart Lung inflammation Subcutaneous fat Visceral fat Vessels	For LAVA, LAVA-FLEX, 3D Dual Echo, from the Navigator tab, click Navigator Signal Enhancement option. Reposition the tracker position to avoid undesired signal. For details about tracker position, see: Navigator: acquire a body scan Navigator: acquire a coronary artery scan