Respiratory motion remains a significant hurdle for widespread clinical adoption of CMR. This lecture will cover key concepts in respiratory motion correction, including respiratory navigators, gating, slice tracking, prospective/retrospective correction and the state-of-the-art in the field. The main focus will be on high-resolution volumetric coronary angiography, which has been an active area of research for 25 years, and the challenges of clinical translation. However, we will also discuss the unique challenges of motion correction for other applications such as quantitative CMR (T1 and T2 mapping) and late gadolinium enhancement.
Respiratory motion has plagued the field of cardiovascular magnetic resonance (CMR) since its origins, causing blurring and ghosting artifacts, and reduced diagnostic yield1. Although breath-holding is the technically most simple and therefore clinically practical solution to avoid motion artifacts in CMR, in many applications the scan time extends beyond that which can be acquired within a 10-15 second breath-hold duration. These include (but are not limited to) high-resolution volumetric imaging such as coronary magnetic resonance angiography2, dynamic contrast-enhanced first-pass perfusion3 and time-resolved cine imaging4. Furthermore, it is desirable to minimize the need for patient compliance as many patients struggle to consistently hold their breath5. For many of these CMR applications respiratory motion correction strategies can be employed to enable data acquisition during free-breathing with minimal motion artifacts. In this session we will be exploring different respiratory motion correction techniques and important concepts such as prospective and retrospective correction, navigators, respiratory gating and slice tracking with different degrees of freedom. It is important to note that, due to the highly variable nature of CMR pulse sequences, there is not one correction technique which can be applied and expected to work robustly for all applications. Rather, each CMR application will have a highly tailored motion correction solution, often combining different compensation strategies, which work within the constraints of the specific pulse sequence6. Here, we will primarily focus on coronary magnetic resonance angiography which historically has been the subject of most research into respiratory motion correction7.
Respiratory navigators, gating and correction
Respiratory navigators are real-time images interleaved with the diagnostic CMR acquisitions8. Navigator images can be used to estimate (using image registration) and correct for the respiratory motion during the scan. The most commonly used respiratory navigator in CMR is the 1D diaphragmatic navigator, which was originally proposed for abdominal imaging in the 1980's9. We will look at why the diaphragmatic 1D navigator has become the dominant method for respiratory motion correction in CMR, its advantages and disadvantages and why it has remained the conventional technique for over 20 years10,11. The navigator motion information can be used for both respiratory gating and correction of the CMR acquisition. Respiratory gating involve adapting which k-space of the CMR scan to sample based on the respiratory motion state or to disable/enable data acquisition12-14, while correcting uses the motion information to modify the k-space data itself to undo the motion assuming some motion model (translation, rotation, affine or non-linear)15,16. In this lecture we will discuss different respiratory gating and correction techniques, including the merits of prospective and retrospective correction.
Recent motion correction techniques and remaining challenges
In the last decade, there has been a move away from diaphragmatic 1D navigator techniques towards measuring and correcting for the motion directly on the heart17-20. The aim of this has been improve motion correction accuracy and to reduce the need for time consuming respiratory gating. To a large extent, this has been achieved by innovations in navigator acquisition techniques, including the use of self-navigation17,18 and image-based navigation19-23, but also more sophisticated gating strategies, often in combination with inherently motion tolerant k-space trajectories24-27. In this lecture, we will survey the current state-of-the-art motion correction techniques, primarily in the field of high-resolution volumetric coronary angiography. However, we will also consider the obstacles towards clinical translation28-30 and the unique challenges of extending these respiratory motion correction techniques to other CMR applications31.1. Scott AD, Keegan J, Firmin DN.Radiology. Motion in cardiovascular MR imaging. 2009 Feb;250(2):331-51.
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