MR Imaging of the Neonatal Heart
Mehdi Hedjazi Moghari1
1Children's Hospital Colorado, Aurora, CO, United States

Synopsis

Keywords: Cardiovascular: Cardiovascular, Cardiovascular: Cardiac, Cardiovascular: Cardiac

Pediatric cardiovascular magnetic resonance (CMR) imaging poses significant challenges due to bulk, respiratory, and cardiac motion. In this lecture, key concepts in motion correction will be covered, including electrocardiogram (ECG) gating, cardiac self-gating, respiratory navigators, respiratory self-gating, and state-of-the-art comprehensive free-breathing 3-dimensional (3D) CMR imaging.

Target Audience

The target audience for this lecture is composed of scientists and clinicians who possess basic to intermediate knowledge of cardiovascular magnetic resonance (CMR) and pulse sequence design and are engaged in either conducting research that addresses motion-related problems or performing CMR exams for children with congenital heart disease in a clinical setting.

Introduction

Pediatric cardiac magnetic resonance (CMR) imaging poses a significant challenge due to the limited ability of younger children to remain still during the scan, and their higher cardiac and respiratory rates that can result in motion blurring (1). Long imaging times and motion artifacts compromise the quality of high spatiotemporal resolution scans of the small heart and blood vessels of children. Due to challenges with motion and long imaging times, faster imaging modalities such as computed tomography (CT) may be preferred over CMR for pediatric imaging. However, despite its shortcomings, CMR offers benefits over CT, which justifies its continued development and clinical use. One important advantage of CMR is its lack of ionizing radiation. Additionally, CMR enables the functional, hemodynamic, and viability assessment of the heart in a single examination. In this talk, we will discuss the challenges of performing CMR examinations in pediatric patients and describe solutions to make the CMR imaging process easier, safer, and more comfortable for children.

Bulk Motion

Most children older than 6 years of age can remain still enough during a CMR scan to obtain a useful clinical study. However, for younger children who are unable to remain still, sedation with general anesthesia or other medications is often necessary. It is worth noting that some studies have suggested an association between exposure to anesthesia and an increased risk of learning disabilities in children (2). Given the potential risks associated with anesthesia, it is important to develop techniques that minimize its use in young children undergoing CMR exams. One commonly employed approach to this challenge is the development of techniques that accelerate imaging without sacrificing diagnostic quality. By reducing examination times, such techniques may enable more children to remain still for the duration of the scan.

Cardiac Motion

To minimize motion artifacts in cardiac imaging, the electrocardiogram (ECG) signal is used for cardiac gating. In order to generate static images, the k-space data is divided into multiple segments, and data acquisition is synchronized with the ECG signal to occur during a specific portion of the cardiac cycle. To generate moving (cine) images of the heart, segmented k-space data is continuously acquired and then retrospectively binned into specific cardiac phases based on their timestamps relative to the ECG signal (3).
The magnetohydrodynamic effect, which occurs during a CMR exam, can create an electrical signal that coincides with the T-wave of the ECG signal and be erroneously detected as an R-wave. This can potentially lead to data acquisition from an inaccurate portion of the cardiac cycle and image corruption (4). Higher magnetic field strength increases the ECG artifact, which poses a significant challenge for higher-field scanners such as 7T (5) and 10.5T (6). Several cardiac self-gating algorithms have been proposed to perform CMR exams without the need for ECG leads, which can simplify patient preparation and address the issues related to the magnetohydrodynamic effect. 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

To prevent image blur and ghosting artifacts, it's important to minimize respiratory motion during data acquisition. One effective way to achieve this is through breath-holding. This approach involves instructing children to hold their breath for 10-20 seconds while the image is being acquired. However, younger or ill children may not be able to hold their breath, necessitating a free-breathing acquisition with respiratory motion compensation. A simple way to address breathing motion is by averaging multiple acquisitions during free-breathing. While this method may still result in some image blurring, it reduces image ghosting (14).
Alternatively, a free-breathing acquisition with a respiratory navigator can be utilized to limit the data used for image reconstruction to end-expiration (15). Respiratory navigators are real-time 1D signals that track motion, often monitoring the position of the diaphragm (e.g., liver-lung interface). Prior to acquiring each segment of k-space data, the position of the diaphragm is recorded. If the diaphragm is in the end-expiratory position, the acquired data is accepted for image reconstruction. If it is not, the data is rejected and must be reacquired in the next cardiac cycle. While this technique reduces respiratory motion artifact, it can increase imaging time by 2-3 times, depending on the breathing pattern. To decrease imaging time and enhance the accuracy of respiratory motion compensation, techniques have been developed to track and correct respiratory-related heart motion directly from the heart, rather than relying on diaphragm motion. These techniques include respiratory self-gating (16-18), image-based navigators (19-22), and more sophisticated motion correction algorithms (23-26).
Self-gating techniques have been also developed to compensate for both respiratory and cardiac motion simultaneously, resulting in a significant reduction in CMR imaging time (27-34). However, some of these techniques require the administration of gadolinium-based or iron-based T1-shortening intravenous contrast agents to achieve a significant improvement in signal-to-noise and blood-to-myocardium contrast-to-noise ratios. These contrast agents are associated with side effects, including deposition in the brain (35) and anaphylaxis (36). As a result, a technique that does not require administration of any contrast agent is preferred for children.

Conclusions

The conclusion of the lecture is as follows:
1) To minimize or eliminate the need for anesthesia in younger children undergoing CMR examinations, it is important to reduce the scan time.
2) Free-breathing acquisitions are essential in CMR examinations for children who are unable to hold their breath. Several algorithms have been developed to compensate for respiratory and cardiac motion, and they are available on commercial scanners.
3) Non-contrast CMR examinations are preferred for children to avoid possible anaphylactic reactions and contrast agent deposition in the brain, compared to CMR examinations that involve the administration of contrast agents.

Acknowledgements

We would like to express our gratitude to all investigators and colleagues who contributed research slides for this educational lecture. We also would like to acknowledge grant support from NIH-NHLBI (R01HL149807).

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Proc. Intl. Soc. Mag. Reson. Med. 31 (2023)