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Respiratory Drift Correction ("Gating level drift = continuous")

Application Tip
Kouwenhoven, Marc, M.Sc. Philips Healthcare

Introduction

In R11 and R1.2, Continuous Respiratory Drift Correction is a new feature of the software.

In the description of Respiratory Drift Correction below, it is assumed that the reader is familiar with the basic principles and techniques of free breathing Navigator scans (see MR Application Guide, Volume 3 Cardiac Imaging)

 

To summarize, the Navigator is a 2D RF pulse that is used to determine the position of the diaphragm during scanning. Navigator gating and tracking assumes that within a small acceptance gate, there is a linear relation between the diaphragmatic respiratory motion and the heart respiratory motion. This linear relation is assumed to have a constant factor of 0.6, meaning that if the diaphragm moves up 10 mm, the heart moves up 6 mm. (with parameter Navigator "position = diaphragm"). This is only a first order approximation (for the average patient), but it has proven to work well in many cases. 

Functionality

Respiratory drift can occur during free breathing Navigator scans. If respiratory drift occurs, the average position of the diaphragm (and heart) slowly shifts upwards or downwards on a time scale of tens or hundreds of seconds. This drift can reduce the Navigator gating acceptance efficiency, because (without drift correction) the Navigator acceptance gate (in expiration) is determined only once, during the preparation phase at the start of the scan. A lower Navigator efficiency will lead to a longer scan time, and therefore in general to lower image quality.

 

Without drift correction, it is implicitly assumed that the average breathing position will remain more or less constant during the entire scan. In most scans over ~ 5 minutes however, significant respiratory drift occurs. For these prolonged scans, the acceptance gate can be adjusted automatically during the acquisition, by switching on the drift correction. Continuous drift correction could play an important role in reducing scan time and failure rate e.g. in long scans like the whole heart Coronary MRA scans.

 

Drift Correction is enabled by the following parameter: Examination parameter (in protocol, "Motion" page): Gating level drift = continuous (This parameter is only visible when the Respiratory Navigators are used).

 

The "continuous gating level drift" allows the Navigator acceptance gate to "drift" with the respiration with a maximum drift speed of 1 mm/min. The Navigator statistics are updated continuously every heartbeat with a latency time of 15 seconds. For every heartbeat, the desired, optimum gating level is determined based on the Navigator history of the last 15 seconds.

 

The actual gating level (for the next heart beat) as based on the desired gating level determined on the 15 seconds navigator history is limited by the maximum drift speed of 1 mm/min. This means that on a time scale of 1 heart beat, assuming an RR interval of 1 second, the gate position can only move 1/60 =0.017 mm. This maximum drift speed ensures smooth changes in gate position and prevents sudden large steps in gate position (back and forth) in case of occasional deep breaths or sighs.

 

Notice that the Navigator Display Window will show discrete, abrupt steps in the acceptance gate position, but this is only because of display limitations; the actual gate position will step continuously.

 

If "gate & track" is used ("Navigator respiratory comp. = gate & track"), tracking is performed taking the drift correction into account, meaning that if drift occurs, the tracking will follow the drift. Thus, it is assumed that diaphragmatic drift implies drift of the heart position (which is a reasonable assumption). Tracking will handle the drift motion similar to normal (non-drifting) diaphragmatic motion.

 

The advantage of using drift correction vs. simply using a larger gate is that with drift correction, more data is acquired in the expiration phase than in the inspiration phase. (With a larger gate, a larger part of the inspiration phase will be included) Since less respiratory motion occurs during expiration compared to inspiration (simply because the expiration phase usually takes longer than the inspiration phase), the (average) instantaneous respiratory speed during the expiration phase is lower than during inspiration. A lower instantaneous respiratory speed will result in a sharper image (Motion during TFE shot readout).

 

 

The image below shows the Navigator Display Window as shown during a free breathing scan. The Thick green bars at the bottom indicate data acceptance (when the red dots are between the blue gate acceptance lines). Notice that in this example there is a slow upward drift leading to unnecessarily rejected data in end-expiration (and thus a longer scan time).

Respiratory Navigator Display Window Example without drift correction. The blue lines indicate the acceptance gate. Every Red dot indicates one RR interval. The thick green bars below indicate data acceptance. Notice that in this example there is a slow drift upwards, leading to rejected data in expiration which could have been accepted with drift correction.
Respiratory Navigator Display Window
Example without drift correction. The blue lines indicate the acceptance gate. Every Red dot indicates one RR interval. The thick green bars below indicate data acceptance. Notice that in this example there is a slow drift upwards, leading to rejected data in expiration which could have been accepted with drift correction.

FAQ (Frequently Asked Questions)

  1. Is tracking still applied over the shift in gate position when drift occurs ?
    Answer : yes; when the gate shifts due to drift correction, the scan volume is also shifted (at least, if tracking is selected in the examination parameter). Thus, it is assumed that if drift of the diaphragm occurs, the heart will drift as well (which is a reasonable assumption). 

  2. What is the benefit of drift correction compared to simply using a larger gate?
    Answer : a) Drift correction assures that data are acquired mainly in end-expiration where the respiratory motion is expected to be least. Using a larger gate will accept more data, including data that are more in inspiration. Respiratory motion during the TFE readout is still an issue since it can create blurring. (A typical TFE readout shot is about 100ms and even a moderate diaphragmatic speed can result in substantial blurring: e.g. 1 cm/s = 1 mm/100ms ! Furthermore; the respiratory motion can be both upwards or downwards; the Navigator only detects the position prior to the TFE shot (with leading Nav))
    b) Drift correction is in principle allowed over a large distance. Only a very large gate will be able to compete with drift correction if significant drift will occur.

  3. Will drift correction affect the image quality?
    Answer: Drift correction will ensure that the scan will be finished successfully in almost all cases. As such, the primary intention of drift correction is to make the scan more robust and to reduce the scan time. In general, the shorter scan times will improve image quality compared to the situation without drift correction and the same gate width. However, in cases with a lot of drift both upwards and downwards during one scan, the result can be a bit more blurred compared to the (much longer!) scan without drift correction. If there is a drift in the beginning of the scan, and the end-expiratory position remains relatively stable during the rest of the scan, the resulting image will most likely be sharper than without drift correction (and the scan faster).


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Jul 12, 2005

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