Cardiac and respiratory motion remains a significant hurdle for
widespread clinical adoption of cardiovascular magnetic resonance (CMR). This
lecture will cover key concepts in cardiac and respiratory motion correction,
including electrocardiogram (ECG) gating, cardiac self-gating, respiratory
navigators, respiratory self-gating, and the state-of-the-art comprehensive 3D
cine CMR and fetal cardiac imaging.
Introduction
Cardiac and respiratory motion in CMR causes blurring and ghosting artifacts which reduces diagnostic yield (1). ECG gating is commonly used to resolve the cardiac motion in CMR (2). This approach, however, fails in a high-field magnetic field (3) and where ECG signal is inaccessible such as fetuses (4). Cardiac self-gating (5), which uses raw CMR data to resolve the cardiac motion, is an alternative but is not yet utilized in routine clinical practice.
Breath-holding is the most common strategy to address respiratory motion (6). Although this is a simple and clinically practical solution, in many applications, such as three-dimensional (3D) acquisitions, the scan time is too long for patients to hold their breath (7). Furthermore, some patients are too ill or young to hold their breath. Alternative approaches including respiratory navigators (8) and self-gating have been developed to enable CMR data acquisition during free-breathing with minimal respiratory motion artifact.
In this presentation, we will be exploring different cardiac and respiratory motion correction algorithms and important concepts such as cardiac gating and respiratory gating. We will also focus on a main challenge of CMR in clinical practice (i.e., long scan time) and discuss recent comprehensive 3D cine CMR solutions. Finally, we illustrate new approaches in fetal cardiac imaging.
Cardiac and Respiratory Motion Correction
Static Imaging of the Heart:
To minimize cardiac motion artifact, cardiac gating is performed by using the ECG signal. Data is first divided into multiple segments, each of which is prospectively acquired one time in the heartbeat during the quiescent period of the cardiac cycle set by the trigger delay. Multiple cardiac cycles (i.e., beats) are required to acquire all data segments, complete data acquisition, and generate a static image of the heart in one phase of the cardiac cycle.
The respiratory motion must also be minimized if static images are to have optimal quality. Patients may be asked to hold their breath or respiratory navigators are used to confine the data acquisition to a designated portion of the respiratory cycle, typically end-expiration. 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 image 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. The result after multiple cardiac and respiratory cycles is an image constructed from data acquired from one point in the cardiac cycle and only in end-expiration.
Cine Imaging of the Heart:
Imaging cardiac and blood flow motion over time or “cine imaging” is often used to derive clinically important functional information. Cine imaging is achieved by creating images at multiple instants or “phases” across the cardiac cycle and then playing the images in a loop to appreciate motion. A segmented approach is typically used whereby the image data for each phase is built up over multiple cardiac cycles. Image data is acquired throughout the cardiac cycle without interruption and retrospectively binned into cardiac phases.
During cine acquisition, respiratory motion compensation is typically done with breath-holding. A respiratory navigator cannot be used because it would interrupt the cine acquisition resulting in a flickering artifact and missing cine data in that part of the cardiac cycle.
Recent Motion Correction Techniques
Static Imaging of the Heart:
The respiratory-induced motion of the diaphragm and the heart are similar but not identical. In recent years, techniques have been developed to track and correct the respiratory-related motion of the heart directly to improve the accuracy of motion correction and reduce the imaging time. This has been achieved by the use of respiratory self-gating (9-11), image-based navigators (12-15), and more sophisticated motion correction algorithms (16-19). We will survey these recent techniques and evaluate their performance for high resolution coronary magnetic resonance angiography.
Cine Imaging of the Heart:
Many cardiac self-gating techniques have been proposed recently to perform CMR exams without ECG leads. This has been achieved by single shot real-time cardiac imaging (5,20), cardiac self-gated imaging with radial, spiral, and Cartesian k-space trajectories (4,5,21-24). We will review these techniques and enumerate the benefit and shortcoming of each algorithm. To compensate for the respiratory motion in the free-breathing 2D cine imaging, two recent techniques, Cine-Nav (25) and respiratory-triggered acquisitions (26) will be discussed. These techniques will be then extended to 3D cine acquisitions (27-32).
Comprehensive CMR and Fetal Cardiac Imaging
At the end of the discussion, we will discuss the potential advantages of 3D cine methods vs. 2D cine methods to make CMR exams free-breathing, fast, easy, and more comfortable for patients. Furthermore, we will explore a novel cardiac self-gating technique developed for fetal cardiac imaging.
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