Overcoming Problems With Motion for Pediatric Imaging
Mehdi Hedjazi Moghari1
1Harvard Medical School, United States

Synopsis

Pediatric cardiovascular magnetic resonance (CMR) imaging is challenging due to bulk, respiratory, and cardiac motion. This lecture will cover key concepts in motion correction, including sedation, electrocardiogram (ECG) gating, cardiac self-gating, respiratory navigators, respiratory self-gating, and the state-of-the-art comprehensive free-breathing 3-dimensional (3D) CMR imaging.

Target Audience

Scientists and clinicians with basic and intermediate knowledge of CMR and pulse sequence design who conduct research to address motion problems or perform CMR exams in clinical practice.

Introduction

Pediatric CMR imaging is challenging due to the limited ability of younger children to hold still, and higher cardiac and respiratory rates giving motion blurring (1). High spatiotemporal resolution scans of the small heart and blood vessels of children are compromised by long imaging times and motion. For these reasons, faster imaging modalities such as computed tomography (CT) may be preferred over CMR for pediatric imaging. Despite CMR’s shortcomings, it has benefits over CT which justifies its further development and clinical use. An important advantage is the absence of ionizing radiation. Moreover, CMR allows for functional, hemodynamic, and viability assessment of the heart in a single exam. In this talk, we will review the challenges of CMR exams for pediatrics and describe solutions to make CMR imaging easier, safer, and more comfortable for children.

Bulk Motion

Many children older than about 6 years of age are able to hold still enough during the CMR scan to get a useful clinical study. For those who are younger and unable to hold still, sedation with general anesthesia or other medications is commonly used. Some studies have suggested that there may be an increased risk of learning disabilities in children exposed to anesthesia (2). Therefore, it is prudent to develop techniques that avoid anesthesia in young children who require CMR exam. The most common approach to this challenge is to develop techniques to accelerate imaging without sacrificing diagnostic quality. This in turn leads to shorter examination times and more children who can hold still for the duration of the exam.

Cardiac Motion

To minimize cardiac motion artifact, cardiac gating is performed by using the electrocardiogram (ECG) signal. To generate static images, k-space data is divided into multiple segments, and data acquisition is prospectively synchronized with the ECG signal so that it occurs during only one portion of the cardiac cycle. To generate moving (cine) images of the heart, segmented k-space data is continuously acquired and retrospectively binned to specific cardiac phases according to the timestamp with respect to the ECG signal (3).
The magnetohydrodynamic effect during a CMR exam creates an electrical signal that coincides with the T-wave of the ECG signal and may be mistakenly detected as an R-wave leading to data corruption (4). Since this ECG artifact increases with magnetic field strength, this presents a major challenge with higher-field scanners (7T (5) and 10.5T (6)). To address this issue and make patient preparation easier, several cardiac self-gating algorithms have been proposed to perform CMR exams without ECG leads. These algorithms include single shot real-time cardiac imaging (7,8) and cardiac self-gated imaging with radial, spiral, and Cartesian k-space trajectories (8-13).

Respiratory Motion

Respiratory motion during data acquisition must also be minimized to avoid image blur and ghosting artifact. One of the simplest approaches for respiratory motion compensation is breath-holding. Using this approach, the acquisition of an image is performed while children are asked to hold their breath for 10-20 seconds. However, children who are too young or ill cannot hold their breath and therefore a free-breathing acquisition with a respiratory motion compensation algorithm is necessary. One easy way to address breathing motion is by averaging multiple acquisitions during free-breathing. This method still results in some image blurring but reduces image ghosting (14).
As an alternative, a free-breathing acquisition with a respiratory navigator can be used to confine the data used for image reconstruction to end-expiration (15). Respiratory navigators are real-time 1D signals that track motion, most commonly the position of the diaphragm (i.e., liver-lung interface). Before the acquisition of each segment of k-space data, the diaphragm position is recorded. If it is in end-expiration, the acquired data is accepted for image reconstruction; otherwise, the data is rejected and reacquired in the next cardiac cycle. Although this technique reduces respiratory motion artifact, it prolongs the imaging time 2-3 fold depending on the breathing pattern. To reduce the imaging time and improve the accuracy of respiratory motion compensation algorithm, techniques have been developed to track and correct the respiratory-related motion of the heart directly rather than the diaphragm motion. This has been achieved by the use of respiratory self-gating (16-18), image-based navigators (19-22), and more sophisticated motion correction algorithms (23-26).

Conclusions

Scan time needs to be reduced to minimize or eliminate the need for anesthesia in younger children undergoing CMR exams. Free-breathing acquisitions are important for children who cannot hold their breath. Several respiratory and cardiac motion compensation algorithms have been developed and are available on commercial scanners.

Acknowledgements

We are keen to thank all investigators and colleagues who contributed research slides for this educational presentation.

References

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Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)