## 2 History and Rationale of Self-Gated Cardiac MRI  
Cardiac motion significantly complicates magnetic resonance imaging of the heart as it adversely affects image quality if not adequately accounted for. If motion occurs, inconsistencies among the acquired profiles in k-space and/or unwanted signal phase accrual during signal readout are among the sources of artefacts. Therefore, the suppression of motion artefacts in cardiac MRI is the most challenging, interesting, and rewarding field of active engineering research. In fact, on one of the very first MR images of the heart that was acquired in Aberdeen by [PERSON] and his team in the late 1970s, heart motion was immediately recognized as one of the sources of artefacts (Figure 1) [1]. In the ensuing years, with the advent of gradient hardware that permitted shortened repetition times, ECG triggering, and k-space segmentation, breath-holding became the minstay for motion artefact suppression in functional and anatomical cardiovascular MRI [2]. Although prospective ECG gating enables signal acquisition during a specific cardiac phase to 'freeze' cardiac motion, and respiratory breath-hold commands can be provided to do the same for respiratory motion, these mechanisms are inherently inefficient and therefore time-consuming. In addition, temporal resolution and spatial image resolution were governed by a patient-dependent duration and quality of the breath-hold. These shortcomings were rate-limiting for 3D imaging with large volumetric coverage, where image acquisition with high temporal and spatial resolution became impractical using breath-holding to suppress respiratory motion artefacts. During the past two decades, an abundance of work has focussed on addressing both the cardiac and respiratory motion problems for improved performance of cardiovascular MRI. With the development of respiratory navigators by [PERSON] et al., this hurdle could be removed and free-breathing 3D imaging of the heart with larger volumetric coverage became available [3]. However, navigator localization and parameter settings remained operator-dependent, and individual respiratory patterns led to various degrees of navigator efficiency and ultimately unpredictable scan times, which complicated the integration of navigator-gated free-breathing 3D scans into routine clinical protocols. Figure 1: One of the first MRI images of the heart was acquired in a volunteer using the Aberdeen NMR imager in 1979, before the spin-warp breakthrough. Although the contours of the body and some internal structures can be recognized, severe artefacts caused by cardiac motion were present preventing its clinical use. Image courtesy of [PERSON]. Interestingly, the concept of cardiac self-gating (SG) emerged for 2D imaging around that time. In their seminal work, [PERSON] et al. demonstrated that the cardiac signal can not only be extracted from continuously acquired k-space profiles but also be used to retrospectively bin data according to their position in the cardiac cycle without the need for an ECG [4]. The insights gained from this work stimulated [PERSON] et al. to exploit SG for the respiratory rather than for the cardiac signal to replace respiratory navigators with a respiratory SG concept [5]. Using 3D radial imaging during free-breathing and a repetitively acquired superior-inferior (SI) profile in k-space, respiratory SG was enabled. This helped minimize operator dependence, scan time was no longer affected by the patient's ability to hold their breath or their respiratory pattern, and whole-heart 3D imaging during free breathing in just a few minutes of acquisition time became available. This concept was then refined by [PERSON] et al., who extended the ECG-triggered 3D radial readout with a phyllotaxis pattern and refined the respiratory signal extraction algorithm for improved image quality [6; 7].