FieldStrength. Using MotionTrak for motion correction in body imaging. Application tips

Transcription

1 FieldStrength Publication for the Philips MRI Community Issue 35 September / October 2008 Using MotionTrak for motion correction in body imaging Application tips This article is part of Field Strength issue 35, Sept./Oct. 2008

2 Application tips Using MotionTrak for motion correction in body imaging MotionTrak Body is a new tool that uses navigator imaging and tracking to compensate for respiratory motion. Slice tracking is implemented to overcome the difference in anatomical position of the liver during multi breath hold or respiratory triggered sequences. There are basically three methods to deal with Jeroen Stout, respiration: breath holding, Philips MR Application. free breathing with respiratory triggering or MotionTrak. If anatomy is at a different position in different breath holds, breathhold techniques will have coverage differences between images. MotionTrak moves the stack to maintain the same position with respect to the anatomy in every breath hold, exactly as planned on the survey. 1 st breath hold 2 nd breath hold with slice tracking What is a navigator? The navigator produces a line image of the anatomy: white is liver tissue and black is lung. Inspiration moves the liver down, and expiration moves the liver up in the navigator display. A trigger point occurs when the expiration enters the gating window in the expiration phase. Because the navigator is a real image of the actual liver/diaphragm/lung, MotionTrak is more accurate than the respiratory belt that only shows in- and expiration which not always correlates exactly with the liver position. Free breathing acquisition with navigator trigger and track. Red dots mark the navigator sample points which follow the patient s respiration pattern. Blue lines outline the gate window, the green dots at the bottom indicate navigator shots within the gate window. Tip 1: Trigger delay in MotionTrak This navigator acquisition shows where a trigger point is detected (yellow circle). A trigger delay of 0 is usually a good choice if tracking is also used. Change the trigger delay only if the patient has a very long expiration phase. Best trigger delay depends on the shot length, but keep it below 100 ms. 42 FieldStrength Issue 35 September / October 2008

3 Tip 2: Setting minimum TR Irregular breathing could cause trigger points to be very close to each other, so that two shots/slices are acquired within one gating window. This may degrade image quality. Setting a minimum TR will avoid acquisition of a second shot when the patient starts to breathe in again. The parameter minimum TR (white arrows) specifies a period where further triggers are suppressed. In this example the minimum TR forces the system to skip a potential trigger point. Tip 3: Scale factor The scale factor (similar to navigator position previously used in cardiac navigators) corrects for the difference between the liver position and the actual slice positioning (tracking factor). The navigator measures the displacement of the liver directly. So, set the scale factor to 1.0 for liver imaging: when 1 cm of liver motion occurs, the slices need also to be moved 1 cm in body imaging. In cardiac imaging the liver motion measured by the navigator is proportional to the heart motion, but not exactly the same. Here a scale factor of 0.6 is recommended (possible values are 0.6 to 1.0). Tip 4: Planning the navigator, setting navigator length Planning is important as it influences the quality of the navigator signal. A clear contrast between liver and lungs is needed for successful tracking. Plan the navigator beam through the right hemidiaphragm. Make navigator length long enough to cover the diaphragm during the complete respiratory cycle. Make navigator length short enough to exclude any static tissue. A shorter navigator also provides a more accurate position, but a larger length is more robust against deep breathing. Recommended navigator length is mm. For most patients 60 mm is enough. The maximum value is 150 mm. In case of shallow respiration the navigator quality improves by choosing a shorter navigator length. Beam length can also be used to change the display of the breathing stroke (difference between inspiration and expiration). FieldStrength 43

4 Tip 5: Selecting coil element for navigator acquisition Preferably use the QBC (Integrated Body coil) for acquiring the navigator. In some cases another coil or coil element close to the navigator beam can be used. When using a multi-element coil, select an element at the right anterior or right posterior part side of the coil. If a pop-up message appears that the navigator signal is too low, change to the QBC, which will provide a good navigator signal. Tip 6: Starting navigator acquisition Always check navigator signal strength immediately when navigator acquisition starts. The navigator s signal should be strong with white liver tissue and black air. Adapt the navigator length if other tissue interferes with the navigator signal. The navigator gating window is placed when the navigator detects the expiration phase. The default window is 10 mm. Check that the patients breathing rhythm in the first preparation phase of the navigator is the same as during the rest of the examination. The gating window is automatically adapted if the diaphragm s position changes during acquisition (gating level drift). For instance, if the patient falls asleep, the window is adapted to the actual expiration. Examples: Example 1: MotionTrak vs. free-breathing, respiratory triggering Respiratory triggered, free-breathing single shot T1-weighted TFE from Panorama HFO. Different anatomy positions due to irregular breathing, lead to slice misregistration as seen on the sagittal MPR of this acquisition. The sagittal MPR of the same acquisition with MotionTrak shows all slices nicely aligned, because MotionTrak detects the actual position of the liver to correct slice position. The navigator display shows the variation in the position of the anatomy. 44 FieldStrength Issue 35 September / October 2008

5 Net Forum Visit for extended version of this application tip. Example 2: MotionTrak with THRIVE 4-dynamic THRIVE without MotionTrak. As the breath hold position varies over the 4 dynamics, lesion comparison between dynamics is difficult. MotionTrak enables slice tracking within every individual breath hold, resulting in aligned images over the 4 dynamics. Example 3: MotionTrak with 8 breath holds Multishot T2W acquisition at 1.5T, 8 breath holds, high in-plane resolution 0.9 mm). The navigator window demonstrates clear differences in liver position between breath holds. Navigator slice tracking ensures correct slice alignment. Scanning this without MotionTrak would significantly increase the risk of missing anatomy. FieldStrength 45

6 2008 Koninklijke Philips Electronics N.V. All rights are reserved. Philips Healthcare reserves the right to make changes in specifications and/ or to discontinue any product at any time without notice or obligation and will not be liable for any consequences resulting from the use of this publication. Philips Healthcare is part of Royal Philips Electronics fax: Printed in The Netherlands Sept 2008 Philips Healthcare Global Information Center P.O. Box BG Eindhoven The Netherlands